Abstracts No.20: Geology & Coal Mining Conference, 1987, Sydney by GSAustralia

Geological Society of Australia

ABSTRACTS 1

Number 20

GEOLOGY AND COAL MINING CONFERENCE

13-15 October 1987 New South Wales Institute of Technology Sydney L

COAL GEOLOGY GROUP Geological Society of Australia Inc.

NEW SOUTH WALES INSTITUTE OF TECHNOLOGY

GEOLOGY AND COAL MINING CONFERENCE Proceedings

1 3 - 1 5 October 1987 New South Wales Institute of Technology Sydney

CONTENTS KEYNOTE ADDRESS Dr. H. Arauner Pivoting of Longwall Faces and Relevant Geological Considerations.

MINING IN AUSTRALIA Mr. A. Galligan Setting the Scene - Coal Resources and Mining in Australia.

Mr. C. Longworth Project Feasibility and Mine Production Costs.

MINE PLANNING Prof. K. Moelle Geological Aspects of Longwall Mining Operations Past, Present and Future.

Mr. A. Kremor & Mr. A. Muir Geology - A Key to Mine Planning - A Case History of the Lochiel Coal Deposit.

Mr. B. Ross, Mr. C. Tobin, Mr. A. Williams Long Hole Exploration Drilling for Mine Planning in BHP Macquarie Collieires.

Mr. D. Fawcett Some intersections between Geological Data and Underground Mine Planning.

Mr. P. Goscombe 104 The Role of Coal Geologists in Open Cut Mine Planning. Mr. K. Whitby & Mr. G. May Exploration Geology and Geophysics - Their Role in Underground Mine Planning at German Creek Mine, Central Queensland.

113

Dr. C. Mallett & R.L. Hammond Prediction of Underground Conditions Using Open Cut Data.

121

Mr. I. Stone Site Characterisation and Forecasting Underground Mining Condtions.

127

Mr. G. McNally Use of Geophysical Logs as Input for Mine Layout Modelling.

136

ENVIRONMENTAL PLANNING Assoc. Prof- L. C. Bell Environmental Planning and Practice in Open Cut Mining.

148

Mr. R. Hughson Environmental Implications Arising from the Underground Extraction of Bituminous Coal.

160

COAL TECHNOLOGY Mr. N. Gibson Coal Preparation, and Possible Future Developments.

172

Dr. A. Waters Implications of Coal Properties for Thermal Coal Exports.

180

Dr. A. Lowe Important Coal Properties for Power Generation.

188

HYDROGEOLOGY Dr. W. Morton Exploration for Groundwater Sources and Hazards in Coal Mining.

198

Mr. J. Brumley An Overview of Geological and Hydrogeological Studies Conducted by the SECV for Open Cut Brown Coal Mining in the Latrobe Valley, Victoria.

205

GEOTECHNICAL STUDIES Dr. J. Shepard, K. P. Walton, Z. Nemes - Nemeth 218 Development of an Australian Colliery Roadway Support Nomogram Using Artifical Intelligence Computing Systems. Mr. N. Godfrey Assessment of Geotechnical Conditions Ahead of Surface Coal Mining - Case Histories from the Bowen Basin.

230

GEOPHYSICS Mr. D. Palmer Surface Seismic Techniques.

246

Mr. J. Doyle In Seam Seismic - A Tool for the Coal Industry.

251

Mr. R. J. Whitely, W. M. Holmes, R. Dowle Downhole and Crosshole Seismic: A New Tool for Detailed Definition of Coal Seams and Geological Conditions.

260

KEYNOTE ADDRESS

CONFERENCE FROM THE 13 - 15TH OCTOBER IN SYDNEY

"Pivoting of Longwall Faces and Relevant Geological Considerations"

by Dr. Hans-Wolfgang Arauner Bergwerksdirektor Eschweiler Bergwerks-Verein AG Federal Republic of Germany

PIVOTING OF LONGWALL FACES MID RELEVANT GEOLOGICAL CONSIDERATIONS

Mining con In the Federal Republic of Germany round about 80 million tonnes (saleable) of coal are produced every year. More than half of this production is burned in power stations, and the rest is either sold to iron and steel industries or used for heating purposes. In 1985 103 longwall faces were advancing ones, 71 retreating faces, and 30 faces were run in Z-configuration. The average daily output per face was of approximately 1700 tonnes (saleable) . This corresponds to a daily run-of-mine production of 3000 tonnes per coalface. The mining conditions are substantially different from those prevailing in Australia, South Africa, or the United States. As recently as 1960, the German collieries produced from an average winning depth of approximately 650 m, whereas the average working depth at present is almost 900 m.

Depth in m 700

800

900

1000

1100

1200

Average

weighted

—

winning depth

\ D e p t h of the deepest level

1300 1400

1500

1600 1960

2000

Fig. 1: Depth increase in the German hardcoal mining industry Although a part of the winning capacity has been transferred into new areas, an overburden thickness increase of approximately 7 m per year is expected for the future. Meanwhile the deepest production level is at 1440 m, where a number of coalfaces are actually operated. The specific gravity of the rock masses of 2.5 t/m3 means a depth pressure of approx. 3500 t/m2 (35 MN/m2) at a depth of 1400 metres.

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The d e p o s i t s of the R u h r d i s t r i c t are c h a r a c t e r i z e d by a large n u m b e r of seams in close v i c i n i t y to each other- T h u s , a s t a n d a r d s t r a t a p r o f i l e f o r the E s s e n area shows a p p r o x . 20 seams of m o r e t h a n 1 m of t h i c k n e s s w i t h a t o t a l c o a l t h i c k n e s s of 25 m .

Depth -700m

-1500m

Seam Karl 2

cm 160

Ernestine Wilhelm

170 270

President

320

Dickebank 2

160

Sonnenschein

220

- — - -

6 Seams 13m Coal W e s t f a l e n Colliery

2 0 Seams 25 m Coal Central Ruhr District

Legend Coal • Sandstone I

F i g . 2: G e o l o g i c a l s t a n d a r d s e c t i o n T h e s e seams w e r e l a r g e l y m i n e d b y the o l d c o l l i e r i e s in this a r e a . In some e x t r e m e cases surface s u b s i d e n c e of up to 20 m has b e e n e x p e r i e n c e d . A t a l l c o l l i e r i e s , s e v e r a l seams are w o r k e d one b y one in the same a r e a , a n d this l a r g e l y a f f e c t s s t r a t a c o n t r o l in the m i n e w o r k i n g s , a n d also i n c r e a s e s risks of rock b u r s t as w e l l as m e t h a n e e m i s s i o n s . The d e p o s i t is f r e q u e n t l y d i s t u r b e d b y m a j o r and m i n o r tectonic faults so t h a t , in m a n y c a s e s , u n i n t e r r u p t e d m i n i n g of l a r g e r areas is s i m p l y i m p o s s i b l e . P i l l a r s and ribs p r e v i o u s l y left r e s u l t in c o n s i d e r a b l e i n c r e a s e of rock p r e s s u r e . Because the R u h r c o a l d e p o s i t s w e r e i n t e n s i v e l y w o r k e d in m a n y seams t h r o u g h o u t the p a s t , and due to the m e c h a n i c a l i n f l u e n c e of ribs and p i l l a r s l e f t , the rock p r e s s u r e s e n c o u n t e r e d at 1000 m d e p t h in the w o r k i n g s c o r r e s p o n d to those n o r m a l l y e x p e c t e d to p r e v a i l at m o r e t h a n 1500 m of d e p t h .

•9—

Excavation and maintenance of mine workings With these prevailing conditions it is obvious that mine openings are expensive to drive and to maintain. Attempts to use rock bolts only for stabilizing roadways driven exclusively "in seam", i.e. without cutting into the roof to obtain an arch profile, were unsuccessful. Only a roof bolt/steel mesh combination in arch-shaped roadways was reasonably successful under favourable conditions at the comparatively shallow depths of 400 to 800 m. Most of the roadways driven in rock or in the seam are supported by yielding arches.

Fig. 3: View of a gateroad with arch support and backfilling This applies to roadway cross-section of 18 to 23 m2 or more. In these cases, the arch centers are of 75 cm or even 50 cm dimension for arch section weights of max. 44 kg/m. Despite this heavy support being used, it is in many cases impossible to maintain a sufficiently large roadway cross-section. Hydraulically set solutions for filling behind the arch supports are used to an ever-increasing extent for improving the supporting function of arch supports. On passage of the face - in advance mining, and in particular in case of medium or large seam thicknesses - the ribs are stabilized by "roadside" packs of 1.5 to 2.5 m thickness, formed by hydraulically setting mortars.

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Fig. 4: Roadside pack after face passage This practice contributed substantially to increased stability of arch supported roadways and to efficient sealing of the goaf against air leakages. These measures made the maintenance of roadways at great depths possible to such an extent that regular use is generally possible, although it is expensive. In particular with very thick seams, the heavy and bulky shield support requires face heading widths of 7 to 9 m. In case of high cover loads and less competent adjacent strata, the maintenance of gate roads is very often extremely difficult and expensive. Under extreme conditions the limits of feasibility are reached in those cases. Implications of the high costs for mine workings Due to the high costs for driving and maintenance of mine openings, the formation of two or more parallel roadways with several intersections is out of the question. The principal roadways as well as the main in-seam roads and the gateroads are headed as individual roadways, so that a coalface is linked to the general mine workings via a tail gate and a conveyor (main) gate. The cross-section of the main gate which, after passage of the face, remains more or less the same, has to cater for the conveyor with a belt generally 1200 m wide belt and for the material transport, generally a rope-driven suspended-monorail system or with traction by diesel or battery-powered locomotives. Since the roadway cross-section also has to be sufficiently large for the supply and disposal mains for compressed air, fresh water, waste water and cooling water, which makes "full use" of the

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c r o s s - s e c t i o n , a n d the r o a d w a y needs to be f r e q u e n t l y d i n t e d o r r e - r i p p e d . T h e s i t u a t i o n is less d i f f i c u l t , as there is n o r m a l l y no b e l t c o n v e y o r i n s t a l l e d w h i c h , h o w e v e r , m e a n s that d i n t i n g w o r k is m o r e d i f f i c u l t . Due to these space constraints in G e r m a n coal mining o p e r a t i o n s - in c o n t r a s t to the c o n d i t i o n s p r e v a i l i n g in other countries non-trackbound "free-steered" special vehicles or corresponding t r a c k - b o u n d m e a n s of transport c a n n o t be u s e d e x t e n s i v e l y f o r s u p p l y a n d d i s p o s a l t a s k s f o r the c o a l f a c e o p e r a t i o n s , n o r can these systems be u s e d f o r m o v i n g e q u i p m e n t to f o l l o w - u p f a c e s . T h i s , in t u r n , m e a n s t h a t a s h i f t of the c o m p l e t e e q u i p m e n t of a c o m p l e t e d face into a n e w face r e q u i r e s u p to 10 000 m a n s h i f t s , a c c o r d i n g to the d i s t a n c e s to be c o v e r e d , and c o s t s 4 to 6 m i l l i o n D M (equivalent of 3 to 5 m i l l i o n A u s t r a l i a n D o l l a r s ) . It takes b e t w e e n 3 a n d 6 m o n t h s to m o v e the e q u i p m e n t of a 200 to 250 m l o n g w a l l f a c e . S i n c e , e v e n w i t h a f a v o u r a b l y thick s e a m , 0.5 to 1 m i l l i o n tonnes of s a l e a b l e o u t p u t c a n be o b t a i n e d w i t h one c o a l f a c e , the m o v i n g of c o a l f a c e s is a c o n t r o l l i n g f a c t o r f o r the o v e r a l l p r o d u c t i o n costs of the c o l l i e r y and f o r the u t i l i z a t i o n rate of the technical equipment. The shifting of coalface equipment alone c o n s t i t u t e s a c o s t f a c t o r of D M 5 to D M 10 (equivalent of 4 to 8 A u s t r a l i a n D o l l a r s ) p e r tonne of s a l e a b l e output. C o n s e q u e n t l y , the l a y o u t of w o r k i n g s has to p r o v i d e for m a x i m u m c o a l b e i n g r e c o v e r e d by one c o a l f a c e . The " s w i n g i n g " or pivoting b y 180° of c o a l f a c e s is a g o o d a p p r o a c h to a c h i e v e that a i m , h o w e v e r , the a d v a n t a g e s and d i s a d v a n t a g e s require careful consideration.

Equipment of West-Germany

pivoted

coalfaces

Westfalen

Colliery,

- N u m b e r of c o a l f a c e s , r e s u l t s The following text r e f e r s e x c l u s i v e l y to the experience g a i n e d at W e s t f a l e n C o l l i e r y , w h e r e I am the chief e x e c u t i v e officer. A t p r e s e n t , W e s t f a l e n C o l l i e r y p r o d u c e s a p p r o x . 9000 tonnes of s a l e a b l e o u t p u t p e r day - this c o r r e s p o n d s to a d a i l y r u n of m i n e p r o d u c t i o n of 17 000 tonnes - from b e t w e e n 4 to 6 l o n g w a l l faces o p e r a t i n g at 1000 to 1200 m d e p t h . A c c o r d i n g to the p r e v a i l i n g g e o l o g i c a l c o n d i t i o n s , the face lengths r a n g e b e t w e e n 125 and 250 m , the l a t t e r b e i n g the d e s i r a b l e " s t a n d a r d " face l e n g t h . The a v e r a g e d a i l y o u t p u t of a l l c o a l f a c e s is at p r e s e n t a p p r o x . 1800 t (saleable) w h i c h c o r r e s p o n d s to a r u n - o f - m i n e p r o d u c t i o n of m o r e than 3000 t / d . As one c o a l f a c e always n e e d s to be run at o n l y 1000 t/d ( s a l e a b l e ) , the average p r o d u c t i o n of the o t h e r c o a l f a c e s is of a p p r o x . 2000 t/d (saleable) w h i c h c o r r e s p o n d s to a p p r o x .

3500 t run-of-mine per day. The highest-performing faces reach 3000 t/d (saleable) corresponding to more than 5000 t run-of-mine per day. it is intended to further increase the average ouput per face in the future. -Conveyors and coalwinning machinery Due to the great seam thicknesses, the highly varying intraseam stone bands, and the necessity to work through faults, drum shearer loaders are exclusively used at Westfalen Colliery. The double-ended ranging drum shearer loaders of the EDW 300 L type supplied by Eickhoff, Bochum, are standard, and the operation of the first EDW 600 L shearer is being prepared. Depending on the seam thickness, high-design or low-design machine frames and ranging arms of 1600 to 2680 mm length, as well as drum diameters of 1600 and 2000 mm are deployed so that seam thicknesses ranging between 2 m and more than 5 m (cutting height) can be accommodated. Faces dealing with strong and "difficult-to-cut" stone bands whose supply gateroad is headed to full section in line with the face, are occasionally equipped with two drum shearer loaders. Cut coal clearance is assured by face conveyors of the EKF III type supplied by Halbach & Braun, Wuppertal, with type E 82 pans with 30 mm bottom plate, 300 t connectors on both sides of the pans, and heavy-duty brackets.

Fig. 5: Controllability of the face conveyor ("diving")

—13—

This particular design assures high stability and operational reliability. Good controllability of the face conveyor is important. The conveyor is run with 30 mm double center chain strands with 2 x 113 t rated breaking load.

Fige 6: Controllability of the face conveyor (climbing) Depending on the prevailing conditions, the conveyor motors and drives' rating varies between 400 and 600 kW. The conveyor ramps are assigned a double function. When the conveyor is advanced they are to serve as static loading aid for the residual coal in the cutter track and, acting as travelling ramp for the shearer loader, they are to assure high stability by affording sufficient support to the machine with up to 50 t of weight. At Westfalen Colliery the Eickhoff-supplied Eicotrack chainless haulage system is in use as well as the Dynatrac system supplied by Halbach & Braun. Both systems are now well established and proven. Power supply and water supply for cooling and spray nozzles are assured by a chain-reinforced trailing cable handler. For financial reasons, a new conveyor design with exchangeable pan top (regarded as wear part) offered by Westfalia Lunen and by Halbach & Braun, has not yet been used at Westfalen Colliery. This design, however, exhibits significant advantages for great working lengths, and is thus particularly interesting for the "swung" faces because of its long service life.

—14—

Shield ^ u The whole development of shield supports for pivoted coalfaces in West German coal mining operations was significantly supported by Westfalen Colliery- Since 1973, 15 more or less complete shield support sets were fielded, and a large amount of experience is reflected in the application of latest design improvements. Shield supports from all three manufacturers (Westfalia Lunen, Hemscheidt and Klockner-Becorit), as run at Westfalen Colliery, allows successful "swinging" of longwall faces. For the German mining conditions the the following requirements:

support needs

to meet

Fig. 7: Lowered shield for large seam thicknesses - For adaptation to the varying seam thickness, a large hydraulic adjustment range of 1 to 2.7 or 1 to 3 is required. By using extendable forepoles with jointed canopy tips, the roof can be supported far beyond the face conveyor, so that, even with coal running off the face, the exposed roof can be supported to a farreaching extent.

—15—

Fig. 8: Extended shield for large seam

thicknesses

- The distance between the canopy tip and the coalface has to be kept small for reducing support lag, however, it should be large enough to allow the machine to cut up to the roof without contacting the support.

The cantilever length of the canopy is specified sufficiently large enough, so that a one-step-back operation is possible, i.e. after passage of the shearer loader the support is advanced. The face conveyor is advanced in a later cycle. - The tilting range of the face sprags must be of 180° ; when used as a jointed canopy tip, they are to induce a minimum of 40 kN into the roof, and, when used as face sprags, they need to induce a minimum of 50 kN into the coalface.

-16-

- The specific supporting pressure needs to be of 700 to 300 kN/m2 and must be directed towards the face side. - The support forces induced by the extended forepoles shall be of at least 300 kN magnitude. - In order to assure that the rated support forces become effective in operation, the shields, once a minimum pressure of 50 to 80 bar is achieved, need to be set by an automatic adjustment control to a pump pressure of 320 bar, independently of human intervention.

Fig. 9: Two-leg-shield with alining cylinders and pusher cylinders

- Dust shields are of particular importance for shields to be used in "swung" faces. The dust shields extend to the jointed canopy tip. They are operated by a total of 6 adjustment rams of 100 kN each. By controlled and directed extension of these rams, positioning of the neighbouring shields can be controlled and affected. - The main directional control of the shields is assured by pushers, installed in the skid system of each shield support unit; they need to apply a minimum force cf 300 kN. These heavy rams always have to be arranged behind the center of gravity of the shields, thus allowing better "steering" by the shields' rear end. - Instead of the pushers, diagonally arranged rams mounted on the skids of neighbouring shields can also serve the same purpose.

F i g . 10: V i e w of the face w i t h e l e k t r o h y d r a u l i c

control

The c o n t r o l of the s h i e l d s n e e d s to m e e t particular r e q u i r e m e n t s , s i n c e the rams in the dust seals f o r the j o i n t e d c a n o p y t i p , the m a i n c a n o p y , the g o a f - s i d e s h i e l d a n d the r e a r strut s y s t e m have to be a c t i v a t e d s e p a r a t e l y . The m o s t a d v a n c e d s h i e l d d e s i g n in a c o a l f a c e , a c t u a l l y in the s w i n g i n g p h a s e , is s u s c e p t i b l e to 18 + 2 c o n t r o l f u n c t i o n s . S u c h a d e s i g n at W e s t f a l e n C o l l i e r y a l l o w e d e l e c t r o h y d r a u l i c c o n t r o l f o r the f i r s t time in p r a c t i c e .

P r e p a r a t i o n of a "swing" p h a s e

of a c o a l f a c e

The faces m a y p i v o t e i t h e r a r o u n d a p o i n t in the c o n v e y o r g a t e o r a r o u n d a p o i n t in the s u p p l y g a t e . In a d d i t i o n , the p o i n t s a r o u n d w h i c h the "swing" takes p l a c e m a y be s i t u a t e d m o r e o r less w e l l b e y o n d the o p p o s i t e r o a d s i d e so that the i n n e r shields do not turn a r o u n d t h e i r v e r t i c a l a x e s , but f o l l o w a 180° s e g m e n t of a c i r c l e . A l l these v a r i a n t s have been practiced successfully at Westfalen Colliery. The e a s i e s t w a y is a "swing" a r o u n d a p o i n t s i t u a t e d in the conveyor (main) gate. This variant should always be p r e f e r r e d , e v e n if the c o n v e y o r s y s t e m n e e d s to be m o d i f i e d to some e x t e n t . The f o l l o w i n g s t a t e m e n t s r e f e r e x c l u s i v e l y to a "swing" a r o u n d the m a i n d r i v e .

— 1 8 —

Pivot point main drive For a 180° "swing" the main drive of a coalface of 200 m length does not change position over 5 to 6 months, as determined by the face advance rate.

Fig. 11: Swinging center at the main drive - old version This positioning therefore needs to be carefully prepared with consideration of the pressure conditions to be expected. While normally the stage loader is bedded not too deeply into the floor, and the main drive is advanced on slide rails, the swing requires an adequately dimensioned "cellar" to support the stage loader. Section

A-B

Section

C-D

Fig. 12: Swinging center at the main drive - new version

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This means that the roadway cross-section is to be increased from 20 to 23.7 m2 , and the floor width is increased to 6.4 m. Around the pivoting point, 2 to 3 m of coal are removed, and the roof strata are consolidated with fully resin-set strata bolts. Subsequently, the space between roadway support and rib sides is backfilled with cement matter, i.e. bulk matter such as anhydrite. Since the face end transfer point remains at the same location, it is extremely important to keep the lower strand of the face conveyor free of coal fines. This is assured by an approx. 1.2 m deep "cellar11. The main drive is kept in its position above the "cellar" by means of a steel structure. The standard fines conveyor is disposed of. For maintaining free access to the face, the main drive is fitted with a spur bevel gear on its goaf side, as in this fashion, the motor can be arranged closer to the face conveyor and consequently requires less space. The swing manoeuvre of a face For correct manoeuvring of a 180° "swing" of a coaface, a well-trained and disciplined crew is necessary. Training has been carried out underground and on the surface by supervisory personnel of the colliery, by expert representatives of the support suppliers, and an engineer of the mines surveying department. Success is a matter of exactitude in the work involved. The following points require particular consideration: 1. K e e p s p r o c k e t in swinging c e n t e r 2. Keep t r a n s f e r s t a t i o n in order 3. Short s u p p o r t tag 4. Install a d d i t i o n a l

supports

5. Lay rails on shields 1-3 6. Pack-building material at swinging center 7. Keep f a c e c o n v e y o r s t r a i g h t 8. Never

ever b a c k w a r d lean

9. Shields v e r t i c a l to c o n v e y o r 10. A d v a n c e shields f r o m s w i n g i n g road 11. S t a k i n g shield

No.1

12. A v o i d spalling of the c o a l face

Fig. 13: Please note in particular

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- During the "swing" manoeuvre the center of the chain sprocket is identical with the pivot point around which the swing takes place. The main drive must not be moved off alignement i.e. the main drive must be re-adjusted consistently and exactly. - The transfer from the main drive side to the stage loader must be kept in its correct configuration. In case of floorheave, dinting works must be done early enough. - The support lag after advancing is to be kept to a minimum. -

Additional

support

set

soon

possible.

- 5 m prior to the beginning of the manoeuvre, steel bars are placed on the shields Nos. 1 - 5 for preventing roof falls and for making use of the lower friction coefficient (steel on steel) for easier directional change (small advance travel, large circumferencial travel). - After re-adjustment of the first shield unit, cavities need to be correctly filled for protecting the pivot point. - It is absolutely essential to keep the face conveyor in a straight alignment. The conveyor line must never exhibit a backward salient. A slight forward salient in the middle of the coalface of up to 2 m, which implies a slight trend of the conveyor pan joints to open on the face side, can be advantageous.

- Over the whole length of the face, the shields must always be kept and "moved" normal to the conveyor line. This means that, in the zone of the pivot point, the shields' angular movement must correspond exactly to that of the remotest shields. Consequently, the outward shields are advanced by several cycles while the shields in the vicinity of the pivot point turn more or less around their rear portion. - To allow turning of the shield, the outward neighbouring shield must first be manoeuvred to create the necessary space. This applies in particular to the shield units Nos. 1 - 30 which always need to be advanced in a "outward-inward" sequence. For the more outward shields the design-inherent gaps between the individual units may be sufficient for manoeuvring. - Whereas for the shield units in the face, the neighbouring shields need to serve as abutment for manoeuvring and readjustment, this does not apply to the first shield on the actual pivot point.

F i g . 14: S t a k i n g of the edge s h i e l d s The f i r s t and the t h i r d s h i e l d u n i t , r e s p e c t i v e l y , c o u p l e d b y chains a n d h y d r a u l i c rams in s u c h a w a y , that f i r s t s h i e l d c a n be p u l l e d v i a skids and c a n o p i e s into r e q u i r e d p o s i t i o n , so that it c a n be u s e d as a b u t m e n t f o r second shield's manoeuvring.

are the the the

- D u r i n g a s w i n g m a n o e u v r e b y 180° the c o a l f a c e n e c e s s a r i l y i s , at a g i v e n m o m e n t , in a p o s i t i o n p a r a l l e l to the d i r e c t i o n of c l e a t s . If c o a l " r u n - o u t " from the face at that stage c a n n o t be a v o i d e d , in spite of s u i t a b l e s h i e l d d e s i g n , it m a y b e c o m e n e c e s s a r y , to some e x t e n t , to take m e a s u r e s a g a i n s t c o a l r u n - o u t , i . e . to use w o o d e n d o w e l s a n d to c o n s o l i d a t e the c o a l f a c e e . g . b y p o l y u r e t h a n e application.

The m o s t serious m i s t a k e s faces a r o u n d

to be a v o i d e d w h e n " s w i n g i n g "

coal

S e v e r a l m i s t a k e s c a n be coalface being stopped.

made,

the

that

could

result

- A " b a c k w a r d s " s a l i e n t of the face line r e s u l t s in the s h i e l d u n i t s b e c o m i n g stuck a g a i n s t e a c h o t h e r so that the face s u p p o r t as a w h o l e c a n n o t m o v e f o r w a r d a n y m o r e . It is e x t r e m e l y b o t h e r s o m e , above a l l in case of long and d i f f i c u l t c o a l f a c e s , to create the n e c e s s a r y c l e a r a n c e , s t a r t i n g from the r a d i a l r o a d w a y , to a l l o w a d v a n c e m o v e m e n t . F o r these purposes, transportable short hydraulic rams are used.

— 2 2 —

- D i s c o n n e c t i n g of e i t h e r one o r s e v e r a l shields from the c o n v e y o r for the a b o v e - m e n t i o n e d r e a s o n s is to be a v o i d e d . I f , f o r saving w o r k and t i m e , the m a n o e u v r e s of the s u p p o r t s - o v e r the i n i t i a l t h i r d of the face's t o t a l lengthis not run w i t h the n e c e s s a r y c a r e , the s u p p o r t w h i c h w o u l d then be in a d i a g o n a l p o s i t i o n r e l a t i v e to the c o n v e y o r , m a y b e c o m e s t u c k , m a k i n g f u r t h e r advance i m p o s s i b l e .

M o n i t o r i n g of the "swing" m a n o e u v r e Since the e x a c t i t u d e in c a r r y i n g out v a r i o u s i n d i v i d u a l w o r k i n g cycles is a k e y f a c t o r for s u c c e s s , early m o n i t o r i n g and i n f o r m a t i o n of the face crew on p l a n n i n g and d e v i a t i o n s from p l a n n i n g , is r e q u i r e d . If the w o r k e d t h i c k n e s s , the face l e n g t h , and the t e c h n i c a l e q u i p m e n t allow use of a l a s e r , s u c h a l a s e r unit on a p i v o t frame of the m a i n drive c a n be h e l p f u l . L a s e r u s e , h o w e v e r , has not y e t b e e n p r a c t i s e d b e c a u s e of the u n d u l a t i n g shape of the seams and b e c a u s e of the size of the s h e a r e r l o a d e r s . The m o n i t o r i n g of the "swing" m a n o e u v r e is c a r r i e d out as d e s c r i b e d in the following: P i l o t shields are m a r k e d as such in a w a y w h i c h c o r r e s p o n d s to the face l e n g t h , and these p i l o t s h i e l d s serve f o r o r i e n t a t i o n of the w h o l e s h i e l d s u p p o r t s y s t e m .

F i g . 15: M o n i t o r i n g a 200 m s w i n g i n g face w i t h p i l o t s h i e l d s In a 200 m long c o a l f a c e , the "outside" s h i e l d (No. 133) needs 10 a d v a n c i n g cycles for 7.50 m of face a d v a n c e p e r d a y .

—23—

F o r s h i e l d N o . 96 the v a l u e r e a d s 7 , for N o . 61 the v a l u e r e a d s 5, and for N o . 36 t h r e e c y c l e s m u s t b e r u n w h i l e s h i e l d N o . 10 o n l y m o v e s by one c y c l e p e r d a y . Per s e c t o r e a c h s h i e l d r e q u i r e s a d i r e c t i o n a l c h a n g e - b y its end- w h o s e o r d e r of m a g n i t u d e d e p e n d s on the s h i e l d ' s l e n g t h and n e e d s to b e d e t e r m i n e d for e a c h i n d i v i d u a l type of s h i e l d equipment.

Fig.

16:

Movement

the

shields

during

"swinging"

action

F o r a 6 m l o n g s h i e l d , the o r d e r of m a g n i t u d e of d i r e c t i o n a l c h a n g e is 0.22 m to be a c h i e v e d b y s h i e l d N o . 133 in 10 a d v a n c e c y c l e s and b y s h i e l d N o . 10 w i t h o h n e s t e p w h i l e s h i e l d N o . 1 is m o r e or l e s s t u r n i n g a b o u t its v e r t i c a l a x i s . Whereas the more remote shields' moves can easily be c o r r e c t e d w h i l e a d v a n c i n g , the s h i e l d s n e a r the p i v o t p o i n t r e q u i r e " c o r n e r i n g " in s m a l l s t e p s s e v e r a l t i m e s p e r d a y . F o r the s h i e l d s N o . 1 to N o . 5 the s t e e l b a r s a b o v e the c a n o p i e s f a c i l i t a t e this m a n o e u v r i n g as the f r i c t i o n c o e f f i c i e n t of s t e e l a g a i n s t s t e e l is l o w e r . For i m p r o v e d m o n i t o r i n g , e a c h p i l o t s h i e l d is f i t t e d w i t h a b o a r d s h o w i n g the a s s i g n e d n u m b e r of a d v a n c i n g c y c l e s , e . g . ten for s h i e l d N o . 1 3 3 .

—24—

Fig.

17:

Monitoring

boards

the

pilot

shields

F o r s h i e l d N o . 36 o n l y 3 cycles are m a r k e d . The b o a r d is s u b d i v i d e d b y fields a c c o r d i n g to the n u m b e r of advance cycles a s s i g n e d to it and the c o m p e t e n t s u p e r v i s o r m a r k s each a d v a n c e cycle r u n b y c r o s s i n g the r e s p e c t i v e f i e l d . A l l fields n e e d to be c r o s s e d on c o m p l e t i o n of a w o r k i n g d a y . According to the advance r e c o r d e d and the professional a p t i t u d e and q u a l i t y of the c r e w , the s u r v e y i n g d e p a r t m e n t runs twice w e e k l y an e x a c t c o a l f a c e s u r v e y . In the course of this s u r v e y , the n u m b e r of cycles r u n is c o n f i r m e d , and the n u m b e r of cycles s t i l l n e c e s s a r y f o r the i d e a l p o s i t i o n of the c o a l f a c e is m a r k e d b y l e t t e r s M (the i n i t i a l f o r M a r k s c h e i d e r e i = mine s u r v e y i n g d e p a r t m e n t ) on the b o a r d s . A c c o r d i n g to the i n s t r u c t i v e d a t a s u p p l i e d by the m i n e s u r v e y i n g d e p a r t m e n t , the c o n v e y o r is again m a n o e u v r e d into the e x a c t l y c o r r e c t p o s i t i o n in s u c h a f a s h i o n that o n l y those sectors of the face are a d v a n c e d , w h i c h have b e e n i d e n t i f i e d as lagging b a c k o r b e h i n d . By the m e t h o d d e s c r i b e d , s e v e r a l s w i n g m a n o e u v r e s have b e e n run s i m u l t a n e o u s l y w i t h o u t the s u p p o r t b e c o m i n g stuck even o n c e . M e a n w h i l e the crews at W e s t f a l e n C o l l i e r y are so w e l l d r i l l e d , that m o n i t o r i n g by the s u r v e y i n g d e p a r t m e n t is u s e d less for c o r r e c t i v e action, but for confirmation only.

—25—

H e a d i n g of the r a d i a l r o a d w a y If the r a d i a l r o a d w a y is n o t " h e a d e d " i n l i n e w i t h the f a c e b u t a h e a d , c a r e s h o u l d be t a k e n t h a t t h e f a c e l e n g t h f r o m the p i v o t p o i n t to the r a d i a l r o a d w a y c o r r e s p o n d s e x a c t l y to the p r e - d e t e r m i n e d l e n g t h . In c a s e of s e a m s w i t h c h a n g i n g d i p s (inclination), the face length necessarily varies with i d e n t i c a l r a d i u s . It m a y t h e r e f o r e b e c o m e n e c e s s a r y to v a r y the r a d i u s a r o u n d the p i v o t p o i n t in o r d e r to m a i n t a i n e f f e c t i v e l y the face l e n g t h p r o v i d e d .

Practical examples and results In a p a n e l of S o n n e n s c h e i n "swing" manoeuvre.

Fig.

18:

Layout

Seam, mining

Sonnenschein

seam

was

planned

without

This would have required 5 or even 7 coalfaces saleable output.

without

swinging

f o r 2.2 M t

B y c h a n g i n g the o p e r a t i o n s ' p l a n n i n g the p a n e l is w o r k e d almost completely (2 M t ) by one coalface and 3 swing m a n o e u v r e s , w i t h o u t m o v i n g f a c e e q u i p m e n t to n e w f a c e s a n d without interrupting production, thus achieving a m u c h more efficient mining operation.

— 2 6 —

F i g . 19: L a y o u t of S o n n e n s c h e i n Seam w i t h

F i g . 20: L a y o u t of W i l h e l m Seam w i t h o u t

"swinging"

The r e s e r v e s c o n t a i n e d in W i l h e l m Seam w e r e to be r e c o v e r e d by 6 c o a l f a c e s . The p a n e l o r i g i n a l l y a s s i g n e d to 4 faces was w o r k e d b y one face r u n w i t h three "swing" p h a s e s b y 180° each.

—27—

Fig. 21: Layout of Wilhelm Seam with

"swinging"

This face commenced in 1980, it is still operational and, after a downtime required by the layout, will work the remaining reserves of No. 8 phase till mid-1988. By then a total of 3 Mt of saleable output corresponding to 5 Mt run of mine will have been produced by one single face.

Fig. 22: Operating results when mining the Wilhelm

seam

—28—

The table b e l o w shows the phases of a n o r m a l face o p e r a t i o n and those of the 180° "swing" m a n o e u v r e s . From the table it m a y be s e e n that p r o d u c t i o n d u r i n g " s w i n g i n g " was on a v e r a g e o n l y 10 % lower t h a n in n o r m a l o p e r a t i o n The highest production of 2927 t per day of saleable output, c o r r e s p o n d i n g to a l m o s t 5000 t p e r d a y r u n - o f - m i n e , was a c h i e v e d d u r i n g a s w i n g m a n o e u v r e . C o n s i d e r i n g the o p e r a t i o n r e s u l t s , h o w e v e r , e m p h a s i s s h o u l d be p u t o n the f a c t , that the m a n n i n g of this c o a l f a c e v a r i e d c o n s i d e r a b l y a c c o r d i n g to the o v e r a l l o p e r a t i o n s i t u a t i o n of the c o l l i e r y . In case of the r a d i a l p r o f i l e r o a d w a y b e i n g h e a d e d b e f o r e h a n d , it m a y be a s s u m e d that d u r i n g the swing m a n o e u v r e p r o d u c t i o n is a l m o s t as h i g h as in a n o r m a l s t r a i g h t f o r w a r d o p e r a t i o n .

R i s k s e n c o u n t e r e d w h e n "swinging" faces A t W e s t f a l e n C o l l i e r y , the g r e a t w o r k i n g d e p t h a n d m i n i n g in s t r a t a c o n t a i n i n g thick sandstone b e d s h a v e r e s u l t e d o c c a s i o n a l l y in h i g h e r s t r e s s e s in the seams w h i c h , in t u r n , h a v e led to sudden coal outbursts. Throughout the "swing" manoeuvres, unfavourable geometric conditions may occur, and this m e a n s "zones of h i g h e r r i s k s " .

603

Ernestine

Seam

4 4

116 Test Drillings ^

6 8 Stress Relieve Drillings

462

r^-rrTTrrTTTTTTTTTTl - Dri" Cuttin9S Spineroad / LJJLllllllllillllAV Wilhelm Seam

/ / X'STAZ A y f t y / jQSCy /

103

«ngs/ / Relieve Drillings V / V s 917m Drill Cuttings / / / ^ 5 9

T e s t

Dri,,

S t r e s s 3

/V /Om / '

50m '

19.1

1 /

F i g . 23: T e s t and stress r e l i e v e m e a s u r e s in the W i l h e l m Seam workings

—29—

By testing and stress-relief measures, i.e. by drilling boreholes of 48 or 96 mm diameter and 20 m depth, coal is removed from the roadside and in some cases also from the face in order to arrive at stress-relief to the strata in the vicinity of the workings. In the Wilhelm Seam 116 test boreholes and 68 stress-relief boreholes were drilled in the spine road, and 463 m3 of coal debris were removed. This is the only way to achieve stress-relief in the strata« In the "swing" roadway more than 900 m3 of coal were removed from 59 stress-relief boreholes of 96 mm diameter. In zones with even higher rock pressure, the advantages and disadvantages of "swing" manoeuvres need to be carefully considered because of such stresses, and also if increased methane release is to be expected. This applies in particular to the working of other seams, thus exposed to additional pressures induced by "edges" and pillar edges of old workings in the seams previously mined. In the past, therefore, many "swing" manoeuvres resulted from constraints and were purposefully run in spite of certain disadvantages likely to occur in the future. If a colliery works only one seam, as is the pratice in many coal mining countries, the advantages of coalface swings seem to be well-established in comparison with expensive shifts of face equipment. In particular with multiple-entry systems, layouts may be realized which allow the gateroads being used only once in retreat, and abandoned after passage of the face. Repeated "swing" manoeuvres are definitely possible. Moderate inclination of the seams make layout work easier, i.e. the direction of coal removal the face can be changed if required.

Summary To sum up, we may state that "swings" of longwall faces offer great advantages if certain prerequisites can be met. The drawbacks need careful consideration. Exactitude in all working cycles is the key to success. Small mistakes, such as backward salients of the conveyor or deviations from the shields1 position normal to the conveyor, which have only nuisance value and can easily be corrected in conventional operation, may result in longer absolute ceasing of all face operations during the "swinging" phase. Face "swings" should only be contemplated by operators who are willing to guarantee the required exactitude of all work to be done during the manoeuvres involved.

•30—

2. MINING IN AUSTRALIA

—32—

SETTING THE SCENE - COAL RESOURCES AND MINING IN AUSTRALIA

By A. Galligan Chief Coal Geologist New South Wales Department of Mineral Resources

—33— BLACK COAL RESOURCES The majority

the identified black coal resources of Australia are

Permian in age and occur in the Bowen Basin in Queensland and the SydneyGunnedah Basin in New South Wales (figure 1). Permian deposits

Queensland,

the Oaklands and Gloucester Basins in New South Wales, the

Arckaringa Basin Australia.

are present

Other smaller, but still

significant

the Galilee Basin in

in South Australia and the Collie Basin

Thick coal seams have been

intersected

in Western

during petroleum

exploration in the Cooper Basin but the coal is very deep. Triassic

coal basins (figure 2) in Australia are limited in extent but

black coals of Triassic age are mined in the Callide, Tarong and Ipswich Basins

in Queensland,

the Telford Basin in South Australia and

in the

north-eastern part of the Tasmania Basin. In recent

years,

intensive exploration of the Jurassic

coal measures

(figure 3) in the Surat-Moreton Basin in Queensland has indentified large resources

of black coal at shallow depths but to date no significant

exploitation both

of these resources has occurred.

Jurassic

and

Cretaceous

Small coal deposits in

Basins have previously

been worked

Queensland and Victoria. The

identified black coal resources of Australia

are given in Table

Figure

shows

the resources for each

state.

surrmary, the

resources are:In-situ Resources Measured + Indicated Resources: Inferred Resources:

9 70 x 10 tonnes 9 57 x 10 tonnes

+ Very Large (Queensland) Measured + Indicated Resources Potential for Opencut Mining:

9 27 x 10 tonnes 9 Potential for Underground Mining: 43 x 10 tonnes

Within

the categories of Measured and Indicated Resources some

cent of the Resources may have potential for open cut mining.

38 per

Age

State

Basin

(1)

QUEENSLAND

NEW SOUTH WALES

Identltied b l a c k c o a l r e s o u r c e s by Basin. S t a t e , a n d potential mining m e t h o d .

(2)

SOUTH AUSTRALIA WESTERN* AUSTRALIA

QUEENSLAND

Bowen Galilee

< S -§ g6

Potential Total Potential Measured Underground Open-cut Inferred Measured t Indicated Indicated 6 (tonnes x 10^ (tonne x 1 0 ) (tonnes x 10*

Sub Total

4177 705 488 2

19732 1660 ~21392~~

23909 2365 26274

Syciney-Gunnedah Oaklands Gloucester Ashford

12^37 1388 101 <1

19^63

31905 1388 143 <1

39 19507

33436

1410

2786

140

2926

9475

Collie Perth Sub Total TOTAL

436 480 916 22513

305 230 535 41574

741 710 1451 64087

Callide Ipswich Tarong

78 17 255

122 630 25

200 647 280 1127

350

777

Tasmania

505

SOUTH AUSTRALIA

Telford

150 525

1282

QUEENSLAND (4)

Surat-Moreton Mulgiidie Styx

3377 20

64 4

Sub Total

3397

3465

SOUTH AUSTRALIA

41920 3260 30

43300 3260 60

490

9965

345

354 u>

fery

sficno Large Large

530 150

300

1807 3441

300 + L a r g e Very Large Small

•20

V p r y f^rrje...

Polda

310

TOTAL

3707

3775

GRAND TOTAL

26745

42924

69669

11204

46055

Queensland Opencut Resources generally based on max. 60m overburden.

(2)

NSW Opencut Resource limit 300m depth and

not ^

10:1 linear overburden ratio. (4)

1-3

(tonnes x 10^)

Large

(1)

References

13929

Sub Total

Total Inferred

V e r y L a r o e (3 1380

Sub Total

TASMANIA

Potential Underground Inferred

Very Large

Arckaringa

Total

a !

Potential Open-cut Measured • Indicated (tonnes x 10^

Very +Large

- ™

n o t —

Minor coal production occurs in Maryborough Basin but resources estimates are not available.

AG-02

—35—

Figure 1.

Permian Basin black coal deposits.

Figure 2.

Trlasslc Basin black coal deposits.

AG-03

—36—

Figure 3.

J u r a s s i c / C r e t a c e o u s Basin black coal deposits.

Figure 4.

Tertiary Basin brown coal deposits.

AG-04

—37— BROWN COAL RESOURCES The

irejor brown coal deposits of Australia are of Tertiary age

found

(figure those

the 4).

Latrobe Valley area of the Gippsland Small

Basin

and

are

Victoria

Tertiary deposits occur in most states and some

South Australia may be mined in the near future

for

domestic

power station coal supply. In

estimating

Geological system

the

which

resources. resources Coal".

brown coal resources for

broadly correlates with that system used In

addition

"Identified Identified

geological depths.

Victoria

(table

2),

the

Survey of Victoria (Reference 4) has adopted a classification for

black

the Survey quantities two other categories

Inferred Sub-economic

Inferred

Resources"

Sub-economic Resources are

and

coal of

"Inferior

determined

interpolation in extensions of known coalfields down to

by 500m

Inferior Coal refers to coal with an ash content (dry basis) of

between 10 and 30 per cent. All

of the resources listed in Table 2 are considered to be

potentially

mineable by open cut methods. Table 2. STATE

Victoria

Identified Tertiary brown coal resources by Basin and State.

BASIN

DEPTH CATEGORY (M)

MEASURED + INDICATED (tonnes x 10 )

Gippsland

0 - 300 300 - 500 0 - 300

Otway Murray

Sub Total South Australia

Western Australia

INFERRED ^ (tonnes x 10 )

INFERIOR + INFERRED SUB-BCONCMIg (tonnes x 10 )

80163 4235

4651 2739

54516 30280

500

15000

80663 4235 84898

39240 2739 41989

970

0 - 300 0 - 300 300 - 500 0 - 500

19599

St.Vincent

2524

Murray

938

160

Sub Total

3462

1130

Bremer

614

760

TOTAL

88974

. 43879

54516 30280 84776

04776

Criteria Used: Victoria: seams

3m thick;

Ash

10% except for Inferior vvhere ash

30%

AG-05

—38—

1970/71

state

Figure 5.

M e a s u r e d and indicated

Figure 6.

resources.

76/76

Year

80/81

86/86

B l a c k c o a l p r o d u c t i o n In Australia.

140n

1070/71

78/70

80/81

86/88

Year

Figure 7. B l a c k c o a l e x p o r t s

1070/71

76/78

80/81

85/86

Year

from

F i g u r e 8.

Black coal usage.

Australia. AG-06

—39—

COAL MINING Black Coal Unfortunately statistics

the

time of preparation

this

paper,

production

for 1986-87 were not available and hence 1985-86 figures

are

used. In

1985-86 production of raw black coal in Australia was

163.6

million

tonnes (Reference 4). Black coal is mined in all states except Victoria. Queensland

and New South Wales dominate production and these two

States

are responsible for all exports of coal. TABLE 3: State

BLACK COAL PRODUCTION BY STATE 1985 - 86

Raw Coal Production

Saleable Coal Production

(Tonnes x 10 3 )

New South Wales

77 186

64 081

Queensland

80 012

63 997

Western Australia

3 750

South Australia

2 167

502

310

163 617

134 305

Tasmania

TOTAL

The tonnage of black coal produced in 1985-86 shows a three-fold increase over

1970

production

which

was

some

million

tonnes.

Saleable

shows a similar trend, increasing from 45 million tonnes

1970 to 134 million tonnes in 1985-86 (figure 6). Saleable

coal production and the method of production for each basin

given in Table 4. Of the 134 million tonnes of black coal produced in Australia in 1985-86, over 65 percent or 90 million tonnes were exported (figure 7). Australia

—40— TABLE 4: SALEABLE BLACK COAL PRODUCTION BY BASIN 1985-86 Black Coal Age

Basin

Saleable Production

Open Cut

Underground

(Tonnes x 1Q3) PERMIAN

Bowen

54 412

51 827

2 585

*Sydney-Gunnedah

64 081

24 474

39 607

Collie

3 750

2 921

829

122 243

79 222

43 021

1 349

TRIASSIC Ipswich

2 965

1 616

Callide

3 403

Tarong

.2 974

2 974

Telford

2 167

Tasmania

310

305

11 819

10 165

1 654

228

129

JURRASSIC- Surat-Moreton CRETACEOUS Maryborough

TOTAL

244

129

131

134 306

89 516

44 790

* Includes Ashford Basin

AG-08

—41— is currently

the largest exporter of seaborne coal in the world and

Australian coal is shipped to most countries with limited coal resources. Both coking and thermal coal are exported. increased

Thermal coal exports have

dramatically in recent years and now constitute

of the export coal.

percent

Most of the export coal is produced from the Bowen

and Sydney-Gunnedah Basins.

Danes tic consumption major use being

in 1985-86 was

42.5 million

for power generation.

tonnes with the

A comparison of

the domestic

consumption and export tonnages is shown in Figure 8. The coal mining industry in Australia is highly mechanised

and overall

productivity as measured by the raw coal output per manshift worked is 21.96

tonnes.

Approximately two thirds of the coal is produced by open

cut mining methods. Most underground mining is concentrated in New South Wales where some 45 1985-86.

million tonnes were produced by this method in

In recent years, many of the underground mines have been

upgraded by the installation of longwall mining equipment with resultant increases in productivity. Brown Coal Brown coal is currently mined in Victoria at three localities and is principally used as a fuel source for electricity generation.

Small

quantities are used for industrial purposes and briquette manufacture. Total production

in 1985-86 was 36.5 million

tonnes.

All coal is

produced by open cut methods. In the larger mines, the coal is recovered by bucket wheel excavators. ACKNaVLEDGEMENTS In addition to the references listed below, the author wishes to acknowledge resource data Western^Australiaf ^ ^

T—nia,

S o u t ^ r ^

^ d

References 1.

GalTigan A.G in prep. Coal Resources in Australia. In Geology of Australian Coalfields Monograph of Bureau of Mineral Resources and Coal Geology Group of P Geological Society of Australia.

Anon

' 4 4. 5.

llff

1986. Queensland Resources of Black Coal, Government Mining Journal, Vol 87 (1016) 257-260 J

1986

June,

1986.

Queensland uueensiana

' Reser^;; 1 9 8 ^ J V n ^ f \ " " » Wales Coal Resources and sta.de T 1986 New Soutl, Wales Department of Mineral Resources Publn. Stanley, D . R , 1986 Brown Coal Resources of Victoria, an Overview. Geological 0giCaX Survey of Victoria Rep. 1986/25 (unpubl.). Joint Coal Board, 1986. Black Coal in Australia 1985-86. A Statistical Year Book.

AG-09

—42—

P R O J E C T F E A S I B I L I T Y A N D M I N E P R O D U C T I O N COSTS

By C , R . Longworth General Manager Planning + Development and Director Clutha L i m i t e d L e v e l 18, 1 York Street Sydney, 2000

The paper outlines the present i n v e s t m e n t e n v i r o n m e n t e x i s t i n g in t h e A u s t r a l i a n Coal Industry and places this in the c o n t e x t of the c u r r e n t w o r l d seaborne coal trade.

Those f a c t o r s w h i c h have the g r e a t e s t i n f l u e n c e on the v i a b i l i t y of c o a l

p r o j e c t s today, namely

c o a l prices

and q u a l i t y , exchange r a t e s , cash cost

p r o d u c t i o n , c a p i t a l and b o r r o w i n g costs, i n f r a s t r u c t u r e and g o v e r n m e n t charges are d e a l t w i t h b r i e f l y . B o t h coking c o a l and s t e a m i n g c o a l prices have d e t e r i o r a t e d s i g n i f i c a n t l y in the last year m a k i n g i t e x t r e m e l y d i f f i c u l t to j u s t i f y the development of new p r o j e c t s t o replace those w h i c h w i l l be exhausted in the n e x t decade. The

significance

cash

costs

are

discussed

and

is concluded

that

the

i n c r e m e n t a l expansion of good q u a l i t y low cash cost e x i s t i n g mines w i t h ample reserves w i l l be the only p r o j e c t s l i k e l y t o stand the c u r r e n t f i n a n c i a l tests. High i n t e r e s t rates and low exchange rates have had a s i g n i f i c a n t a f f e c t on the c a p i t a l cost components of selling p r i c e s .

L o w exchange rates on the other hand

have assisted the i m p r o v e m e n t in the A u s t r a l i a n dollar r e t u r n but prices in US dollars

have

continued

weaken

during

this

period

devaluation

the

Australian currency. The i n f l u e n c e of t r a n s p o r t and i n f r a s t r u c t u r e as w e l l as g o v e r n m e n t charges and taxes are discussed.

T r a n s p o r t and i n f r a s t r u c t u r e costs now represent a very

s i g n i f i c a n t p r o p o r t i o n of the t o t a l F O B T selling p r i c e and savings need to be made in these areas i f the i n d u s t r y is to r e m a i n c o m p e t i t i v e . The paper concludes by g i v i n g a schedule of those f a c t o r s of a l o c a l c h a r a c t e r w h i c h can be changed to render p r o j e c t s more c o m p e t i t i v e and also presents some external

factors

beyond A u s t r a l i a ' s c o n t r o l w h i c h w i l l ,

d e v e l o p m e n t of the industry in the next decade.

in p a r t , influence

the

•43—

INTRODUCTION C o a l is a source o f carbon f o r e i t h e r d i r e c t energy use or as a c o m b i n e d e n e r g y / r e d u c t a n t in s t e e l m a k i n g .

I t is a basic c o m m o d i t y w h i c h in A u s t r a l i a

today sells f o r f r o m 3.0 t o 6.50 per k i l o g r a m f r e e on board (FOB) ocean vessel*.

The paper r e v i e w s the p r i n c i p a l f a c t o r s w h i c h c u r r e n t l y i n f l u e n c e

the v i a b i l i t y o f e x p o r t c o a l p r o d u c t i o n in A u s t r a l i a , most are adverse and f a r o u t w e i g h those t h a t are f a v o u r a b l e .

In s p i t e of the poor o u t l o o k f o r t h e

i n d u s t r y , e x p l o r a t i o n and e v a l u a t i o n must c o n t i n u e in order t o m a i n t a i n an i n v e n t o r y of prospects to r e p l a c e mines as they become exhausted. Most of the w o r l d ' s m a j o r c o a l producers r e l y on the home m a r k e t t o provide stability to their industry.

A u s t r a l i a is an e x c e p t i o n and depends very m u c h

on the e x p o r t m a r k e t f o r v i a b i l i t y .

The i n d u s t r y is s t i l l the N a t i o n ' s l a r g e s t

e x p o r t e r earner w i t h sales in excess of 5 b i l l i o n dollars, even though c u r r e n t p r i c e r e d u c t i o n s have slashed i n c o m e and g r o w t h prospects h e a v i l y . While many of t h e f a c t o r s discussed in t h i s paper are c o m m o n to d o m e s t i c coal p r o d u c t i o n f o r i n d u s t r i a l and power g e n e r a t i o n uses, c a p t i v e d o m e s t i c mine

evaluation

industries.

heavily

the

viability

the

end

user

Many p o l i t i c a l issues also i m p a c t on d o m e s t i c c o a l p r o j e c t s and

t h e y are best considered in i s o l a t i o n . T o t a l A u s t r a l i a n coal sales in 1986 a m o u n t e d to 5.914 b i l l i o n dollars w h i c h was a rise on the previous year of 13%. now

operates

oversupply.

extremely

In spite of t h i s rise, t h e i n d u s t r y

precarious

situation.

There

gross

The m a r k e t is unstable and s t e a m i n g c o a l sells below the c o a l

costs of p r o d u c t i o n .

There is a low l e v e l of e q u i t y p a r t i c i p a t i o n in the

industry w h i c h has a n e g a t i v e r e a l r e t u r n on funds e m p l o y e d . There has been a qradual increase in b o r r o w i n g s , now in excess of 4 b i l l i o n dollars R e f (1), and a c o m m e n s u r a t e

decrease in shareholders 1 e q u i t y .

D u r i n g the

1985

p e r i o d the d e b t / e q u i t y r a t i o o f the industry increased f r o m 0.95 t o 1.13 and e x p l o r a t i o n is c u r r e n t l y at a d i s t u r b i n g l y l o w l e v e l .

A l l of these f a c t o r s

point t o a sick i n d u s t r y .

A l l m o n e t a r y sums are in A u s t r a l i a n c u r r e n c y unless o t h e r w i s e n o t e d .

CRL-01

—44— A f u r t h e r f a c t o r of deep concern is t h e steep decline in e x p e n d i t u r e on f i x e d assets w h i c h cover e q u i p m e n t r e p l a c e m e n t .

A n i n d u s t r y w h i c h does not

m a i n t a i n sound modern e q u i p m e n t , w i l l i n e v i t a b l y become r u n down, and this f a c t o r is now c l e a r l y e v i d e n t in c e r t a i n sectors of the i n d u s t r y . The industry is c y c l i c a l and has been in states of depression b e f o r e but has r e c o v e r e d to achieve increased p r o d u c t i o n . Those f a c t o r s having the g r e a t e s t i n f l u e n c e on p r o j e c t v i a b i l i t y are c o a l prices and exchange r a t e s , cash costs of p r o d u c t i o n , c a p i t a l and b o r r o w i n g costs, i n f r a s t r u c t u r e and g o v e r n m e n t charges.

E n v i r o n m e n t a l f a c t o r s can

r e s u l t in t i m e delays and s e r v i c i n g costs in p r o j e c t i m p l e m e n t a t i o n and w h i l e i m p o r t a n t , these are e x c l u d e d f r o m the paper, because of t h e i r s u b j e c t i v e character. 2.

G E N E R A L OBSERVATIONS O N W O R L D C O A L T R A D E A r e v i e w of h i s t o r y R e f (2) t e l l s us t h a t sound r e c o v e r i e s in t h e industry have o c c u r r e d f o l l o w i n g m a j o r s t r u c t u r a l and t e c h n o l o g i c a l change. The conclusion t o the paper deals w i t h some of the changes necessary t o render l i k e l y prospects v i a b l e , and mentions some of the e x t e r n a l f a c t o r s w h i c h w i l l a f f e c t the p r o f i t a b i l i t y o f f u t u r e p r o j e c t s . The amount of black c o a l w h i c h moved in w o r l d c o a l trade in 1986 was 336 m i l l i o n tonnes R e f (3) or about 10% of t h e t o t a l w o r l d o u t p u t of 3.68 b i l l i o n tonnes.

The decline in s t e e l o u t p u t of the m a j o r i n d u s t r i a l i s e d nations has

c r e a t e d a reduced demand f o r the higher p r i c e d higher q u a l i t y coking coals. Crude s t e e l p r o d u c t i o n in the n o n - c o m m u n i s t developed c o u n t r i e s decreased by 18.7 m i l l i o n tonnes in 1986 Ref.(4) w h i l e modest increases have o c c u r r e d in the developing c o u n t r i e s of R e p u b l i c o f K o r e a , T a i w a n , India, P a k i s t a n , T u r k e y , B r a z i l and A r g e n t i n a .

A u s t r a l i a n e x p o r t s in 1986 amounted to 92

m i l l i o n tonnes or about 27 p e r c e n t of the world's t o t a l seaborne coal t r a d e . 3.

COAL QUALITY In US dollar t e r m s , c o k i n g and non c o k i n g coals have shown a steady p r i c e decline, since the early e i g h t i e s .

The p r i c e v a r i a t i o n s are shown in Figs 1,2

and 3 and have had the g r e a t e s t i m p a c t on p r o j e c t v i a b i l i t y in the last 2 years.

The Japanese c o k i n g coal m a r k e t is an established b e n c h m a r k f o r

w o r l d prices, although w i t h c u r r e n t oversupply, prices in m a r k e t s other than Japan are on average l o w e r .

T h e r m a l coal prices to m a r k e t s , other t h a n

Japan, are at present up t o 10 p e r c e n t l o w e r on average.

CRL-02

—45— When viewed in Australian dollar terms, prices did not go into steep decline until mid 1986, as evidenced by the stabilising influence of the long progressive decline in the Australian dollar, shown in Fig.4. Extreme oversupply of all types of coal has existed in world trade during this period, and most of the recent projects in Australia, producing steam coal for export, are just covering cash costs with nothing l e f t for debt servicing and i n t e r e s t , let alone the shareholder. The steep fall in the Australian Dollar in early 1985 improved producer cash flows and in part staved off the crisis which now exists in the industry. Australia isn't unique with difficult conditions existing in Canada and South Africa. The prediction of coal prices for new projects will always prove difficult and the industry has experienced the use of various accepted test criteria by customers over the years to establish prices. The steel and power generation industries have learnt to use a wider variety of coals and have achieved skills in changing coal sources relatively quickly. They now watch closely the consuming p a t t e r n s of their competitors. There now appears to be a close correlation between coal prices in US dollars per tonne of net carbon. Fig.5 illustrates the price variations of hard, s o f t and weak coking coals and thermal coals supplied from N.S.W. to the Japanese market over the past 8 years. Acknowledgement is made to the Australian Coal Report for this d a t a . The weak coking coals and thermal coals achieve the same price on a net carbon basis, regardless of whether the coal is used for metallurgical or electricity generation, and hard and s o f t coking coals have also achieved price equality on this same basis. Pricing based upon net carbon content may seem very obvious at a time of oversupply of all types of coal. For coal quality and price predictions constant attention to the international market scene remains the best advice. In the last 3 years customers have demanded higher quality coals. Average ash levels for coking and thermal coals have been progressively reduced. Future export projects will t h e r e f o r e need to reach lower ash levels if they are going to p e n e t r a t e new m a r k e t s . This f a c t o r will influence yield assessment and washing plant capital costs. Washing plants will become more sophisticated. The influence of yield cannot be stressed too highly in coal prospect evaluation. A number of productive mines in Australia have had to cease operations primarily due to the influence of yield on their production costs.

CRL-03

MINING COSTS AND PRODUCTIVITY An analysis of the distribution of costs comprising the F O B T selling price of coal from all existing Australian mines is shown in Fig.6. Mining and washing costs represent an average from 29 to 46 percent of the selling price. The distribution figures guoted are not sufficiently recent to reflect the significant improvements being effected by the new longwalls. Most Australian open cut mines face progressively increasing strip ratio over the mine life resulting in an increasing cash cost of raw coal production. There is thus a constant need for further new capital to maintain production. Some multiple seam Hunter Valley mines are exceptions and have relatively stable strip ratios over the mine life. The increased productivity from underground operations equipped with longwalls is gradually closing the gap between the open cut and underground cash costs. Fig. 7 shows the change in raw coal output pattern over the past 35 years and serves to illustrate the influence of certain physical, technological and industrial factors on productivity and hence mining costs. Fig. 7 is based upon industry averages. While underground operations have much lower outputs than open cuts the best new longwall is better than many of the mature open cut mines. When capital requirements are taken into account, it will be concluded that high production underground mining of the right type of coal, favourably situated to the port will remain competitive. This is in contrast to Western Canada where underground mining has ceased. It is not possible to generalise on raw coal cash costs of production but with the new generation of longwalls, working under ideal conditions, cash costs in the region of $15 per tonne can be achieved. The costs are higher than ideal open cut operations of say $9 per tonne, when capital costs are added some underground mines will prove more competitive. Deeper mining with greater support requirements and the likelihood of gas drainage, add significantly to underground cash costs which can rise to $24/tonne or more. Labour costs impact heavily on mining costs. If the industry is to remain competitive in the world scene then these costs have to be reduced by a combination of lower levels of manning, wage reductions through more

CRL-04

47— a p p r o p r i a t e bonuses, the e l i m i n a t i o n of p e n a l t y r a t e s and a wide range of work practices.

These steps in c o n j u n c t i o n w i t h new technology w i l l reduce

cash costs and c o n t r i b u t e to the r e s u m p t i o n of a h e a l t h y s t a t e of g r o w t h in the i n d u s t r y . 5.

C A P I T A L COST I N T E R E S T A N D E X C H A N G E R A T E S R e f e r e n c e to F i g . 6 shows t h a t t h e c o n t r i b u t i o n t o F O B T selling p r i c e s f r o m b o t h c a p i t a l and r e p l a c e m e n t c a p i t a l varies f r o m 12.2% t o 21.7%. In r e a l i t y , f o r new p r o j e c t s the c a p i t a l c o n t r i b u t i o n is s i g n i f i c a n t l y higher t h a n this and in the present m a r k e t e n v i r o n m e n t , i t w o u l d be e x t r e m e l y d i f f i c u l t to f i n d a grass roots p r o j e c t w i t h m i n i n g and c a p i t a l costs s u f f i c i e n t l y low to m e e t financial acceptance. Most underground and opencut m i n i n g e q u i p m e n t has s i g n i f i c a n t manufacturing component. has

therefore

become

overseas

The i n f l u e n c e of t h e reduced A u s t r a l i a n d o l l a r

a two

edged s w o r d ,

i m p r o v i n g cash r e c e i p t s

but

increasing the c a p i t a l cost c o m p o n e n t o f p r o d u c t i o n and thus m a k i n g new p r o d u c t i o n d i f f i c u l t to j u s t i f y .

By way o f e x a m p l e , t w o h y p o t h e t i c a l c a p i t a l

cost dissections are p r o v i d e d , one f o r a new underground mine and the o t h e r an opencut m i n e , b o t h w i t h s i m i l a r saleable c o a l o u t p u t s .

Longwall underground mine producing 3.3 million tonnes of saleable coal per annum $M E x p l o r a t i o n and p r e l i m i n a r i e s

L a n d purchase

Service f a c i l i t i e s

Construction camp

D r i f t s and s h a f t s

Longwall mining equipment

Conveyors

Electrical

Conventional miners, FCTfs etc.

Underground transport equipment

5.3

Surface e q u i p m e n t

2.5

Coal p r e p a r a t i o n and handling

R a i l loop

Spares

Engineering and p r o j e c t m a n a g e m e n t

Total

$263.8m

CRL-05

—48— The capital cost per saleable tonne of output is djo/7 Rrr» = $80 per annual tonne of production 3.3 Opencut Mine Producing 3.3 million tonnes per annum of saleable coal

$M 12

Exploration and preliminaries Service facilities

1 35

Construction camp Mining and surface equipment

385

Coal preparation and handling R a i l loop

66 15

Land purchase

Spares

Engineering and project management

20 $554m

Total

The capital cost per annual tonne of output is <t» c c/, __ _ = $168 per annual tonne of production 3.3 It can be seen from this example that the capital cost per annual tonne of output for the underground mine is approximately surface mine.

one half that of the

Surface mines are far more capital intensive unless the strip

ratio is low such as exists at Blair Athol. In the capital cost examples selected, no provision has been made

for

infrastructure such as townships, water and power supply, contributions to railways and ports.

These items are significant in relation to a number of

Australian project mines awaiting a more favourable development climate. Underground mines are generally less capital intensive but have higher cash costs.

general

underground

mines

reguire

less

additional

capital

throughout their life than opencut mines to maintain the designed level of production. With underground mines a need arises over the mine life to extend district coal conveying systems and to provide additional man access and surface facilities to reduce travelling times to the face as well as constructing extra ventilation facilities.

With opencut mines overburden removal represents

the largest ongoing capital requirement.

CRL-06

—49— F i g . 8 i l l u s t r a t e s the v a r i a t i o n in c a p i t a l i n v e s t m e n t in t h e A u s t r a l i a n C o a l Industry in the 5 years to 1985.

The low cash cost mines c o n s t r u c t e d in the

last decade are now t h e only ones w h i c h can c o m p e t e in the c u r r e n t w o r l d market. In

the

underground

mining

example

guoted if

a cash cost of

raw

coal

p r o d u c t i o n o f $15 was adopted, a washing cost o f $4 and a y i e l d of 80%, t h e n the p i t head cost of saleable c o a l w o u l d be $23.75. I f a 15% c a p i t a l f a c t o r is adopted t h e n t h e p i t head selling p r i c e of t h e p r o d u c t i o n w o u l d be $35.75. A d o p t i n g a s i m i l a r approach f o r the opencut mine e x a m p l e and using a cash cost of p r o d u c t i o n o f $9 w i t h a washing and handling cost of $4 and a y i e l d of 70%, ( l o w e r y i e l d due to o u t - o f - s e a m d i l u t i o n ) the cash cost of p i t h e a d p r o d u c t i o n is $18.57 and a p p l y i n g a s i m i l a r c a p i t a l f a c t o r gives an e x - m i n e selling p r i c e o f $43.77.

Unless p o r t and t r a n s p o r t charges are e x t r e m e l y low

n e i t h e r o f these mines w o u l d be c o m p e t i t i v e , at p r e s e n t . The overseas c u r r e n c y c o m p o n e n t in the cost of m i n i n g e q u i p m e n t plays a very m a j o r p a r t in the c a p i t a l cost c o m p o n e n t and in t h e case of l o n g w a l l e q u i p m e n t can amount t o 70% o f the t o t a l costs.

R e f e r e n c e to F i g 4 gives

the v a r i a t i o n in A u s t r a l i a n d o l l a r U.S. dollar r a t e and t h e 90 day bank b i l l r a t e , as an i n d i c a t i o n of b o r r o w i n g costs. Exchange rates and debt f i n a n c i n g have had a m a j o r i n f l u e n c e on the v i a b i l i t y o f a l l the new mines established t h i s decade.

A s i g n i f i c a n t i m p r o v e m e n t in b o t h o f these r a t e s w o u l d lead t o

an e n t i r e l y d i f f e r e n t p e r s p e c t i v e on the economics of c o a l m i n i n g . u t i l i s a t i o n is too low in the i n d u s t r y .

Capital

E v e r y e f f o r t should be made in the

e x i s t i n g c l i m a t e of w o r k p r a c t i c e r e v i s i o n to e x t e n d p r o d u c t i o n to 7 days and r a t i o n a l i s e manning r o s t e r s , thus o b t a i n i n g a more e f f i c i e n t use o f c a p i t a l . TRANSPORT A N D INFRASTRUCTURE The t w o h y p o t h e t i c a l examples of c a p i t a l costs f o r underground and o p e n c u t mines given in Section 5 e x c l u d e d i n f r a s t r u c t u r e i t e m s .

The provision o f

w a t e r supply power systems, townships and c o n t r i b u t i o n s to r a i l w a y s ' r o l l i n g stock and p o r t f a c i l i t i e s , can increase the c a p i t a l c o n t r i b u t i o n by 50% or more depending upon p r o j e c t l o c a t i o n .

I t is c l e a r , in these c i r c u m s t a n c e s ,

that

take

the

only

developments

expansions of e x i s t i n g

mines

likely

and t h e n only

place

are

the

incremental

when these mines have

low

t r a n s p o r t and p o r t costs. F i g . 9 i l l u s t r a t e s the a p p r o x i m a t e c u m u l a t i v e e x p o r t mineable reserves of coal d e p i c t e d in r e l a t i o n to distance f r o m the p o r t . appear

attractive,

is only those

While the reserves

prospects w h i c h are associated

with

CRL-07

—50— e x i s t i n g mine d e v e l o p m e n t and i n f r a s t r u c t u r e t h a t can possibly c o m p e t e and t h e n only i f endowed w i t h high p r o d u c t i v i t y , f a v o u r a b l e labour r a t e s and good i n d u s t r i a l p r a c t i c e s . A u s t r a l i a n r a i l f r e i g h t r a t e s f o r c o a l have t r a d i t i o n a l l y been m u c h h i g h e r t h a n those o f C a n a d a , South A f r i c a and The U n i t e d S t a t e s .

I t is g e n e r a l l y

a c c e p t e d t h a t the NSW and Queensland r a i l f r e i g h t r a t e s have been used as an i n d i r e c t f o r m o f S t a t e t a x a t i o n .

In N e w South Wales t h e r a i l f r e i g h t r a t e s

on a tonne k i l o m e t r e basis are m u c h h i g h e r f o r the s h o r t e r haul r o u t e s and are at least 100% h i g h e r t h a n t h e C a n a d i a n and A m e r i c a n r a t e s .

Short haul

r a i l f r e i g h t r a t e s in N.S.W. are in the o r d e r of 9.8 c e n t s per tonne k i l o m e t r e , w h i c h is c o m p a r a b l e t o r o a d t r a n s p o r t w h i l e l o n g e r r o u t e s are in t h e order of 4.4 c e n t s a l t h o u g h

the r a t e s are on a p r o g r e s s i v e l y

r e d u c i n g scale

with

d i s t a n c e , and g e n e r a l i s a t i o n c a n n o t be a d o p t e d . In c o m p a r i s o n the W e s t e r n C a n a d i a n r a t e s are in t h e o r d e r o f 1.65 c e n t s per tonne k i l o m e t r e and t h e East C o a s t U n i t e d States r a t e s are 2.5 cents per tonne k i l o m e t r e .

The n e w e r Queensland c o a l p r o j e c t s are also be burdened

w i t h v e r y high r a i l f r e i g h t r a t e s w h i c h w i l l i n h i b i t d e v e l o p m e n t of f u r t h e r projects. I t is u n l i k e l y t h a t a new m i n e c o u l d j u s t i f y t h e c o n s t r u c t i o n o f a stand alone e x p o r t c o a l l o a d i n g t e r m i n a l in t o d a y s c o a l m a r k e t .

T h e r e is s t i l l s i g n i f i c a n t

surplus

and N.S.W.

terminal

capacity

existing

Queensland

Terminal

charges f o r c o m m o n user f a c i l i t i e s are h i g h e r t h a n our e x p o r t c o m p e t i t o r s . N e w t e r m i n a l s h a v i n g i n s u f f i c i e n t t h r o u g h p u t are e n c u m b e r e d w i t h excessive c a p i t a l s e r v i c i n g charges, K o o r a g a n g c o a l being an e x a m p l e . Port

Kembla

high

due

over-manning

f a c i l i t i e s in A u s t r a l i a n and a b r o a d .

when

The t a r i f f a t

compared

with

other

The annual c a p a c i t y in tonnes per m a n

e m p l o y e d at P o r t K e m b l a is less t h a n 40,000 c o m p a r e d w i t h 120,000 tonnes at t e r m i n a l s such as R o b e r t s Bank in Canada and Hay P o i n t in A u s t r a l i a . There is thus scope f o r a r e d u c t i o n , in r e a l t e r m s , in loading c h a r g e s . I n t e r n a t i o n a l shipping r a t e s w i l l play an i m p o r t a n t p a r t in t h e f u t u r e trade pattern.

coal

In the l a s t 12 m o n t h s f r e i g h t r a t e s to A R A posts f o r capesize

vessels have been as l o w as US$7.00 per t o n n e .

The hardship e x p e r i e n c e d in

c e r t a i n areas of the ocean f r e i g h t business is as bad as the c o a l i n d u s t r y . The

rate

construction.

scrapping

old

tonnage

is now e x c e e d i n g

new

bulk

ship

This f a c t o r when c o m b i n e d w i t h increased o i l prices w i l l lead

t o a m o r e r a p i d r a t e o f increase in f r e i g h t r a t e s w h i c h w i l l i n f l u e n c e c o a l t r a d e p a t t e r n s in the n e x t decade.

CRL-08

—51— 7.

TAXES The

coal

industry

in A u s t r a l i a

is now

burdened

with

a wide

range

G o v e r n m e n t charges and taxes w h i c h w e r e able to be absorbed during t i m e s of higher prices and positive r e t u r n s on funds i n v e s t e d .

For r e p l a c e m e n t

p r o j e c t s to proceed i t w i l l be necessary to r e v i e w a l l o f these charges in order to e f f e c t a r e d u c t i o n c o m p a r a b l e w i t h t h a t t a r g e t e d in the area o f cash costs. rental,

The charges and taxes c o m p r i s e excise, r o y a l t i e s , lease fees and

fringe-benefits

tax

on housing, w h a r f a g e , harbour

dues

research

levies as w e l l as the i n d i r e c t s t a t e t a x a t i o n t h r o u g h excessive t r a n s p o r t and p o r t charges. 8.

THE N E X T D E C A D E OF COAL PROJECTS To conclude a number o f observations and r e c o m m e n d a t i o n s f o r change are made to c a t a l y s e the e x p l o r a t i o n planning and d e v e l o p m e n t of soundly based coal mines. i)

A buyers 1 m a r k e t w i l l e x i s t f o r some t i m e , as a p r i c e t a k e r A u s t r a l i a must reduce costs in a l l q u a r t e r s .

ii)

Development

will

be l i m i t e d

to established c o a l areas served

infrastructure. iii)

The i n d u s t r i a l r e c o r d in the i n d u s t r y is 3600 p e r c e n t worse than the r e s t of t h e A u s t r a l i a n w o r k f o r c e .

I t has average w e e k l y earnings of

$875 w h i c h is s i g n i f i c a n t l y higher than o t h e r mine w o r k e r s in more profitable

metal

mines.

Labour

costs

must

reduced.

Poor

i n d u s t r i a l r e l a t i o n s add s i g n i f i c a n t l y t o c a p i t a l and o p e r a t i n g costs and destroy c u s t o m e r c o n f i d e n c e . iv)

Infrastructure

costs

and f r o n t

end p r o j e c t

loading costs must

reduced. v)

R a i l f r e i g h t r e d u c t i o n s are r e q u i r e d f o r a l l p r o j e c t s and n o t r e s t r i c t e d to those w i t h long distance hauls.

vi)

New p r o j e c t s w i l l need to be h i g h l y p r o d u c t i v e and of low cost, yield.

capable

speedy

development

capital

w i t h good c o a l q u a l i t y

The a u g m e n t a t i o n of e x i s t i n g mines or the d e v e l o p m e n t

and of

nearby deposits w i l l be the most l i k e l y candidates. vii)

Reduction

government

charges

and

taxes

as an

incentive

revitalisation. viii)

R e m o v a l of the F l a g o f Convenience ban on N.S.W. c o a l e x p o r t s ,

ix)

P r o d u c t i o n o f an increased range o f c o m p e t i t i v e A u s t r a l i a n produced

R e d u c t i o n in i n t e r e s t r a t e s .

imposed by M a r i t i m e Unions. mining equipment.

CRL-09

—52— xi)

Maintenance of devalued currency until industry is revitalised and servicing equity participants adequately.

Reduction in the inflation rate.

It is appropriate to record some external factors which will have positive and negative influences on Australian coal projects.

Factors of positive

influence are: a)

The reduction in coal production in Japan, France, Belgium, Britain

The reduction in the enormous subsidies paid to the West German

and West Germany. industry which could occur in the next decade. c)

Reduced exports from South A f r i c a , Canada and U S A .

Continuation of high inflation in South A f r i c a as well as industrial

disruption and high financing costs. Increase in oil prices.

Increase in coal fired power plant construction

Decrease

nuclear

power

plant

construction.

High

cost

decommissioning old nuclear plants. h)

Increased growth and financial stability in developing nations.

Factors of negative influence are: i)

Increased exports from China, Colombia and Indonesia.

ii)

Increased ocean freight rates.

iii)

Further reductions in steel output from developed nations.

iv)

Embargos and trade restrictions.

More efficient use of electricity e.g. the advent of super conductors in the next 2 decades.

CRL-10

—53— References 1.

Coopers & L y b r a n d - A u s t r a l i a n C o a l A s s o c i a t i o n , I n d u s t r y R e p o r t 1986,

E l l i s M . H . - A Saga o f C o a l - Angus & R o b e r t s o n .

F i n a n c i a l T i m e s I n t e r n a t i o n a l C o a l R e p o r t - C o a l Y e a r 1986.

I n t e r n a t i o n a l Iron & Steel I n s t i t u t e regular r e p o r t s .

CRL-11

—54—

N.S.W. HARD COKING COAL PRICESReproduced TO JAPAN from data from

Australian Coal Morketmg Monual Supplied by Australian Coal Report

FIG I

64 - AUSTRAl IAN S

M —— mmmm-

U.S. S

56 i i i

—r f i

44 A

40 J V v /

-Y

\ -v

VAA , A* / V M

i 1• i/ \ %i i\ V

t\ \P

J / Ar If ft < Jt iI a N l! M I%/n# I' , p^ \ 1

V V

1978

1979

t i —t

\1/ V

f 32

1980

1981

19 82

1983

1984

/985

1986

N.S.W. SOFT COKING COAL PRICES TO JAPAN

1987

Reproduced from data from Australian Coal Marketing Manual Supplied by Australian Cool Report

FIG 2

—55—

AVERAGE EXPORT VALUES FOR AUSTRALIAN NON COKING COAL FIG 3

1-20

' 1978 '* 1979' "1 i'seo

1 1981 ' ' 1982 1 1983

1 1984

90 DAY BANK BILLS FIG 4

1985

1986 1987 EXCHANGE RAT£

CRL-13

—56—

HARD COKING SOFT

COKING

WEAK COKING THERMAL

IS79

I990

1981

1982

1983

1984

1983

1986

1987

DATA PROVDED BY AUSTRALIAN

COAL PRICE VARIATION WITH NET

COAL

REPORT

CARBON

FIG 5

12 2 2 % LABOUR

28-71%

LABOUR

285%

16-90% MATERIALS POWER FUEL OThER

14-32% Q-90%

WASHING

COSTS

16-68%

PORT

4-30% 4-47% 3-64%

_G0V7"

12-35%

9 10%

co^ 8-84%

27-68% FREIGHT

FREIGHT 23-52%

MATERIALS POWER FUEL/ OTHER ^

26-86%

PORT 8 00% 4-43%

ilfM

UNDERGROUND (jONGWALL

CAPITAL

2 66%

379% 19 0 1 % 8-45%

CAPITAL

CONVENTIONAL UNDERGROUND

OPEN CUT > 2 0 MTPA SOURCE: AUSTRALIAN COAL CONSULTATIVE COUNCIL- COST STUOY 1985

FOBT SELLING PRICE COST DISTRIBUTION AUSTRALIAN COAL MINE CATEGORIES FIG 6

FOR CRL-14

—57—

O h-

X CO

q: uI

CL 30 <1 LU > O t K D

I— ZD O

§ CJ

1950

I960

1970

TRENDS IN OUTPUT PER FIG

1980

1990

MANSHIFT

CRL-15

—58—

I9QI

I9Q2

(963

1904

1985 SOURCE•• AUSTRALIAN COAL ASSOCIATION INDUSTRY REPORT 198 6 DATA ANALYSIS COOPERS & LYBRAND

VARIATION IN CAPITAL INVESTMENT AUSTRALIAN COAL INDUSTRY FIG 10000 J

— • TOTAL OLD a 1f SW - NSW • OLD.

mmmm

si < a 5

g 6000)

- RESERVES QUOTED ARE CONSIDERED MINEABLE BY PRESENT METHODS WITHIN 30 YEARS

••

-RESERVES DEDICATED TO POWER STATION STEELWORKS OR OTHER DOMESTIC CONSUMERS HAVE NOT BEEN INCLUDED

rA •f •

3 § §

NOTES

4 0 0 0)

-CUMULATIVE RESERVES SHOWN AGAINST A RAIL DISTANCE TO PORT INDICATES THE TOTAL MINEABLE RESERVES CONTAINED WITHIN THAT DISTANCE TO A PORT

2000

500 RAIL DISTANCE

RAIL DISTANCE

PORT

TO PORT VERSUS CUMULATIVE MINEABLE RESERVES NSW a QUEENSLAND FIG 9

EXPORT

CRL-16

3. MINE PLANNING

—60—

GEOLOGICAL ASPECTS OF LONGWALL MINING OPERATIONS PAST, PRESENT AND FUTURE

KONRAD 11.R. MOELLE DEPARTMENT OF GEOLOGY TELE UNIVERSITY OF NEWCASTLE

—61—

GEOLOGICAL ASPECTS OF LONGWALL MINING OPERATIONS

PAST, PRESENT AND FUTURE

K.U.R. MOELLE DEPARTMENT OF GEOLOGY THE UNIVERSITY OF NEWCASTLE

ABSTRACT The geologist assists the mining engineer during the planning of the colliery layout and during the production period. The elimination of geological "surprises11 is one of the main tasks of the geologist. Faults, igneous intrusions, washouts and palaeochannels, sedimentary dykes and "stonerolls" are the most common geological features that may cause delays in development and production if encountered unexpectedly. The geological setting of a colliery holding usually provides a considerable amount of information pertaining to the system "floor-seam-roof." The large number of geological variables that commonly occur in such a system make it very difficult to assess the system algebraically. The mining engineer is concerned with geological phenomena that result from several processes and conditions. It is an engineering task to extract the coal, sometimes with difficulties. The geologist must analyse the environment in which the engineer operates and these analyses are designed to define the geological "environment11 of the mine workings.

Such a definition of tasks should ideally lead

to a close co-operation between the engineer and the geologist toward the choice of an efficient and highly productive mining method.

INTRODUCTION

Many mining methods have been successfully used in the Permian Coal Measure sequences of the Sydney and Bowen Basin structure.

The development of Australian longwall mining practice in the early f60s was marked by two contrasting events.

Whereas the first installation

could not successfully operate in the Bulli Seam of the Permian Illawarra Coal;Measures, a longwall face commenced not long after achieved a production record, mining also in the Bulli Seam.

Longwall faces were brought into an efficiently operating industry that relied almost exclusively on continuous miner-based bord and pillar mining methods with subsequent pillar extraction on the retreat.

Mining operations

took place at relatively shallow depths ranging from approximately 150 metres to 250 metres in flat lying coal seams occasionally disturbed by moderate to small scale faulting.

Igneous intrusions in the form of dykes

and, less frequently, sills, have impeded development and extraction work in most mines.

Other geological features and phenomena have proved

troublesome to the evolving Australian mining practices as well as to the rather rapidly introduced methods, which revolutionised the extractive processes on occasions.

Each major phase of innovation has brought improvements and increases in productivity, because the introduction of modern machinery and technologically advanced auxiliary equipment did not only optimise their performance but it also often considered the geological aspects of the coal measure strata.

THE MINING METHODS AND GEOLOGICAL ASPECTS

High and low capacity coal mining operations are extractive processes in a mechanically anisotropic, layered and heterogeneous geological discontinuum. The response of a rock body with such attributes to the removal of one of its "layers" is a complex petrophysical "reactionary" sequence of events, which cannot always be mathematically assessed and categorised.

The

characteristic response patterns of rockmasses to mining operations depend on the mining methods used, rate of extraction and type of support, just as significantly as on their own scalar and vectorial attributes.^ KHRM-01

A study of mine plans, drawn in the hand mining era, reveals that early miners "understood" the concept of directional stability as well as modern rockmechanics researchers.

Plans of pillar extraction patterns in

conventionally driven panels clearly show the varying mining conditions in specific areas, which can now also be defined as geological domains.

The extreme difficulty of the initial bord and pillar operations at Appin Colliery demonstrated that "mining engineering" had to be practised in specific and typifiable geological environments and that mining methods had to "interact" with those attributes of the rockmass that are mechanically significant.

The bord and pillar mining method was replaced by the longwall method which resulted in greater stability, much improved productivity and in an "interaction" response from the rockmass rather than in a "reaction" leading to instability. The geological setting of the Permian Coal Measure sequences in the Sydney Bowen Basin Complex ( 2 ) ( 3 ) has created very favourable conditions for retreating longwall mining operations (Figures 1 and 2).

The specific

anisotropies that determine the total response of a rockmass to the inevitable disturbance by the mining process are:

The planar discontinuities (bedding planes, including cross-bedding and foreset bedding, joints and faults and sheetlike intrusions).

The linear elements (intersections of planar discontinuities).

The petrofabric patterns in the sedimentary rocks, notably imbrication and the preferred dimensional orientation of oblong clasts.

The type and nature of stratification surfaces separating different lithologies (propagation and modification of mining induced stresses through layered media).

The differential compaction structures and textures in areas affected by primary non-diastrophic features (stonerolls, "wants," erosion channels). KHRM-02

- 64 These anisotropics have vectorial and scalar attributes, spatial attitudes and geometric relationships, that can be related to the mechanical processes of longwall seam extraction, namely the coal cutting, support and convergence phases.

It is evident that longwall mining can achieve record breaking productivity in the currently mined areas in the Sydney and Bowen Basin structure under the prevailing geological conditions; consequently that mining method is applicable and suitable.

While a number of longwall faces performs at

satisfactory levels, some faces have not produced the expected tonnages. It is suggested that such instances do not indicate a need for a different mining method, but rather that the "right'1 geological questions have not been asked during the assessment, planning and development stages of the longwall face (Figure 3).

The concept of longwall mining "engineering" in a block of coal with floorand roofstrata, that is, in a geological body, must include an initial test for the susceptibility of that body to "engineering."

The affected

parts of the geological body must be structurally integrated into the mining process.

Such an integration may not always be achievable, because the inherent self-supporting capacity of a rockmass segment is insufficient and cannot form a beam.

It is also often observed that a previously formed "beam"

over a gateroad has been converted into two cantilever segments by a penetrative mining induced fracture set.

The consequent response of the

gateroadfs roof can consist of a marked deterioration in roadway stability affecting eventually a portion of the face.

The interaction between the stresses generated by the mining process and the existing regional stressfield is a critical link in the sequential events that accompany the progression of a longwall face in a layered geological discontinuum.

The generated and existing stresses, their

superposition on one another and the mechanical involvement of the "floorseam-roof" system with its anisotropies must be assessed and analysed in a geological context, and mathematically modelled as well as possible. The geologically interpreted results should then be used as criteria KHRM-03

- 65 for the dimensions and layout of the face and gateroads, selection of supports, cutting machinery and conveyor equipment.

The gateroads should receive particular attention.

The concept of "in-seam11

single entry merits increased research and development efforts.

GEOLOGICAL FEATURES AND LONGWALL MINING

The geological conditions are generally ideal for longwall mining; the assessment of geological bodies and the delineation of relevant attributes improve rapidly with advances in technology and instrumentation available to the geophysicist and petrologist.

Economic pressures resulting from

increasing international competition for a non-expanding market will prompt support for sustained research efforts.

Methods are being developed that can identify and define those geological factors and phenomena restricting the capacity for dimensional increases of longwall panels.

The processes of violent gas emissions and outbursts, that may seriously threaten the development of future large panels, are not fully understood at present; a considerable advance, however, is indicated by recently

(4) reported findings

that the chemical and isotopic composition of seam

gases emitted depends on physical disturbances associated with the sampling method.

Of particular interest in this context is the observation by

Gould (opc cit.) that coal measure sequences, with increased CO2 contents due to invasion by externally derived gases, emit gases with different compositions in a time dependent fashion.

As future and larger longwall

panels are likely to cause brittle deformation of considerable rockmass volumes, the areal extent of increased fracture porosity should be researched. The production capacity of longwall panels can be further restricted by loss of coal integrity at the face, the presence of igneous intrusions, sedimentary dykes and "stone-rolls.11

Spalling at the face is commonly

due to unfavourable angles between the face alignment and the spatial attitudes of cleats, but can also be caused by an inability of the coal to support the load at the face line after cutting, but phenomena have geological causes. KHRM-04

—66—

The floor- and roofstrata have ideally a rectangular relationship with the face; this should be maintained by use of appropriate machinery and support systems for the specific geological conditions in the panel.

The floorstrata perform three principal functions in a longwall mining operation which depend for success and efficiency on geological attributes; floorstrata provide a support and glide horizon for the shield and/or chock-shield supports.

They provide the base for the armoured face conveyor

and stage loader, and they provide movement planes for lateral adjustments of the coal seam.

Floorheave and high coefficient of friction values at the interface of seam and floor will invariably lead to loss of production time.

Floorheave is frequently due to high shear stress concentrations.

The

presence of layers with expanding clayminerals in the floor is occasionally responsible for floorheave when water from the shearer reaches the claystone layers through the opening joint sets in the floor and causes expansion.

It is advisable to regard the roofstrata as an anisotropic and layered cantilever, consisting of particulate matter in three size and "function" categories, namely clasts, matrix and cement.

A distinction should also

be made between the "immediate" roof and the main roofstrata, as defined by N a t a u . ^

The "immediate" roof is of great mechanical significance as

it propagates the applied supporting load from the support system; it fractures by shear (Fig. 4) and is thus a vital element in the convergence process.

The "immediate" roof is commonly twice as thick as the seam

mined.

The application and propagation of supportive forces through a

bedded (layered) geological unit over a short, but variable time span causes specific response patterns in a "layered" fashion and does not develop uniformly.

The strength, rigidity and stiffness of the floor- and roofstrata have to be determined in addition to those moduli measured in the coal.

KHRM-05

—67— The phenomenon of bedding is an important feature in the development of sedimentary rock sequences and also in the study of their behaviour under load, may it be applied vertically, obliquely or horizontally.

Bedding, in the context of longwall mining, is to be regarded as a product (Pettijohn et a l . ) ^ of

and

(a)

composition

(b)

size of particles

(c)

shape of particles

(d)

orientation in space of particles

(e)

packing of particles.

It now becomes interesting from a "mechanical behaviour and response" viewpoint, to examine if such a product is also a mechanical functional feature in a load situation or not.

Its anisotropic nature has been

established, consequently there is reason to search for such a functional aspect.

The active functional role of the individual bedding plane surfaces

and units is well established in model work and prototype behaviour, as well as in fabric studies, that have established the co-existence of functional and morphological fabrics.

The superposition of a mining induced stressfield on to an anisotropic rockmass leads to a ndeformational response."

The rockmass has been "loaded"

before; the residual stress may be called primitive stress or geologically, a "finite state of strain."

It is thus suggested that a load redistribution

is directly linked to a deformational response in a "composite material" characterised by planar discontinuities in two principal attitudes, namely horizontal and vertical.

Bedding planes constitute the horizontal element and should be regarded as planar discontinuities along which varying deformational processes take place.

Initially, a stress pattern variation across a bedding plane may not be demonstrable geometrically on a specimen scale, although it may be visible under a microscope.

The failure types occurring in rockmasses along bedding KHRM-06

—68— planes vary considerably in situ and also in testing devices, as they do separate "unlike" (to varying degrees) fabrics and lithological entities.

The mechanical aspects of joint systems in the roof- and floorstrata of longwall panels have been defined and discussed previously ( 1 9 7 9 a ^ and 1979b). (8)

Recently initiated microscopic analyses of roofstrata specimens sampled in goaf areas indicate the possible presence of "fatigue" cracks in the highly anisotropic sedimentary fabrics of fluvially deposited sandstones. Fatigue cracking of "brittle" sandstones with siliceous cement as a result of low frequency loading and unloading cycles, produced by the advancing support systems, would be a failure mode not previously considered in the mining context.

Such a failure mechanism, if it can be demonstrated,

could provide explanations for certain stress distribution patterns observed along longwall faces under strong roofstrata.

FUTURE DEVELOPMENTS

The "future" longwall panel is likely to be considerably larger than at present.

Panel lengths of up to 5000 metres are probably possible in

the geological settings of the Sydney and Bowen Basin.

The control of Energy Release Rates (E.R.R.) will probably become possible over the next 10 years by advances in the study of interaction processes between geological anisotropies and mining induced stress fields, and taking gas type and distrubution patterns into consideration.

The "pivoting" of longwall faces will probably become an economically interesting alternative to the "shifting" of faces under deep cover.

Mining geologists should develop special maps of the potential longwall mining areas in the individual coalfields, showing "tectono-depositional" trends and domains.

Such maps could be compiled from available data with

little manipulation necessary. KHRM-07

- 69 It may also be very useful to develop a set of maps that may become known as "Anomaly" plans.

Such plans should highlight any significant deviations

from the conceptual model of a basin development, sedimentation and modification in the conventional geological sense.

The preliminary, interim and final geotechnical models of all types assisting the mining engineer in the choice of design criteria should be based on such an "anomaly" map,

The mining geologist of the future should be able to interact with the mining engineer on questions of production capabilities, possible dimensions of longwall panels and "reaction" patterns of the rockmass to high volume mining.

Geologists must have the intellectual courage to think innovatively within geological frameworks to match the engineering progress; geologists and engineers should think together and not merely rearrange "failed" prejudices.

REFERENCES

(1)

Moelle, K.H.R. 1978, Directional mining in rocks of the Sydney Basin, N.S.W., 3rd Australian Tunnelling Conference, The Inst, of Engineers, Australia, Canberra, pp. 7-10.

(2)

Branagan, D.F., C. Herbert & T. Langford-Smith 1976, The Sydney Basin, Sydney Science Press.

(3)

Murray, C.G., 1985, Tectonic setting of the Bowen Basin, GSA Bowen Basin Symposium, pp. 5-16.

(4)

Gould, K.W., A.J. Hargraves & J.W. Smith, 1987, Variation in the composition of seam gases issuing from coal, Bull. Proc. Aus.I.M.M., Vol 292, No. 5, pp. 69-73.

(5)

Natau, 0., 1975, Quantitative Gebirgsdrucktheorie fur den oberflachennahen

(6)

Pettijohn, F.J., Potter, P.E. & Siever, R., 1972, Sand and sandstone,

Strebbetrieb, Erzmetal, Vol. 28, pp. 225-228.

Springer Verlag, New York, 618 p. KHRM-08

(7)

Moelle, K.H.R., 1979a, Fracture"ysterns in roof rock strata and goaf formation, 4th Tewksbury Symposium, University of Melbourne, p. 4. 1-4. 13, Melbourne.

(8)

Moelle, K.H.R., 1979b, On the interaction of geological fractures and mining induced fractures, in G.C. Sih & S.R. Valluri (eds.) Fracture mechanics in engineering application, pp. 505-509, Alphen aan den Rijn, Sijthoff & Noordhoff.

ACKNOWLEDGMENTS

Thanks are expressed to Messrs B. White and R. Phillips for many very useful discussions on ideas and mining engineering aspects. Mrs J. Pitts kindly typed the manuscript.

KHRM-09

SOURCE AREA ROCKTYPES EROSIONAL A C T I V I T Y TRANSPORTING MEDIUM 1 D E P O S I T I O N A L AREA PALAEOENVIRONMENT PHYSIOGRAPHY-MORPHOLOGY CLIMATE RATE/MODE OF D E P O S I T I O N PRIMARY NON DIASTROPHIC ANISOTROPIES I (PROCESS ELEMENTS) I DIAGENESIS STRUCTURAL/TECTONIC CHANGES

(RESPONSE ELEMENTS) i L I T H O L O G I C A L SYSTEM I * HETEROGENEITIES ANISOTROPIES I RESPONSE, BEHAVIOUR PATTERN UNDER LOAD

EROSION,

REPLACEMENT

Figure 1.

Processes and events d e t e r m i n i n g response p a t t e r n o f rockmasses under l o a d .

HETEROGENEOUS

A P P L I C A T I O N OF

ANISOTROPIC

DIRECTED FORCES

GEOLOGICAL BODY

CHANGE OF PHYSICAL CONDITIONS

STRESS D I S T R I B U T I O N 1 MOVEMENTS OF COMPONENTS 1 HISTORY-STRUCTURE-STRESS/STRAIN

DEPENDENCY

J STRUCTURAL ANISOTROPY STRENGTH ANISOTROPY F i g u r e 2.

Development o f s t r e n g t h

anisotropy.

- 72 -

GEOLOGICAL JUDGEMENT

FORMAL KNOWLEDGE

RISK

CAPITAL EXPENDITURE

MAIN DEVELOPMENT ROADWAYS

PIT BOTTOM ;

PANELS

DRIF TS SHAFT S

SUPPORTS

MINING MACHINES TRANSPORT

PRIMARY NON-DIASTROPHIC ANISOTROPIE5

COAL TYPES

ROCK TYPES

DYSTROPHIC AMISOTROPIES

SEAM THICKNESS

PHYSICAL PARAMETERS

PROFITS

PRODUCTION

FUTURE MINING OPERATIONS

Figure

OVERBUR D E N IMMEDIATE

"SEAM NOW G O A F

SEAM HMHIlinninnimi " kN '

2c cos 3 , I'M. s AW=1 0 C1-XJ-C1+XDsin S K - 1

AW=1 0

2c cos9

C1-X) sinC2^-3D-C1+X)sin9'

•C1)

m2J

| _

K-i J

t = D E N S I T Y 2. 5 t / m 3 M"HEIGHT 2.2m K " L O O S E N I N G F A C T O R 1.5 S " F A C T O R OF S A F E T Y 1.6 S-4-0F INTERNAL FRICTION C - C O H E S I O N OF I M M E D I A T E

kN 1

Q o .

m 2

C2)

30 ROOF

Figure 4 DETERMINATION OF SUPPORTING LOAD CAPACITY CAW) Caccording lo N a t a u . 1 9 7 5 ) SHEARING IN THE IMMEDIATE ROOFSTRATA AFTER GOAF FORMATION

ROOF

- 74 -

APPENDIX CHECKLIST OF ITEMS TO BE CONSIDERED FOR LONGWALL MINING

STRUCTURAL INTEGRITY OF THE COAL "BLOCK"

(i)

Mining time in the "block" of coal

(ii)

Developments in adjacent areas, future deeper mining

SUPPORT SPECIFICATIONS '

(i)

What information is required to enable "informed" selection of the most suitable support systems?

(ii)

Po stulated caving mechanism (conceptual model)

(iv)

The role of modelling

MINE LAYOUT CONSIDERATIONS (i)

Directional stability

(ii)

Pillar calculations

(iii)

Width of face

(iv)

Length of coal block

CAVING AND GOAF FORMATION (i)

Hydrological problems

(ii)

Subsidence, surface effects

MONITORING (i)

(GEOL. & ENG.)

Chocks and shields

(ii)

Pillars

(iii)

Roadways

(iv)

Gas emissions

MINING DEVELOPMENT (i)

Geotechnical assessment of the stratigraphic column

(iii)

Roof supports

(ii)

Rate of advance

(iii)

Rate of development work

EFFICIENCY (AVAILABILITY) OF THE LONGWALL (i)

Use of machinery at 90% or better. Which part is not always avaialble at that rate?

(ii)

Geological reasons for non-availability?

—75—

GEOLOGY - A KEY TO MINE PLANNING A CASE HISTORY LOCHIEL BROWN COAL DEPOSIT, SOUTH AUSTRALIA BY A. G. KREMOR & A. MUIR

•76—

Abstract The

Lochiel

mid-north

Deposit,

a T e r t i a r y s o f t b r o w n c o a l d e p o s i t in the

of S o u t h A u s t r a l i a , is c u r r e n t l y being

investigated

the E l e c t r i c i t y T r u s t of S o u t h A u s t r a l i a as a p o t e n t i a l s o u r c e of fuel for a base load p o w e r The

sedimentary

station.

sequence

consists

fluvio-lacustrine

and

m a r g i n a l m a r i n e s e d i m e n t s and the o v e r b u r d e n s h o w s a c o n s i d e r a b l e variation

lithology comprising unconsolidated, soil

c l a y , s i l t , sand and The

low

s t r e n g t h and h i g h pore w a t e r p r e s s u r e of the

sediments and of

combined

sedimentry mining

analyses

Geological models defined

geophysical

bee n

used

These

provided

to a

log

define means

interpretation

of p r e d i c t i n g

T h i s p a p e r d e s c r i b e s the studies

correlation

sediment

regimes. properties pit.

detailed have these

been

c a r r i e d o u t and

how

were

integrated with

concurrent

g e o t e c h n i c a l and

and

test h o l e s and a trial

which

hydrogeological

studies

questions

relating

answer to

specific highwall

stability.

Introduction In

1982 the E l e c t r i c i t y T r u s t of

South

Australia

carried

out

major exploration programme led

Lochiel

which

the

discovery

Coal

Deposit,

located

130 km n o r t h of A d e l a i d e

the

(Fig. 1).

set

from bore core

geological/geotechnical

and a b a s i s to site g e o t e c h n i c a l

and s p o i l p i l e

tectonic

d i s c o n t i n u i t y p l a n e s have c r e a t e d an u n u s u a l

have

geological

overburden

w i t h the p r e s e n c e of s u b h o r i z o n t a l ,

conditions. and

strength

coal.

KILOMETRES

Figure 1 - Locality

Plan

—77—

The

deposit,

basin

(Figure

some

the 2).

12 km long and 6 km wide occurs in a faulted

northern

part

within the Kooliata Coal Zone. to

but

the

classified

lignite

Reserves

the Tertiary St Vincent Basin

Within the resource area coal occurs in three seams average B

The combined seam thickness is up in

the order of 12 m.

The coal is

(ASTM) or soft brown, class 15 (ISO).

exceed 550 million tonnes in the measured and indicated

categories.

Insitu

coal quality for the reserves are presented

on Table 1.

Mine

planning

walking

removal method. 1982

date

draglines a

has

evaluated

bucketwheels

the use of either large

as the principal overburden

At this stage, draglines are preferred and since

number

combined

geological,

geotechnical

and

hydrogeological investigation programmes have been carried out to assess highwall and spoilpile stability.

The

sediments

outcrops on

subsurface

divided

in the deposit are essentially flat lying with no

and therefore geological evaluation has relied entirely on

investigatory

the

response

from

boreholes

drilled

methods.

The

sequence

has been

basis of lithology and geophysical borehole log interpretation at

and

spacing

correlation

less

over

300

than one kilometre,

Table 1 Coal Quality Specific Energy

23.4 MJ/kg

Total Moisture

61.5%

Ash

15.1%

Sulphur

2.89% (d.b.)

Sodium

0.95% (d.b.)

Chlorine

0.48% (d.b.)

(d.b.)

Figure 2 - Regional Geology

AK-AM-0 2

—78— i n c l u d i n g a n u m b e r of fully and p a r t i a l l y cored h o l e s . holes

provided

signatures lateral

means

relating

(Figure

3).

aid

The cored

l i t h o l o g y to g e o p h y s i c a l

c o r r e l a t i o n and

understanding

l i t h o f a c i e s m o d e l s w e r e d e v e l o p e d and u t i l i s e d to d e f i n e

depositional

environments

depositional

environment

p r e d i c t i o n of s e d i m e n t

and

processes

(Figure

m o d e l s w e r e then a p p l i e d

4).

The

to a s s i s t

the

properties.

Geology The s e q u e n c e has b e e n d i v i d e d on F i g u r e 5 . The

b a s a l B u m b u n g a Sand has b e e n d i v i d e d

facies

which

channel

through

member

cyclic

sequence

equivalent end

into the s e v e n f o r m a t i o n s as s h o w n

represent

distributary

overlain of

floodplain

into three

depositional

a g r a d a t i o n of e n v i r o n m e n t from the

and

Condowie

lacustrine and

floodplain

fluvial

peatswamp.

This

S i l t w h i c h c o n s i s t s of a and

their

laterally

peatswamp deposits.

sediments

A t the

northern

of the a r e a , the top of the C o n d o w i e S i l t has been e r o d e d

the u p w a r d fining f l u v i a l D a r n l e i g h Park

Sand. SELF" P O T E N T I A L RESISTANCE

F i g u r e 3 - L i t h o l o g y and G e o p h y s i c a l Log

Signatures AK-AM-0 3

BUMBUNGA sZl^DS - LITH0FAC1ES MODEL

Distal Backswamp

Interdistributary Baekswomp

Carbonaceous Sand BU3

Interbedded Sand BU2

~ j Clean Sand (course) | Clean Sand (fine-medium)

Carbonaceous Sand 9» Silt

(

\ Coal

GEOPHYSICAL LOG RESPONSE

DEPOSITIONAL ENVIRONMENT

Condowie Silt Member Sand Lithofacies

DISTAL BACKSWAMP WITH INTERNAL DRAINAGE SYSTEM

Bumbunga Sand

FLUVIAL SYSTEM

Weathered Quartzite Basement

Natural Gamma API

Neutron - Neutron CPS

23 1.7 i—i— Apparent Density gms/cc

Darnleigh Park Sand Member

BACKSWAMP OVERBANK

CHANNEL OVERBANK

Bumbunga

STACKED CHANNEL

Sand

FLUVIAL SYSTEM

CREVASSE SPLAY INTERBEDDED SPLAYS AND DISTRIBUTARY CHANNELS Rosehaven Clay Basement

Figure 4 - Bumbunga Sand Lithofacies Model The

Kooliata

Coal

Silt

and

Darnleigh

bearing

the

Zone

conformably overlies both the Condowie Park

Sand

and consists of three coal

horizons separated by well defined but thin interbeds of

sand and silt.

AK-AM-04

—80— These

interbeds

are

single

lacustrine

cycles

which comprise

marginal swamp, shoreline and deeper water facies. The sequence to the top of the Kooliata Coal Zone is Upper Eocene and

this

overlain

01igocene/Miocene

marginal

Silt

highly variable in lithology, consisting of a number of

supratidal

Warrindi Silt and Tarella Silt.

marine

interdigitating

the

sediments

lithologies

depositional

representing

facies.

The Warrindi

intertidal

then

and

overlain by the

transgressive subtidal Tarella Silt. The Tertiary sequence has the northwest.

been regionally tilted at

about 1° to

The overlying Hindmarsh Clay equivalent consists of mottled and fissured silty and sandy clay deposited in an oxidising lacustrine environment. This formation is overlain by Recent sediments consisting of alluvial fan clay, sand and gravel and gypsum rich aeolinites.

STRATIGRAPHY

LATERAL CONTINUITY

AQUIFER DISTRIBUTION

CHARACTERISTICS OF AQUIFER SAND DEPOSITIONAL ENVIRONMENT

Holocene

AVERAGE GEOMETRY THICKNESS

lacustrine

tabular

LATERAL VARIABILITY

high very high

Pleistocene

Oligocene

Late Eocene BUMBUNGA SAND (sand, silty sand, coal)

estuarine channel

shoestring

estuarine-channel, shoreline, washovers

variable

I -10

high

deltaic

wedge

moderate

lacustrine shoreline

elongate

moderate

lacustrine shoreline

elongate

1-5

moderate

fluvial - meander belt

tabular

low

crevasse splay

wedge

0-5

high

fluvial system -crevasse splays -meander belts

tabular

low

fluvial undifferentiated clay, quartzite siltstone

Figure 5 - Stratigraphy and Sediment Characteristics

AK-AM-0 5

—81—

Application of geological data to geotechnical and hydrogeological aspects Following

preliminary and brief geotechnical and hydrogeological

assessment, grained presented aid

soil

was

recognised

strength

material

that

the

and

high pore water pressures

combination

of fine

a particularly difficult geotechnical environment.

selection

divided into geological regimes based on:overburden thickness aquifer thickness proportion of Tarella Silt in the overburden. Fully

cored

completed with

holes

major tests

testing

within

each

formation.

These

confirmed

strength and

were

each regime

engineering

performed

the

low

of the materials

also

number

identified

subhorizontal

defects in the overburden. As well

field

these

tests,

depressurisation

tests

were

regimes

conducted

determine

the

pumping

water

of geotechnical investigation sites, the area was

and

pressures

effects the

pore

the

Tarella Silt.

Figure 6 - Geotechnical Regimes and Trial Pit Site Selection Criteria

AK-AM-0 6

— 8 2 —

These

regimes

represented

were

chosen

for hydrogeological testing as they

variation in sedimentary sequence from thick silt

with

no surrounding aquifer sand to silt with a thick aquifer at

the

base.

identified

Slope a

stability

number

analyses

which required further investigation.

the

based

these

tests

of critical aspects common to most regimes These were:

rate and extent to which the pore water pressure in the

silts is reduced by dewatering and prestripping. 2)

the large scale strength of in situ materials.

the strength and characteristics of subhorizontal defects.

was considered that the best method to examine these features

was

trial

effective

essential

that

the

trial

being of

excavation.

deposit

order

such

for this method to be

variable

lithology it was

the location of the trial pit was representative

aspects pit

with

previously defined. site

present at least

subhorizontal

was

9 metres

defects,

The technical selection of

based amongst other criteria on there of

Tarella Silt, the presence

and aquifers below

the Tarella Silt

( see Figure 6).

Relationships between geological and engineering properties. Depressurisation of Tarella Silt The rate and extent to which pore water pressure within sediments is reduced is related to vertical and horizontal permeability and the proximity of surrounding

aquifers.

The

stratigraphic

location and spatial distribution of aquifers

has

been

defined to

specific

detail

over the deposit and these can be

depositional

environments.

The processes

acting to form the aquifers has resulted in predictable geometry, thickness and lateral continuity (see Figure 5).

AK-AM-07

—83—

Prior

to the excavation of the trial pit it was assumed that the

Tarella

Silt

could

be considered homogenous for application of

consolidation theory.

Detailed line mapping demonstrated that an

intensive

pattern

shear

which

sand

aid

are the

rate

infilled. of

and tension defects exist, some of The existence of these features will

depressurisation

and future analyses will be

modified to include effects of these features.

Subhorizontal defects Subhorizontal defects in the overburden are weak zones which have the

potential

genetic

types

act

subhorizontal defects exist - sedimentary and

as failure planes for block slides.

Two

tectonic• The tectonic defects are confined to a polished shear zone within the

Tarella

coincides In

the

common

Silt,

The stratigraphic location of the shear zone

with

the clay rich base of a coarsening upward cycle.

trial

pit area this shear was shown to exhibit features

to other shear zones.

These include riedel and conjugate

riedel shears, thrust shears and a principal displacement shear.

was

also

demonstrated that the principal displacement shear

was essentially planar but that the validity of shear box testing was dependent on the selection of 2-

the principal shear plane for testing the direction of shear testing on the sample the frequency of subvertical displacement shears intersecting the principal subhorizontal displacement shear. Sedimentary to

20 mm

defects

are of two types; horizontal planar beds up

thick and occurring at the top of discrete sedimentary

cycles and as beds dipping at up to 17°.

AK-AM-08

—84—

These

features

o c c u r at the base of the T a r e l l a Silt and w i t h i n

the W a r r i n d i S a n d .

The h o r i z o n t a l p l a n a r beds are u b i q u i t o u s and

are

considered

representative

sedimentation

cycles

depositional

environment.

predicted

that

formed

individual

vertically

accreting

Because

beds

c o n t i n u o u s , extensive and

dipping p a r a l l e l to surrounding The

s u b a q u e o u s l y in e s t u a r i n e / l a g o o n a l are

this

process,

sediments.

dipping clay beds are c o n f i n e d to a p a r t i c u l a r m e m b e r of the

Warrindi Silt. and

occur

sand.

The beds are c o n t a i n e d w i t h i n e s t u a r i n e

sediments

as d r a p e s on a b a n d o n e d c h a n n e l s and on storm w a s h o v e r

P r e l i m i n a r y a n a l y s e s of p a l a e o f l o w data indicate that the

c h a n n e l flow d i r e c t i o n was to the south east and the shoreline of the

sea

was

orientation geophysical programme

also of

the

dipmeter which

c u r r e n t l y being

the

south.

dipping tool

used

aid

determination

b e d s , a test p r o g r a m m e utilising a was

completed.

The r e s u l t s of this

b o t h S c h l u m b e r g e r and BPB instruments are

assessed.

Conclusion The

assessment

integration

hydrogeological geotechnical

the

Lochiel

Coal

D e p o s i t has relied on an

understanding

aspects.

The

geology

which

enabled

regimes,

geological,

g e o t e c h n i c a l and

has been used to define optimal

siting

g e o t e c h n i c a l core holes and t r i a l d e p r e s s u r i s a t i o n

wells.

demonstrate

completed

and

understanding

the

geotechnical

assumptions

t r i a l pit was

the

extrapolation

based

the

results

geological

processes

thank

Electricity

which

on an

formed

the

South

s e d i m e n t a r y sequence . Acknowledgements The

authors

wish

the

Trust

A u s t r a l i a for t h e i r a s s i s t a n c e in preparing this p a p e r .

We w o u l d

also like to thank Mick O ' B r i e n for his c r i t i c a l a p p r a i s a l of the manuscript.

AK-AM-0 9

•85—

LQNGHQLE EXPLORATION DRILLING FOR NINE PLANNING IN BHP MACQUARIE COLLIERIES 1 1 by B. Ross , C. Tobin

2 and A. Williams

ABSTRACT In-seam longhole drilling is now routinely practiced as a means of geological exploration for mine planning in the BHP Macquarie Collieries mines in the Newcastle Coalfield. The technique is seen as complementary to our own in-seam seismic surveys for locating and interpreting structural anomalies affecting longwall layout planning.

A judicious combination of these techniques,

drawing on the particular advantages of each can provide the most cost effective and reliable geological assessment. A wide variety of geological environments have been investigated within our mines, with the longest hole having reached 1008m in Pacific Colliery.

The highest metreage rate is 330m per seven hour drilling

shift, although 150m - 200m per shift is more typical. Planning, supervision and interpretation of the drilling programmes is the responsibility of the mine geologist.

Drilling information,

often of a subtle nature, can then be integrated with other facets of geological knowledge, eg. changes in seam, roof, floor and splitting characteristic s or predicted structural anomalies based on in—seam seismic or surface geophysical surveys.

1. GEOLOGIST

GROUP OPERATIONS GEOLOGIST

GEOLOGY DEPARTMENT BHP MACQUARIE COLLIERIES GROUP P.O. BOX 171, BELMONT, NSW 2280

•86—

LONGHOLE EXPLORATION DRILLING FOR MINE PLANNING IN BHP MACQUARIE COLLIERIES 1.

INTRODUCTION Long-hole drilling has been trialled or systematically practised at J

various Australian collieries since the early 1980 s.

At most of these

mines, in-seam drilling has been primarily for seam gas pre-drainage, to reduce problems associated with instantaneous outbursts and high gas makes (Williams et. al., 1986). At the four mines of the BHP Macquarie Collieries Group in the Newcastle Coalfield (Fig. 1), long-hole drilling has been used specifically for geological exploration.

The prime need has been to accurately

locate faults and dykes to assist longwall layout planning.

Longwall

mining has been introduced at each of the mines, except Lambton Colliery. A trial long-hole drilling programme, the first in the Newcastle Coalfield, commenced in January, 1986 at John Darling Colliery. The initial one month trial was set up and supervised by ACIRL, and a Longyear LMJ7 drill rig was used for the first time for this type of work in coal seams. Following the successful fig I.

Location of BHP Macquarie Collieries

introduction of the technique and operator training, the drill rig

and ancilliary equipment were purchased and have been in continual use on geological exploration programmes to the present time. A key element in the success achieved with the method has been that the planning, supervision and interpretation of drilling programmes is the responsibility of the mine geologist.

Drilling information, often of a

subtle nature, can then be integrated with other facets of geological knowledge, eg. changes in seam, roof, floor and splitting characteristics or predicted structural anomalies based on in-seam seismic or surface geophysical surveys etc. 2.

GEOLOGICAL AND MINING SETTING

Sedimentary and Structural Environment

The Macquarie Collieries Group of mines work seams within the Lambton SubGroup, in the lower part of the Newcastle Coal Measures. BR-CT-AW-01

—87—

The Borehole seam is mined at Stockton Borehole, Lambton, and John Barling Collieries with thicknesses typically 1.4m? 1.7m and 2.3m respectively.

The Young Wallsend seam is worked at Stockton Borehole and Pacific

Collieries, although over most of Pacific Colliery the seam is split into an upper Nobbys component and a lower Budley component.

Thickness ranges

from 2.1m - 3-0m. The mines lie within the northern part of the Macquarie Syncline. Bips on the flanks of the syncline are relatively gentle, upto 3°, although fairly steep rolls and flexures may affect seams locally.

The dominant

direction of faults, dykes and major joint and cleat sets in this part of the coalfield is approximately NW - SE. Bykes are common and vary in thickness but are almost always hard, so even with the current generation of longwall shearers even thin dykes (say <1m) invariably present cutting problems. 2.2

Mine Planning Requirements The accurate prediction of structural anomalies ahead of the workings

has assumed a far greater importance in recent years due to the introduction of longwall mining. The use of long-hole drilling, especially when coupled with the complementary technique of in-seam seismic reflection, has markedly increased the confidence level of geological prediction into virgin areas. Face-to-face in-seam seismic surveys have limitations with penetration distance (generally upto 400m, depending on circumstances), precision of reflector location and identification and, sometimes, survey geometry.

On the other hand, the method allows a broad lateral coverage

and may detect subtle features and variations such as faults with minor (less than half seam) displacements, discontinuous or en-echelon zones and changes in trend. The current long-hole drilling rig has a greater distance potential (generally upto 800m) than seismic reflection and, with appropriate survey control, can locate anomalies accurately. dykes can be determined.

The thickness and hardness of

However, it is possible that minor faults, or

even major faults, with coal to coal contacts, may not be detected.

Also,

the level of confidence in interpreting linear continuity of structures depends on borehole spacing and trajectories. It is clear that a judicious combination of available investigation techniques can provide the most cost effective and reliable geological assessment. BR-CT-AW-02

—88—

THEORY AND PRACTICE OF ROTARY IN-SEAM DRILLING A relatively large diameter hole is drilled in relation to rod size

so that deviation in a vertical plane can be initiated when required.

our case an 80mm hole is drilled for BQ (56mm dia.) rods. Downward deviation is achieved using higher revolutions relative to feed pressure and upward deviation by higher feed pressure in relation to revs.

This is fairly successful in practice, but the ratio between the two

for a particular deviation varies according to penetration rate, strata cutting resistance, type of bit and bit sharpness.

In soft strata there is

a strong tendency for the bit to dive down and increasing pressure alone is sometimes insufficient to bring the bit up - revs must be dropped considerably and penetration rate reduced. In hard strata it is difficult to bring the bit down as higher feed pressure is needed to achieve penetration which counteracts the effects of higher rotation.

If the bit has been in hard strata above the seam for

several metres with no or minimal indications of it being brought back into the seam and the hole is less than 300m, it may well be worthwhile pulling out and starting a new hole.

A new hole may be drilled to the same dis-

tance in a few hours, and slight alteration to the drilling parameters may keep the hole in the coal seam.

In most cases a second hole would be

needed to determine if a fault was the cause of the penetration into hard strata.

A further factor in an early pull out decision is bit wear, which

will be considerable in hard strata when using high revs. An alternative way to deviate the bit is with a down the hole motor (Dynadrill or Slimdrill).

This gives control over vertical and horizontal

direction, compared with pure rotary drilling which has no control over horizontal direction (although stabilisers and/or reduced hole/rod size can produce a straighter hole with reduction in vertical deviation capability). In rotary drilling, lateral deviation appears to be controlled by the planes of weakness in a coal seam, seam dip and the rotation of the bit. Drilling a hole down-dip at more than 5° for any distance creates problems with cuttings returns as the heavier fragments tend to settle out.

A more

powerful water pump or different flushing medium may be needed. Drilling up-dip presents no problems with cuttings, but most of the drilling water is lost when changing rods so, in a long hole, there are delays while water is pumped up to the bit. A good knowledge of lateral and vertical variations within the coal seam are essential if the bit is to be kept within the coal seam. BR-CT-AW-03

This

—89— knowledge can be used for feed pressure/revolution adjustment whilst the bit is in the seam rather than when it penetrates roof or floor.

It has

been a major factor in our success with longhole drilling. When a drill bit exits the coal seam, a decision must be made whether it has gone into the roof or floor, and thus how to change drilling parameters.

If this does not return the bit into the seam within 10 - 20m then

a decision must be made whether to core (if the new strata cannot be positively identified) or pull out and drill a new hole, changing to slightly different parameters 10 - 20m before the point of exit. The identification of fine borehole cuttings is a considerable problem, particularly when they are not bright coal.

They typically are pro-

duced over long distances, due to grinding by rods, and also in hard strata. Hole surveying is a relatively long process, taking a minimum of 15 minutes at short distances and over 30 minutes at greater distances.

When

30m can be drilled in 30 minutes, this represents a considerable loss of drilling distance during a day.

The frequency of surveys needs to be

assessed by the geologist as the hole progresses and will depend on the constraints of time and the need to locate the hole accurately. Surveying can be carried out as the hole progresses or after completion*

The latter is better for time saving as the furthest survey gives

the geologist a good idea of total horizontal deviation and thus the frequency of surveys needed to ascertain the holes position with reasonable accuracy.

If post completion surveying is planned, it is still wise to

survey occasionally during drilling to ascertain that the hole is within target limits. 4* 4-1

SELECTED CASE STUDIES Lambton Colliery The main area of future development in the Borehole seam at Lambton

Colliery lies offshore.

In undersea mining, the thickness of solid cover

over the seam is critical for calculation of coal extraction ratios and pillar sizes, and thus has major influence over method of extraction.

Lambton Colliery the feasibility of longwall mining in this area was being considered. Depth to bedrock was known from marine seismic surveys, but levels from development headings being driven into this area indicated the seam was shallower, and thus the thickness of solid cover less, than expected. A longhole drilling programme was undertaken to investigate seam BR-CT-AW-04

—90—

REDHEAD (old

COLLIERY

workings) TYPICAL SECTION • BOREHOLE SEAM

Middle Band

Bottom Band Miners Floor

102

LONGWALL

Ponel

No. 5

LONGWALL TYPICAL SECTION-BOREHOLE SEAM 104

Panel

Middle Band

Bottom Bond

100

200m

Legend Borehole seam workings —

70mu

Longhole Seam structure contour

Longhole drilling in Lambton (fig 2.)

Fault

and John Darling (fig 3.) Collieries,

Dyke

Borehole searn.

JBR-CT-AW-05

—91— levels beyond the developed area, by maintaining the boreholes in the seam and taking frequent inhole surveys. Four holes were drilled (Fig. 2) with starting directions varying from 165° to 245°.

Each bore converged to within 4° of 219° at some stage

in its length indicating the influence of seam structure over drilling direction.

This degree of longhole convergence had been anticipated, so

the holes had been started from the most divergent points within the area. The drilling programme proved that the seam rolled over steeply to the S.W. as shown in Fig. 2.

This knowledge has assisted in planning for

development heading directions and extraction panel layouts.

However, the

thickness of solid cover would restrict longwall blocks to uneconomic widths and extraction ratios. 4.2

John Darling Colliery An extensive long-hole drilling project was carried out over six

months, involving 23 bores and 5,300m of drilling.

The objectives were to

investigate the position and extent of several dykes and faults, inferred from marine magnetometer surveys or projected from existing workings, in the area of possible future longwall blocks.

Part of the programme is

shown in Fig. 3. The programme proved a dyke (target A, Fig. 3) to the SW of, and parallel to, existing longwalls, but disproved the existence of intervening dykes suspected from marine magnetometer surveys. A known dyke and large fault which had been restricted to pillars during initial gateroad development were proved to deflect to the south (hole 23) permitting the continued development for longwall 5 block, but curtailing the adjacent planned longwall 6 to the south.

Three drilling problems encountered were:(a)

Graben - like troughs in the Borehole seam containing soft coal and

severe changes in dip.

The soft coal required the use of very low revs,

moderate feed pressure and a low penetration rate to maintain the bit in the seam. (b)

A very soft roof of shale containing swelling clays.

If this was

penetrated the bit would sometimes preferentially remain in this band, and after several hours hole collapse and closure would occur. (c)

The tendency of the bit to deflect to the S.W.

This was initially a

problem, but was used to great benefit in hole 21 where a suspected dyke presented a very naxrow end - on target.

The hole was drilled out para-

llel to strong jointing and eventually deflected through what was thought BR-CT-AW-06

-92would be the centre of the dyke.

The non-existence of the dyke was proved

and the hole was continued to dyke target A to become one of the longest holes drilled in Australia, at 795m. The Borehole seam at John Darling Colliery proved to be a good longhole drilling medium, with several claystone bands, a contrast between the bright main upper section and the dull basal plies, and a strong contrast between the sandstone floor and shale roof. 4-3

Stockton Borehole Colliery The area of Stockton Borehole Colliery shown on Fig. 4 was inten-

sively investigated in an effort to accurately delineate any faults and dykes which may have been present.

Both the Borehole and Young Wallsend

seams are being longwalled in this area, by Stockton Borehole and Pacific Collieries respectively, so there is very little margin for error in gateroad positioning. Fault zone A, a series of normal faults with a combined displacement of approximately 3m, was intersected in workings to the south but its north-westerly continuation was unknown. An initial interpre-

I I L%

tation (1) was fairly tentatively based on brecciation observed in core from a surface borehole. An initial trial survey of the in-seam

! I I I — H SEAM WOnKINGS • TOING W/\lLSENO BOREHOLE FAULT INJEnrnETAMONS OniGINAl BASED ON I ss — BASED ON LOfJGHOLE DRILLING

seismic (iSS) reflection method from the Borehole seam had led to an upgrading of this interpretation (2) and also provided some targets for in-seam drilling. The results of the drilling led to a further revision (3), which enabled the position and

fig 4.

Longhole drilling in Slocklon Borehole and Pacific Collieries Borehole and Young Wallsend seams

extent of the fault to be BR-CT-AW-07

—93— fairly accurately plotted prior to gateroad driveage.

It also demonstrated

that the ISS reflectors which had been interpreted as the north-westerly continuation of the fault, were probably due to very minor faulting or a zone of intense jointing. Two of the boreholes continued past the initial zone A to intersect another significant fault (4) and, towards the north, a thin and probably soft dyke (5), Most of the boreholes in this area were drilled in the Young Wallsend seam which has proved to be quite a good medium for in-seam drilling.

This is mainly due to the lateral persistence of tuffaceous bands,

both those within the seam and also the immediate roof, the Nobbys Tuff (Fig. 5).

Having a very distinctive roof was essential when exploring for

faults such as zone A which was likely to have a relatively small displacement as it died out towards the north-west.

Therefore, by staying within

the upper part of the seam (by repeated deflections from the base of the tuff), any small downthrown fault could be detected. On the other hand, NOBBYS TUFF

the Borehole seam in this area, presents greater I

interpretative problems

YOUNG WAltSENO SEAM

due to its highly variable roof and floor strata. 4.4

Pacific Colliery The area of Pacific

SIRAHGRARIIC SECTION

Colliery shown on Fig. 5

j<U

I'D

1 ^

provides another good example of the meshing

together of ISS and long-

FAULT IN IERPRE TAT IONS

hole drilling.

BASED ON I S S BASED ON LONGIKXE DnilLING

ISS anomalies (1) and (2) were interpreted with moderate confidence to be faults and lined up with a known normal fault to the south (displacement 1.7m). Anomaly (3) was of low

K?Q

290

3<?0

METRES

fig 5.

Longhole drilling in Pacific Colliery Dudley / Young Wallsend spams

4^0 5pO

confidence and was tentatively interpreted to line BR-CT-AW-08

—94—

up with a small normal fault (displacement 0.2m - 0.3m), also known to the south. The drilling results to date have confirmed the presence of faulting at the locations of anomalies (1) and ( 3 ) .

However, further drilling will

be necessary to fully define these features. A third structure shown as (4) was found by drilling alone and because of the number of intersections of this feature a high confidence is placed on both its presence and amount of displacement.

At the conclusion

of the drilling programme the seismic profiles were re-examined and a faint anomaly was found at the location.

This was of benefit for future

ISS interpretation in the area. The drilling in this area has been quite successful, mainly due to the presence of a shale split in the Young Wallsend seam (Fig. 5). The presence of this horizon, when coupled with the tuffaceous bands, allows a chosen stratigraphic position within the seam to be readily maintained. Because of this, a displacement of 0.5m - 1.0m could be confidently predicted for structure ( 4 ) . CONCLUSION Long-hole drilling has been successfully carried out in a wide variety of geological environments, with the longest hole having reached to 1008m in Pacific Colliery.

The precise location of geological

structures has been beneficial for mine planning, especially for longwall operations.

For the industry at large, various recent technological developments will enable improvements in routine hole distances, guidance and structural interpretation.

With the increasing adoption of longwall mining and the

constraints this imposes on layout planning, undoubtedly long-hole drilling, in conjunction with in-seam seismic techniques, will be increasingly used for geological exploration. 6.

ACKNOWLEDGEMENT

The authors wish to thank BHP Macquarie Collieries for permission to publish this paper. 7.

REFERENCE WILLIAMS, R. J., ALLAN, J., TRU0NG, D., BEAMISH, B.B. AND HUNGERFORD, F. (19B6). Hole Drilling.

An Evaluation of Seam Gas Drainage by Long-

The Coal Journal, No. 10.

BR-CT-AW-09

—95— SOME INTERACTIONS BETWEEN GEOLOGICAL EXPLORATION DATA AND UNDERGROUND MINE PLANNING D.H* Fawcett Fawcett Mining Consultants Pty. Limited 7/281-285 Pacific Highway Crows Nest. NJS.W. 2065 Ph (02) 959 3077 Fax (02) 959 5067 Abstract Attention is drawn to some basic factors which constrain underground mine viability and some problems which can arise from initial geological data are outlined. The primary needs are for well founded resource estimates for seams of adequate quality, thickness, and uniformity within a well defined structure and lithology. Problems noted, whilst trying to establish these, include resource and seam thickness anomalies, definition of major faulting, minor faulting, folding and dykes, interpretations of rock strength from lithology, and coal quality sampling. It is concluded that current mine development costs justify closer surface exploration than in the past but that in some environments trial mining may prove necessary as the final exploration stage.

DHF-00

—96—

SOME INTERACTIONS BETWEEN GEOLOGICAL EXPLORATION DATA AND UNDERGROUND COAL MINE PLANNING 1 JO INTRODUCTION Mine planning progresses from broad brush to detail and is a screening process which aims to identify the most profitable development of a resource. That profit is the final measure of success is inevitable and essential. It is impossible to fund a non-profitable development; subsequent employment at the mine cannot be secure unless the venture shows a positive cash flow. In seeking the best profit, underground mine planning should maximise revenue, (tonnes x price), and minimise costs and it is perhaps pertinent to outline basic factors which influence the subsequent mining costs in Australian coal mines since these can form constraints to future development, and sometimes be inferred early during explorations Coal price is dependent on coal quality and market demand. Minehead price is dependent on transport and port costs. Distance from the port is important for export projects; closeness to the consumer is important for domestic sales. Product cost is dependent on the washery yields. Most mining costs arise in run of mine* terms and are inflated by low yields. 'Run of Mine mining costs have a high labour content. Productive units are essential; successful longwall costs are much lower than continuous miner costs and indeed no sizeable new underground mine is likely to be developed unless longwall can be applied. Successful longwall requires adequate seam thickness, - preferably more than 2m sufficient mineable reserves to cover the capital expenditure, and a geological structure which allows layout of long panels. Deep cover increases the cost and time needed to get access to the seams and usually requires higher revenue, (i.e. more tonnes per year, and/or higher price), to render a mine profitable. New projects compete for markets and for finance. Within this background, some problems which can arise from the initial geological data are outlined and some of the wild cards in the pack are mentioned. The very necessary geotechnical and coal analysis work often undertaken by the geologist is excluded only by the scope of the paper. DHF-01 T

—97—

2J) RESOURCES/RESERVES - SEAM ISOPACHS The estimate of resources/reserves arising from the exploration data forms the basis for all future mining assessment. The recoverable reserves must allow sufficient life to justify the capital required for a new mine. The seam isopachs must allow layout of highly productive mining systems. The Australian Code for Reporting Identified Coal Resources and Reserves, (February 1986) , clearly states that the points of observation should provide sufficient confidence to support the various levels of mine planning/resource classification identified and further gives limits for the maximum distance between these points for various categories. T

Experience suggests that these maximum limits should often be treated as exceptions. Examples encountered include, (worst first),r-

Outcropping Seams at 70° dip (overseas)

A hole drilled 1 km from outcrop might encounter the seam at 2700m depth - the overseas limit of 100m between points of observation still takes the seam from outcrop to 270m depth between boreholes giving 'measured status. Failure to convert the seam thickness for the dip allows a 1m seam to show an apparent thickness of 2.9m. 1

Lenticular Deposits

These can show considerable variation over short distances in some areas and gradual change in others. In the Greymouth Coalfield, N.Z., a lm seam near outcrop may be 7m thick 400m away.

Washout Areas (not expected)

In one area in Australia an apparently uniform seam was adjoined by an area containing old workings which were subsequently found to be subject to numerous washouts. Drilling a check borehole 10m from an original exploration grid hole showed a reduction in seam thickness from 1.9m to 0.8m in a seam previously shown as uniformly 1.9 - 2.2m thick.

Washout Areas/Seam Anomalies - expected

o Drilling of a 42 knr area showed a seam with uniform trends except for one anomaly arising from an old borehole. During subsequent search for potential washery reject sites the borehole was accidentally found some 1 km from its stated grid location. Replotting removed the anomaly. The search for borehole sites is not always restricted to old boreholes - other DHF-02

—98—

cases are known where new boreholes were inadvertently drilled on a neighbour's area. Seam Splits Seam splitting can cause a mixture of the problems given by lenticular deposits and washouts when trying to interpolate seam isopachs and from them calculate resources/reserves. Often a problem can occur trying to identify correlations - is it a split? or is it an adjacent seam. Seam Correlation Without distinctive characteristics allowing seam correlation, reserves may be miscalculated but, perhaps more importantly, the structure may be badly misinterpreted. Mining Recovery In some Australian conditions - flat, uniform trends, no washouts, (expected or not), no anomalies or splits, and no correlation queries - the resource can be mathematically assessed with accuracy. To convert it to a reserve a mining recovery must be assessed. Views on recovery vary from optimistic at the top end to unproven. Some basic factors are however often misjudged:1) Wongawilli Extraction (Open ended lifts) This gives the highest mining recovery if practised systematically and poor recovery, (from self induced reasons), if not. 2) Longwall Extraction This looses coal against natural boundaries that do not match a rectangular layout and looses the pillars between faces and the safety pillars left to protect the main/trunk headings. Recovery is generally less than for systematic Wongawilli extraction, (and better than a poorly practised Wongawilli system). 3) Bord and Pillar Splitting/Recovery This generally gives the lowest recovery but not always - it depends upon the conditions. 4) Thick Seam Recovery (i.e. > 4m seam thickness) This is frequently low. Even with sophisticated, but economic, systems the recovery from say 12m seams is rarely greater than 30%. 5) Surface Constraints Recovery will be limited under a number of surface features including flood plains, escarpments, railways, dams etc. It may be limited also by conditions arising under old opencuts, talus of unproven depth etc. A DHF-03

—99— barrier must be left against the lease boundaries, and must also be left against outcrop or opencut walls, 'Wild Card As an example, the careful drilling of a uniform 40m grid over a shallow seam, (overseas), may be quoted. Coal was proved consistently. Resources, and for the shallow depth, opencut reserves were calculated "accurately". Unfortunately subsequent opencut working disclosed previous underground working with pillars on 40m centres. 1

GEOLOGICAL STRUCTURE Having, "hopefully", gained sufficient confidence in recoverable reserves and seam thickness from analysis of geological data and having thus identified "target seams" which may support economic mining operations - (reserves sufficient to allow capital recovery, seam thick enough to give economic production rates and uniform enough to allow longwall layout) - it follows that the geological structure must be assessed. Major Faulting Often the major faults have been determined during the exploration but this has not always been so - the existence of a 30m fault between boreholes 1000m apart would only change the gradient by 1.7 degrees and this could be well be interpreted as a flexure rather than a fault. Problems met in the field include: Tahmopr The decision to sink this new deep mine within a 42 km area was made when only 20 boreholes had been completed. At that stage one major fault - the Nepean - was firmly identified and a second possible fault postulated. Review after the drilling of a further 23 boreholes was made by 5 different individuals and five different structures, each fitting all the known facts, resulted. The final exploration tool - the continuous miner - has since shown that the simplest interpretation using monocline folds rather than faults for most of the change in gradient has proved correct though it was not the interpretation accepted when the mine was sunk. 2

DHF-04

—100— Regional Studies in N e w Zealand A number of regional studies have been made in N e w Zealand in areas where the

coal

was

indicated 1

inferred 1

status.

Whilst

the

exploration

boreholes and surface mapping had located many major faults, the frequency shown in the more closely drilled areas was higher than in the less well drilled

sections

the

coalfields.

order

allow

realistic

assess-

ment of mining results and costs it proved necessary to construct simulated structures for parts of all three coalfields. spacing areas,

Guides to the possible fault

are given by structural interpretation

of the more closely drilled

by the frequency of faults found in boreholes, by review

lineaments,

and

of course by surface mapping.

boreholes

included

structure

plans cannot

defects

suggesting

be expected

of photo-

(In one region 32% of the

faults).

The

subsequent

simulated

to locate faults precisely, - this can

only be economically done during underground drivage; the plans can however allow the mining economics to be more realistically assessed. Ljjgpite ExplpratiQn Exploration originally Only

the

Mae

requested

Moh

Lignite

assuming

basin

reasonably

northern

consistent

Thailand

basin

of the boreholes which 'finger printed1

the geophysical logging

was

structure. each

main ply, allowed the extremely complicated structure given by the closely spaced

major

confidence

normal

and

all.

Though

reverse faults

to be mapped with

final definition

any level of

the structure will

require

extensive forward drilling the fore knowledge of the complexity means that mining

plans

Although

can

this

framed

particular

example

cope

with

the

is, in fact,

variations

opencut

finally

a similar

found.

situation

could well occur in underground resources.

Minor Faults The

determination

very

rare;

even

of a

underground

resources

the

often

surface

drilling,

mines

and from

the

very

at one

frequency (to

majority

measured

comparison

have been

pattern

Queensland small

by surface drilling the

relatively

mine leaves some

faulting

status),

and

must

not

shallow

uncertainties.

indicated

be deduced

of regional structures;

proven by drivage underground. simulations

of small faults

close drilling

all by

from

nearby

it is invariably finally

Many instances have been experienced where

necessary

to allow

realistic mine costing.

Using

practical classification of faults as:Pit Stoppers DHF-05

—101—

Panel Stoppers and Face Stoppers it has been necessary to simulate virtually all the expected face stoppers', perhaps some 'panel stoppers , and very occasionally some pit stoppers . The resultant structures whilst clearly not ultimately correct, nevertheless allow the effect of faulting on mining methods, production, and costs to be assessed. f

An interesting example was a correlation of fault frequency and direction between two mines sited on the Muswellbrook Anticline. When allowance was made for a change in the anticline direction between the mines a close correlation was found. The simulated underground structure showed frequent small faulting which significantly constrained the methods of work which could be applied. Again only drivage underground can finally confirm this. Folding Major folding is not common in Australian colliery holdings but has caused some problems; in particular mis-interpretation of the exact nature of the 'Muswellbrook anticline has caused subsequent opencut mining difficulties. 1

An extreme exists in the Greymouth Coalfield in New Zealand. Exploration of deep resources in this field, where mining has been carried on for over 100 years, is constrained by the difficult surface terrain requiring helicopter installation and servicing of drill rigs. Even if the grid could have been reduced to 400m spacing between boreholes, it could not ensure detection of the folds shown in existing mines in which a complete anticline and syncline, with gradients up to 30°, could be found in a 250m span. In this case structures from existing mines were used to construct three models of different complexity and these models were then applied, as regional environment suggested, to the multiplicity of fault blocks (56) created by frequent major faults (> 50m throw). The resulting mine assessments seek to show the effects of the extreme complexity existing in this coalfield. Dykes and Sills Whilst sills may be delineated by surface exploration the location of dykes can only be finally confirmed by underground drivage. Underground mines formed from the highwalls of opencuts will benefit from the exposures DHF-06

—102—

made in the cuts, others may be able to trace dykes on the surface, from vegetation changes or from geophysical surveys. Many however are only finally proven during underground drivage. A knowledge of the rock properties, the possible variation in thickness and nature, and the frequency of dykes or swarms is however necessary for any realistic mining assessment. Areas subject to dykes have been successfully mined by Bord and Pillar method for many years and more recently have been mined by longwalls. The production results are however obviously affected, particularly by bad roof conditions adjoining weathered dykes, or at the other extreme by the difficulty cutting very hard dyke material. 'Wild Card

Wild Cards' include shafts/drifts driven through major faults, a difficulty recorded more than 200 years ago and still reoccurring from time to time - it is highly desirable to ensure good drivage conditions for the formation of expensive stone drivages and also for the pit bottom layout. T

4.0 LITHOLOGY

The lithology and the existence of rock defects, (bedding planes, joints etc.) go to determine the rock strengths subsequently reviewed by geotechnical testing, and may also effect ground water, (again separately reviewed later). They may also indicate depositional structures and trends. Whilst the final assessment will normally be based on geotechnical testing the initial geological data input remains important particularly for early reviews and may also prove important for anisotropies arising from depositional structures. 'Wild Card

Lithology and sonic logs suggested good immediate roof conditions in one area of a prospect. Core photographs showed grossly laminated strata immediately above the seam (shown later not due to drilling techniques).

5.0 COAL QUALITY

The coal quality will finally be established by detailed laboratory analysis and perhaps by large cores, bulk samples and even washing plant tests. Early review is much assisted however by seam logging of the visible coal properties. More important perhaps is the assessment of sources and possible extent of dilution DHF-07

—103—

during mining. The nature of roof and floor and their partings with the seam can be very important as equally can be the existence of marker bands in the seam. A seam without marker bands and with a floor which grades gradually from coal to mudstone will suffer much increased dilution. Early identification of such a problem allows floor material to be included for subsequent laboratory analysis - a necessary but often neglected practice. 'Wild Card

A bulk sample of more than 2000t sent for plant washing tests showed all the characteristics of the H seam, (not planned to be worked), rather than the A seam in which workings have since been entirely sited as planned. Washery design and associated capital was inflated by the difference. f

CONCLUSIONS

The increases in the capital costs and productivity required of new underground coal mines has produced ever closer exploration grids over the years - the days of sinking a new mine on one good borehole are long gone. The primary needs are for well founded resource estimates for seams of adequate quality, thickness, and uniformity within a well defined structure and lithology. Whilst closer surface exploration than in the past is inevitable, some features can only be finally determined by continuous miner drivage. The aim should be to eliminate exploration of non viable areas early and to concentrate efforts on increasing the knowledge of apparently viable target seams. In doing this no available data should be neglected and certainly early collection of coal quality data is vital. The selection of target seams is also much helped by early broad brush mining cost estimates - estimates which can be progressively upgraded as exploration proceeds. Where the environment is judged to include features which cannot be adequately defined from surface exploration then trial mining may prove to be the final exploration stage. 'Wild Card

Few of the mistakes are new. We should try to avoid our grandfather's blues and yet hopefully inherit their instincts for 'good coal . T

DHF-08

—104—

THE ROLE OF COAL GEOLOGISTS in

OPEN CUT MINE PLANNING

PETER W. GOSCOMBE MANAGER - COAL EXPLORATION & EVALUATION CSR LIMITED

ABSTRACT

Open cut coal mines are capital rather than labour intensive. The magnitude of loans needed for their development places considerable pressure on owners to maintain operations, even if the return is below expectations. This demands that all aspects of such projects are thoroughly evaluated before committing funds to their development. Uncertainty surrounding the nature of the resource is probably the main contingency in the development proposals for a new mine and as a consequence, geologists have a major role to play in their formulation. Geological uncertainties are reduced as information on the resource accumulates. This leads to an improvement in the reliability of cost estimates and a corresponding reduction in the economic risk associated with mine development. The technical evaluation therefore tends to involve the appraisal of very large amounts of data. The geologist must ensure these data are analysed in such a manner as to minimise economic risk. This is being achieved by the application of specialised computer techniques and the use of geostatistics. The evaluation is carried out by a multidisciplinary team of professional mining people, which includes geotechnical and mining engineers, coal technologists and environmental planners. To be effective, the project geologist must have strong communications skills and understand the role and requirements of the other members of the team. In this respect, there are few jobs for geologists which offer comparable, mult idisciplinary teamwork opportunities, making it one of the attractions of coal geology.

—105— Introduction

The single most of development

important

factor to influence the preparation

proposals

for open cut coal mining

operations

is the very large capital investment needed to bring them into production. capital

The magnitude of the loans needed

investment

places

considerable

for this large

pressure

on owners

maintain operations and therefore the cash flow to service the loans.

This

expectations

the

because

case to

even

the

return

the

mine

may

below

effectively

bankrupt the company.

The

financial

problems

export market

caused

for coal

the current downturn

have highlighted

thoroughly

evaluating

committing

funds to their development.

mining

geological

all

and

economic

risk.

The

responsibility

aspects

risk

coal

provide

the

in the

importance

of such p r o j e c t s

The largest

project

before

component

development

geologist

therefore

has

the

planners

and

project

mine

vital

managers with clear and accurate information on the resource.

Project Management

The key management issues are:(1) (2)

The strategic impact of the project on the company, The commercial risks inherent in the development of the project,

(3)

The

technical

evaluation

the

project

and

the

reliability and accuracy of the cost estimates, and (4)

The financial evaluation of the proposed

Decisions

taken

interrelated

and

each

jointly

these

four

impact on whether

development.

key

issues

to commit

are

funds to

the development of the project.

PWG-02

—106—

Technical

The

Evaluation

technical

with

the

evaluation

successive

resource

improving

the project to

the accuracy

development.

whether

proceed

additions

review

the

progresses

the

stages on

the

the cost estimates

for

its

findings of each

further

information

with

the

stage

decides

evaluation;

market

opportunities decide when the further work is carried out.

The stages of the evaluation are typically:Exploration Review Conceptual Study Prefeasibility Study Feasibility Study Preproduction

Assuming

Planning.

for the purposes of this paper that the resource

is a

new discovery, a point is reached in the primary exploration of the deposit

when

justification assessed

level

Exploration postpone

further

Review

further

prospects,

consideration

for

must

economic

the

either

relinquish

pending

improvement

broader

technical

economic

decision

the

commence

the

Depending

risk,

may

work

be given

investigation.

the

project,

its

economic

evaluation

with

view eventually of developing a mine.

The

first

Study.

stage

This

the

broader

evaluation

the

Conceptual

involves:-

an assessment of the likely scale of production, the

development

of concepts

for removing

the

overburden

and mining the coal, a preliminary appraisal of the likely utilisation of the coal and its market opportunities, and the assembly of order-of-magnitude

capital and

operating

cost estimates.

PWG-0 3

—107— The cost estimates are analysed against forward price to determine return on

the

required

investment

selling

is likely

price

scenarios

for a satisfactory

to be competitive

in the market

place.

The

studies

stage

and

comprise

evaluation

mine

output,

with

the

investigations

the

geological

the

detailed

deposit

product

geological tests of

aimed

quality

and

the

expected

study

commences

price.

The

resource

coal.

the

Following

Study

engineering

at establishing

investigation

the product

the P r e f e a s i b i l i t y

and

the

market

development

of the geological model, the mining and geotechnical

engineers

prepare

engineers

mine

plan,

civil/mechanical/electrical

design the infrastructure and environmental planners assess the impact of the proposals on the physical and social A financial evaluation opportunities proceed

The

the

time,

a decision

is made

whether

further with the evaluation of the project.

Feasibility

Study

has been identified to

environment.

is carried out and, depending on market

reduce

carrying

the

out

is carried out when a market

for the coal. risk

further

opportunity

The aim of the study is

associated

with

studies

investigations

and

the

resource

the

utilisation

the

information

from

geology, geotechnology, and coal quality, to o p t i m i s e equipment,

the mine possibly

trial mining

plan

with

and

the

aid

operation,

to match the production schedule to expected

sales,

quality

and

including

pilot

conduct

detailed

studies

the

utilisation characteristics of the coal,

scale studies on bulk samples, and to market the planned output, to

assemble

capital and operating

accuracy

and

to evaluate

costs of a high

the financial

project against contractural/forecast

order

return of

the

prices,

PWG-04

—108—

to secure

all Government

consents,

including

acceptance

of the environmental impact of the development proposals, and to prepare a scope document, arrange finance, and obtain management approval to commit funds to the development of the project. The Preproduction

Stage

is part of

the project.

Commitment

involves still

further studies

the construction

phase

to the development of the project and

investigations

in order

provide accurate technical information for the final design and specification well

of equipment,

the detailed

facilities

design

and

infrastructure,

scheduling

the

and production

first few years of mining.

Role of the Geologist Irrespective of whether the resource is a new discovery or the subject of a tender, geologists play a primary role, together with mining engineers, in the Exploration Review. Because of their prior involvement with the project, geologists also have

introducing

important

role

to play

in the Conceptual

Study,

the resource to specialists from other disciplines

and, quite possibly, to the management of the company. The

detailed

Prefeasibility

studies

Study

require

and the

investigations

establishment

team.

The structure of the team will depend

nature

people

the project

qualified

utilisation,

but will geology,

environmental

of a

the

project

on the size and

include an appropriate mix of mining

assessment,

engineering,

infrastructure

coal

design,

financial evaluation and administrative support. As

the

evaluation

progresses

through

the

Prefeasibility,

Feasibility and Preproduction Stages and the size of the

PWG-0 5

—109— project

team expands,

hierachy

uncertainty the most

the

the position of the geologist

team

usually

surrounding

significant

because of

thisf

the nature of aspects

the

diminishes.

the resource

the entire

importance of

in the

However,

the

is one of

evaluation

and

the role of the geologist

does not diminish. From

the

required

start to

the

Conceptual

communicate

with

Study, ever

the geologist

increasing

technical people, either professionally qualified or

specialist

outside

consultants.

Effective

range

team members

communications

are vital if those charged with the responsibility of designing and

operating

the

mine

are

appreciation of the resource.

have

perfectly

clear

Good teamwork and communication

skills should therefore complement the geologist's professional and technical ability. offer

comparable

There are few jobs for geologists which

multidisciplinary

teamwork

opportunities,

making it one of the attractions of coal geology. The Project Team To communicate effectively it is necessary for the geologist to understand team.

the

The

engineer

role and

requirements of other members of

geotechnical

and

engineer,

environmental

coal

planner

requirements which must be recognised geologist

technologist, all

have

the

mining

different

and addressed

by the

in the course of mapping, drilling and sampling the

resource .

For

example,

geologist

the

geotechnical

engineer

would

expect

the

to make a start on the definition of the in situ

geotechnical

conditions

during

exploration

drilling.

The

occurrence of joints, faults and other planes of weakness which lower

the strength of the rock mass should

geologist during

core

logging.

Groundwater

be noted by the inflows should

always be measured and recorded as the pressure and volume of

PWG-06

—110—

groundwater difficult

can

result

saturated

to handle, highwall

spoil pile instability. geologist

from

lithological

geomechanically

which

are

floor heave and

Depositional models constructed by the

geophysical

control

sediments

instability, logs

for effective

suspect

materials

the

holes

provide

sampling

and testing

overburden

and

the

interburden strata.

The

work

the geologist all

and

the

coal

stages

the

technologist

closely

linked

quality

is such

important

parameter

project,

quality

profitability

consideration

planning

Selective the

sampling

distribut ion

correlatable

exploration

the

physical

subsections

advantageous choice of working likely

coal

in determining trends

are

drilling

the quality

and

chemical

the

major

programmes.

properties

to be assessed

and

the

sections to be made.

to be mixed

also sampled and analysed. matching

of coal core on a ply-by-ply basis enables

seam

floor material

is very

evaluation.

most

Roof and

with coal during mining

Where core holes are widely spaced,

of the coal to the depositional

model

enables the prediction of properties between data points.

The geology of the deposit is the most significant variable to be

considered

mine

planning

and

the

therefore depends heavily on the geologist formulate

the plan.

mining

engineer

for information

Differing geological conditions alter the

technical and economic feasibility of a given mining method and it is therefore and

the

overburden the

necessary

predictions

coal

thickness seams,

geometry

strata,

and

for

the

reliable. and

the

volume,

tonnage

and

information The

to be

accurate

information

includes

the number quality

and of

thickness

the

coal,

the deposit, structural discontinuities the distribution

and

lithology

of the

in the

of overburden

and

interburden materials.

PWG-07

—Ill— The

geologist

assists

the

environmental

planner

providing

drillhole samples for tests of:the

suitability

the

overburden

and

interburden

mine

use

and/or

meet

anti-

is also likely

to be

materials for revegetation, the

suitability

groundwater

for

discharge off-lease, and the

ability

pollution Because

co-opted

the

his field

and

the

role, the geologist

to m e a s u r e ,

aspects of

coal

ash

regulations.

sample

monitor

various

the background environment, at least in the

initial

stages of the monitoring

and

otherwise

programme.

Data Management

The

technical evaluation of coal resources and the preparation

of development proposals for open cut mining operations require the

appraisal

especially more

detailed

very

today

large

amounts

proponents

investigations

of new

counter

data.

This

projects

carry

out

the

added

technical

risks associated with deposits with less than ideal geology and coal

quality.

techniques

The

and

availability

greater

environmental

specialised regulation

evaluation of

open

cut

coal mining have also added considerably to the amounts of data requiring

appraisal.

retrieving

this

A computer

data

computer programmes

and

the

is essential

for storing

application

is now established practice

and

specialised

for just about

every aspect of the technical evaluation.

The

largest

component

of mining

development is geological risk. ensure

that all available

and

economic

risk

The geologist should

information

project

therefore

is collated and analysed

in such a manner as to minimise

this risk.

Where

adequate

data

techniques

should

used

quantify

the

variables,

eg. coal

seam

and

set,

statistical

reliability

the

overburden lithology thicknesses, key coal quality

there is an

parameters,

PWG-08

—112—

etc.

The increasing

confidence

limits

application of geostatistics to determine in

resource

and

reserve

estimation

removing the subjective bias of estimates and enabling the coal geologist

to avoid

unnecessary

risk

through

underdrilling

waste through overdrilling.

For

example,

information

uncertainties are reduced. of cost estimates associated

with

be nefits

uncertainty, economic

and

development

reliability

the economic

uncertainty

reduced.

information

ie.

expected

higher are

subject

accumulates,

the

geological

the

additional

risk,

information

is improved mine

accumulates,

As a consequence,

However,

reducing

profitability diminishing

reliability

the

economic and

lower

returns.

the

profitability

estimate improves, but at a decreasing rate.

Exploration

normally

carried

out

stages

with

each

successive stage becoming progressively more expensive and time consuming

as the grid dimensions of the drill hole layout are

reduced.

The costs of drilling, assaying and data analysis to

obtain

the additional

of the profitability the benefits

improve

initially

but at some point, marginal

marginal costs. lowers

information and estimate will

Further exploration

the expected

profitability

reliability

benefits will

beyond

and

the

be outweighed this point

increases

the

equal simply

economic

risk of the project.

Summary

Geologists have an important responsibility open cut coal mining operations. have

strong

technical skilled

teamwork

and

the

application

and

communications

professional handling computer

in the planning of

To be effective, they should

ability. large

skills

They

amounts

techniques

and

as well

should of

data the

also

the

use

geostatistics.

PWG-0 9

—113—

THE G E O L O G I C A L SOCIETY OF A U S T R A L I A COAL GEOLOGY GROUP GEOLOGY AND COAL MINING

CONFERENCE

"EXPLORATION G E O L O G Y AND GEOPHYSICS THEIR ROLE IN UNDERGROUND MINE PLANNING AT GERMAN CREEK M I N E , CENTRAL Q U E E N S L A N D "

AUTHORS:

KERRY J , WHITBY

(1)

GAVIN P . MAY

(2)

Senior Associate

McElroy

Bryan & Associates

(2)

Senior Project

Pty.

Ltd.

P.O. Box 34 Willoughby.

N.S.W.

Geologist

—114—

INTRODUCTION

German Creek Coal Mine, in the Bowen Basin of Central Queensland comprises a four dragline open cut operation and one longwall underground mine at Central Colliery (see Figure 1.1). A second underground mine in the south of the deposit will soon be established. Open cut operations commenced in July 1981, followed by underground production in January 1984. Exploration of the German Creek deposit was initiated with a major programme of drilling in April 1977. The management of Capricorn Coal Management Pty. Ltd. (Capcoal), operators of the project on behalf of their joint venture participants, have at all times demonstrated a commitment towards exploration techniques, which has enabled the complex nature of the geology and the ensuing mining operation to be effectively resolved. The technical success of the mining operation as a whole, and in particular Central Colliery longwall mine, is testament to the role played by geological and geophysical exploration in providing Capcoal engineers with reliable and accurate data on which to base mine planning.

EXPLORATION TECHNIQUES

From the beginning, it was clear that the German Creek mining operation would initially be open cut oriented with the subsequent introduction of underground operations, to supplement at first, and eventually take over from, open cut production. While much exploration was directed specifically at the open cut or underground aspects of the development, the overall objective has been the evolution of a comprehensive assessment of the sedimentological and structural features of the entire coal deposit and its relationship within the regional framework. The general philosophy applied to all stages of exploration at German Creek has been to obtain data of the highest quality, particularly in relation to drilling. The significance of many results obtained during exploration is KW-GM-01

—115— often not fully appreciated at the time, and it was considered essential that all data be recorded in such detail that, allowing for unforeseen changes of project emphasis in the future, the data collected be acceptable for a variety of uses. The following geological and geophysical exploration techniques have been applied at German Creek over the past 10 years. Geological Drilling Open hole Fully cored diamond-vertical and inclined Large diameter Regional geological mapping Highwall and pit mapping

Geophysical Down hole geophysical logging High resolution magnetics High resolution surface seismic In seam seismic Ground resistivity

The relative merits of these techniques for locating geological hazards to mining and their application to the determination of geotechnical aspects relating to the prediction and monitoring of overburden characteristics is addressed (see Figure 2.1).

G E O L O G I C A L AND STRUCTURAL MODEL

The complex interaction of igneous features (dykes, sills and plugs) with numerous normal, reverse, strike-slip and KW-GM-02

—116—

bedding plane faults at German Creek (see Figure 3.1) has necessitated the development of a structural and sedimentological model to assist in the planning of projected underground mining operations with a view to: (i)

increasing the accuracy of predictions related to ground conditions and the location of geological hazards such as dykes.

(ii)

determine target areas for detailed investigations.

The model is constantly under review, as additional data becomes available, and establishes a geological base from which further exploration targets are identified (see Figure 3.2).

KW-GM

G E R M A N CREEK O P E N CUT M I N E D OUT AREAS. TOWN OF MIDDLEMOUNT

AtoP 315C

AREAS SCHEDULED FOR M I N I N G ,

G E R M A N CREEK LEASE M L . 1306

MIDDLEMOUNT A l o P 315C

BLOCK

—117—

TRALEE

G E R M A N CREEK EAST A to P 4 M C

TRALEE SLOT

PARROT CREEK

-SURFACE LEASE BOUNDARY

BLOCK

bAK PARK BLOCK

SOUTHERN COLLIERY RESERVES

KILOMETRES

LOCALITY MAP GERMAN CREEK MINE FIGURE I I

—118—

Figure 2 1

—119—

CAPRICORN COAL MANAGEMENT P T Y

LTD

SOUTHERN COLLIERY MINE AREA

STRUCTURAL FEATURES/ Southern Colliery

kilometres

•—

Dyke, position approximate

•

Dyke, position inferred

: : ' ! ' / . : . ' . Intruded and heat affected Aquila Seam /£ —— lllllltlll

Woshout, German Creek Seam thickness < 1 • 7m

Intruded and heat affected Tieri Seam //

—JOO-- Overburden Isopoch, German Creek Seam U 0 Uo

y •

Normal fault T

Reverse fault Strike slip foult

Prepared by: McElroy Bryan & Associates Pty Ltd Figure 3-1

—120—

g "O . o

TECTONIC STYLE REGIONAL

LOCAL

Basement Rifting

COAL

MEASURE

SEDIMENTATION

PERMIAN - TRIASSIC

Igneous Boundary

Fault

@D1

A Sill Injection

v 02

Normal Faults Partial Reactivation of Rift

Reverse Faults

Total Reactivation of Rift Zones

CZD 12 045°

Dyke Emplacement Controlled by Basement Fracturing

<v"

controlled by Joints

Stabilisation

Dyke emplacement

315°

Riftlm

ES3 070°

Regional Compression

D4 E3

Regional Relaxation

Prepared by: McElroy Bryan & Associates Pty Ltd

SEQUENCE OF TECTONIC EVENTS

Figure 3 2

—121—

PREDICTION OF UNDERGROUND CONDITIONS USING OPEN CUT DATA BY C.W. Mallett & R.L. Hammond

CSIRO Division of Geomechanics Private Bag 3, INDOOROOPILLY. QLD. 4068

CM-RH-1

—122—

Underground Mining Needs

Data

from

surface

underground

pits

planning

can

and

important

design

for

a variety

requirements.

In the

pre-

development phase the feasibility of specific mining techniques need to be established

before major

equipment

commitments

can

be made. New underground developments are now concentrating on longwall

mining

economical

systems,

production

which

have

much

rates, but d e m a n d

higher

very

and

strict

ground

c o n d i t i o n s over the w h o l e area of a panel. The overall

mine

layout

with

panels

to be extracted

must

intergrated

service, access and transport systems. The layout is dependent on local geological

conditions,

which

are

usually

interpreted

from widely spaced b o r e h o l e s and any a d j a c e n t exposures. The area covered amounts to tens of square kilometres, and requires detailed

knowledge of location of structures with half

to one

seam thickness displacements throughout the area.

During

development

and ongoing

mining

forecasts p o s s i b l e of c o n d i t i o n s

operations,

required. Geological features such as sedimentary washouts, intrusions and faults, mining

and their

unexpected

the

best

in advance of w o r k i n g s are channels and

can be of major importance to

intersection

seriously

disrupt

operations. They are h o w e v e r , often surrounded by g e o l o g i c a l features

that

give

encountered.

warning

Exposures

of the nature

underground

are

changes

severely

limited,

usually to within the coal seam. It is difficult to develop an understanding

of the interconnection bewteen the components of

a g e o l o g i c a l feature in this e n v i r o n m e n t and larger exposures are

Many

features

surface

required.

that

are

important

for

strata

control

are

confined to i n t e r s e a m s and only affect the coal in a minor way. These features in highwalls can be completely exposed and their effects within the coal seam documented,

and used to

interpret

d e f o r m a t i o n m e c h a n i s m s in roof and floor strata underground. Zones of high gas and water

inflow are localised by geological

CM-RH-2

—123— d i f f i c u l t i e s of this nature at the surface, their nature and distribution ated

patterns

can be established

there f

and

extrapol-

underground.

Building Predictive Geological Models

Sedimentary models

The scale of Australian surface strip mines usually exceeds the dimensions of sedimentary depositional systems within the coal m e a s u r e s . It is t h e r f o r e p o s s i b l e to see the edges of d e p o s i tional units and determine their gross dimensions. Strip mines are linear features, so the section obtained is e s s e n t i a l l y a single slice through the sediment pile.

Directional

f e a t u r e s can be d e t e r m i n e d

outcrops. These are obtained

from

readily

the overall

highwall

orientation

and

s u p e r p o s i t i o n of beds, and internal f e a t u r e s such as current bedding.

Depositional processes and the hydraulics of sediment transport can

obtained

from

the

lithologies

and

textures

the

sediments. Lithologies reflect both the sediment source and the energy of the d e p o s i t i o n a l zone. The a s s o c i a t i o n s of beds can be built up to d e t e r m i n e a n u m b e r of c h a r a c t e r i s t i c facies of deposition for an stratigraphic

unit or geographic area.

are related back to p h y s i o g r a p h i c

These

m o d e l s such as a channel,

channel mouth bar, levee, splay etc, which in turn indicate the relative position of standing water levels or sea levels.

initerative

comparative

confidently

build

depositional

environment,

and

distribution

widely

scattered

association

process,

a picture and

varies

The

the

sedimentary

of l i t h o l o g i e s data.

is p o s s i b l e

units.

and

The

of is

a a

local

geometry

can then be p r e d i c t e d

continuity

dramatically

regional

from

lithological major

deltaic

channel sand, a feeder for a splay, delta m o u t h bar or a tidal channel sand. On a local scale the l i t h o l o g i e s could be very

CM-RH-3

—124— similar, and it is only by knowing what depositional

system

that you are dealing with, that successful correlation between boreholes can be made. Rock mass properties One

the

most

important

aspects

of m i n e p l a n n i n g

and

development, and of the safe operation of a mine, is defining the characteristics operating.

of the rock mass

The performance

in which the mine

of the rock

mass

is in

general

directly related to its structure, and to a lesser extent the material properties of the different constituent rock types. Structures in a rock mass might include: 1. boundaries between different rock types 2. joints & cleats 3. faults 4. folds and flexures 5. cleavage Under normal circumstances these

structural

it should be possible to integrate

f e a t u r e s into

a mine

scale

geological

conceptual model. Ideally this involves an understanding of how the various structures in the mine area developed and how they are

inter-related.

By necessity, the development of such a model involves more than a passing understanding of the geology of the surrounding region. It is simply not possible to develop adequate models of mine geology without referring to parameters like; regional deformation histories, or the distribution of major regional structures,

or indeed the general character of the

terrane

(e.g. a thrust belt). In many cases the developers of a proposed underground will have access to an abundance of highwall exposure provides

exact

information

about;

joint

density,

mine that joint

orientations, joint set inter-relationships, fault style, fault

CM-RH-4

—125— p a t t e r n s , u p - s e c t i o n or lateral continuity of faults, and so into

areas

where there is no exposure, provided there exists a mine

on.

This

information

can be used

to e x t r a p o l a t e

scale

conceptual model that offers a basis for careful An additional,

and equally

models

way

which

about

the

3-dimensional

the

predictions

extrapolation.

significant attribute of they

allow

conceptual

geologists

character

make

fault

system, and hence the mining performance of the immediate rock mass.

particular

associations

importance

between

different

the

establishment

structures

that

allow

of the

p r e d i c t i o n , say, the o c c u r r e n c e of a fault b e c a u s e a roll in the floor is encountered, or perhaps because a systematic swing in cleat trends is noted. Associations can only be

established

with careful observation of geology wherever it is exposed.

Application of Mine Scale Modelling

Detailed interpretations can be made on highwall exposures and there are many examples where mines have done excellent mapping and

interpretations

for

strip

mined

areas.

Very

few

have

persisted long enough to develop a three dimensional picture of d e p o s i t i o n a l s y s t e m s from s u c c e s s i v e cuts, a l t h o u g h this is quite

feasible.

The further you go from the h i g h w a l l h o w e v e r , the sparser

the

information

and

less

reliable

the

prediction

l i t h o l o g i c a l d i s t r i b u t i o n s and faults. The p r o c e d u r e in m i n e scale modelling is to

1. Establish the best facies distribution and structural

model

possible based on all evidence from

lithologies,

bed

etc

extend

incorporate

from

borehole

highwall

exposures,

and

it to

geometry

data.

Determine

the

relationship

between

structure

and

lithological distributions. This includes not only the controls

CM-RH-5

—126—

on l o c a l i s a t i o n also

the

joint

type and

of structure always exerted by l i t h o l o g i e s ,

internal

structures

characteristic

but

b e d s such

spacing.

3, Use i n d i c a t o r s of f e a t u r e s to p r e d i c t t h e i r l o c a t i o n

where

associations

slips

often

can be expected.

terminate

in v e r t i c a l

For example, reverse

faults

bedding which

d i s r u p t one s e a m , but d i e out to a s m a l l r o l l seam,

or even l i t h o l o g i c a l

usually

change l a t e r a l l y

transitions

into channels

plane

significantly

in an

overlying

such as l e v e e s

which might

lie

which

between

boreholes.

is t h i s

i n t u i t i v e geology that makes it d i f f i c u l t

remove the g e o l o g i s t from the process and depend on analyses

of a

as y e t ,

statistical

database.

The D i v i s i o n

of Geomechanics

sites

surface

where

underground p l a n n i n g .

has been

geology

has

involved

been

with

applied

These i n c l u d e Moura, South

many to

mine

assist

Blackwater,

German C r e e k , G o o n y e l l a and Oaky C r e e k , w h i c h c o v e r b o t h bord and p i l l a r and l o n g w a l l mining.

Examples from these s i t e s

predictive

presented

illustrate

g e o l o g i c a l modelling a p p l i e d to mining

capabilities

practice.

CM-RH-6

will of

—127—

SITE CHARACTERISATION AND FORECASTING

UNDERGROUND MINING

Ian J .

CONDITIONS

Stone

BP Coal A u s t r a l i a ,

Picton

ABSTRACT

S i t e s p e c i f i c mining Coalfield,

conditions

NSW have been studied

and forecasting mining

stressfield

The range of roof

of the s t r e s s f i e l d

components.

for each area of the mine to define conditions,

in the Southern

to provide a framework for

conditions.

were caused by variations ratio of the horizontal

at Tahmoor Colliery

correlating

conditions

orientation,

Roof F a i l u r e Curves were the r e l a t i o n s h i p between

o r i e n t a t i o n and r e l a t i v e

noted

magnitude

stressfield

and

constructed roof magnitude.

Forecasts of roof conditons were made by matching areas with similar Roof F a i l u r e Curves and projecting rates and roof conditions.

trends

support requirements

into unmined a r e a s .

Development

can be associated with forecast

roof

-128-

INTRODUCTION

High horizontal stressfields are one of many geological contributors to difficult roof conditions in underground coal mines.

This paper

outlines an approach to defining the in situ stressfield and its effect on mining conditions in Tahmoor Colliery in the Southern Coalfield of NSW.

Initial roof investigations suggested that the variations of

stressfield orientation and stressfield intensity caused the observed range of roof conditions (1).

Subsequent work involved characterisation

of the stressfield and roof conditions at specific sites, and correlating between sites to develop trends to forecast the stressfield and roof conditions in future workings. Studies which focus at specific sites, for example, monitoring roof strata - roof support interaction, must be aware of limitations on results from one site when transferred to other areas of the mine. Geological characterisation of sites will provide the necessary framework for transferring site specific data.

Tahmoor mine presents an example of

the interaction of a variable stressfield and mine roof conditions (2). The characterisation of the mining conditions in an area has two main aspects: 1.

determination of the stressfield orientation, and

determination of the roof conditions typical of that area.

Methods required to gather, interpret and use this data for predicting expected roof conditions are outlined below.

STRESSFIELD ORIENTATION. The orientation of the apparent principal horizontal stress is determined from the pattern of mining induced shear fractures and tensile fractures in the immediate roof strata at the mining face.

This stress

direction generally reflects the in-situ virgin stress field but in some areas it is modified by mine development.

Variation of the apparent

lateral stress direction in Tahmoor can be either gradual or very rapid (Fig.l), for example, in one area the dominant stress direction changes some 90° within 100m.

The stress direction measured from fracture

patterns (Fig.l) are comparable to those established by in situ stress measurements which indicated the maximum horizontal stress component between 18.0 and 21.3 Mpa (3).

IS-01

—129— Three factors influence the roof fracturing: 1.

Roof fracturing is more clearly developed in the finely interbedded strata, as exists at Tahmoor,

than in massive

sedimentary rocks. 2.

A simpler roof fracture pattern occurs if there is one dominant horizontal stress direction.

Two principal horizontal

stress directions of almost equivalent magnitude, have a more complex roof fracture pattern.

Both situations exist at

Tahmoor. 3.

The intensity of roof fracture patterns varies in roadways dependent on the order of drivage in that mining cycle.

Table 1. lists the features that can be used to determine the apparent principal horizontal stress direction, for a dominant lateral stress direction and for lateral stress of nearly equivalent strength.

CHARACTERISATION OF ROOF CONDITIONS Short Term Roof Conditions The roof conditions at Tahmoor can be distinguished as short term and long term roof failure types (Table 2).

Short term failure

essentially occurs at the mining face, whereas long term failure develops after the placement of roof supports. Short term roof conditions are used to characterise the type of roof failure in an area because they are most easily compared to the stressfield intensity, and least affected by mining practices or methods. Likely long term roof conditions can be determined by the short term roof fractures, and some relationships have been reported (1).

In Tahmoor the short term roof conditions are affected by: 1.

The horizontal stressfield orientation,

The angle between the principal horizontal stress direction and

The stressfield magnitude, and

relative strength of both in situ horizontal stress components.

the mine roadway (9sr),

Each of these four factors vary in different areas of Tahmoor and produce a different set of roof conditions which may have different roof support requirements.

Short term roof conditions vary with Gsr, being

worst when 0sr = 90°, but also vary with changes of stressfield intensity.

IS-02

TABLE 1 DETERMINING PRINCIPAL HORIZONTAL STRESS DIRECTION FROM ROOF FRACTURES.

Relative condition of adjacent headings and cut-throughs

DOMINANT LATERAL STRESS

LATERAL STRESS MAGNITUDES NEARLY EQUIVALENT

One roadway in significantly worse condition

Similar condition in both directions

(gives quandrant of str

s direction)

Short term roof fractures, (refer to Table 2) (a)

Orientation of low angle conjugate shears relative to roadway direction

Oblique shears mostly oriented normal to horizontal stress directions. At higher stress magnitudes, or at certain Osr no oblique shear present.

Very few oblique shears, Normally parallel shear.

u> o

(gives stress direction)

(b)

Mining induced tensional fracture (m.i.f. ! s).

Where present preferential location to one side of roadway.

Where present occur both sides of roadway.

(gives quadrant of stress direction)

(c)

Location of short term term shear failure

Preferential location of shear to one side of the roadway.

No preferential locationcommon in centre of roadway.

(gives quadrant of stress direction)

IS-03

—131— TABLE 2. ROOF FAILURE TYPES Short Term Arch Severe parallel shear Parallel shear Parallel and oblique shear Oblique shear Good

Long Term Roof Fall Cantilever Gutter Sag Good

NOTES: 1. Shear refers to shear failure of immediate roof strata. 2. Oblique shear trends across the roadway and parallel shear runs along the roadway. 3. Mining induced fractures may occur with the short term failures types.

Fig.l. Orientation of the dominant lateral stress. In situ measurements indicated by bold lines.

IS-04

-132-

Stressfield Magnitude The stressfield magnitude can only be determined from direct measurement. At Tahmoor twelve types of short term roof failures are recognised and ranked by increasing severity of failure (Table 3). The relative intensity of short term roof failure has been used as a guide to the relative stress magnitude in different mining areas.

TABLE 3 SHORT TERM ROOF CONDITION SCALE AND SUPPORT REQUIREMENTS (Decreasing Severity) SHORT TERM ROOF CONDITIONS

SUPPORT REQUIREMENT (Bolts per W-strap)

Scale Failure Type Number 12 - Arch 8 11 11 - Severe parallel shear, mif* across roadway 11 10 - Severe parallel shear, mif 1 side 11 9 - Parallel shear, mif across roadway 8 - Parallel shear, mif 7 1 side 11 7 - Severe parallel shear 11 6 - Oblique and parallel shear, mif 11 5 - Parallel shear 4 - Oblique shear, mif 6 or 7 11 3 - Oblique, parallel shear 2 - Oblique shear 6 1 - No shear * mif - mining induced fractures (tensional)

Method of Comparing Roof Conditions The scale of relative stress magnitude, based on short term roof conditions, must be used in conjunction with 0sr because roof conditions vary as 0sr changes from 0° to 90°.

On a graph of short term roof

conditions versus 0sr, a curve can be drawn which will indicate the short term roof conditions expected to occur as the angle between the principal horizontal stress and the roadway changes (e.g. curve 2, Fig.2 ).

This

curve is referred to as a Roof Failure Curve (RFC) and is used to define the range of roof conditions possible in a specific area of mine development.

An area of the mine which is represented by one RFC has

consistent stressfield magnitude parameters.

IS-05

—133— Curve 2 in Fig.2 shows a number of characteristics: 1.

A distinct change of roof conditions either side of 0sr = 45°, reflects the different roof conditions commonly noticed between headings and cut-throughs.

The relative intensity of roof failure, and relative intensity of the stressfield, is easily represented for a range of mining directions.

A guide to the relative intensity of the two principal horizontal stress components is given by comparison of failure intensity greater and less than 45°.

COMPARISON OF ROOF CONDITIONS IN TAHMOOR Five different RFC's have been identified to describe the range of roof and stressfield conditions noted in Tahmoor development panels (Fig.2), The RFC ! s, curve 2 apart, are defined only for a limited range of 0sr (25° to 65°), because no mining has taken place at the extreme values of 0° and 90°.

Curve 2 shows the rapid deterioration of roof

conditions as 0sr approaches 90°.

In Fig.2

curve 1 represents the best

roof conditions and curve 5 the worst roof conditions.

The areas of mine

development in Tahmoor which are represented by RFC's 1 to 5 are placed on the mine plan to highlight trends, changes or patterns of roof conditions.

Roof conditions in adjacent planned workings can be

projected from such a mine plan.

PREDICTION OF ROOF CONDITIONS AND ROOF SUPPORT Prediction of Roof Conditions Mining conditions within future development workings are predicted by projection from adjacent workings.

Predicted roof conditions will be

best characterised by assigning an RFC to each planned development area. The number of different RFC's required to characterise one or more development panels will be determined from adjacent workings. To determine the expected roof conditions in headings and cut-throughs of a new area, the following is needed: 1.

expected sigma 1 trend in the area

orientation of headings and cut-throughs

determine Gsr (from (1) and (2))

use the roof failure curve typical of the area to determine the expected roof conditions for the Osr values of headings and cut-throughs.

- 134 Roof Support Options Essentially three options of support are used - 6, 7 or 8 bolts per W-strap.

Spacing between W-straps is reduced as conditions deteriorate.

Experience at Tahmoor has enabled roof support requirements, or bolt density, to be related to the intensity of short term roof conditions (Table 3).

Bolt density may differ in headings and cut-throughs

dependent on short term roof conditions (Fig.2).

r f

UJ -J

r CD

o H H

a z o

cn 73

U) H X) >

Li.

o o

i 30

l— 60

ANGLE OF STRESS TO ROADWAY (6sr) (degrees) FIG 2. Range of Roof Failure Curves (RFC 1 s) at Tahmoor.

SUMMARY OF PREDICTION OF ROOF CONDITIONS IN TAHMOOR Predicted roof conditions are determined for each new area based on conditions in adjacent workings, for which an RFC has been determined. The RFC for each new panel allows the expected roof conditions, and roof support requirements for headings and cut-throughs to be determined for appropriate 0sr values.

The production rate for the likely roof

conditions can be estimated. At Tahmoor roof conditions, and production rates are assigned to the twelve month development plan.

The roof conditions of some panels within

the development plan are expected to change during mining and therefore have a number of RFC's each with a different development advance rate.

IS-07

—135— Acknowledgement Acknowledgement is made to BP Coal Australia for permission to present this paper.

REFERENCES

Gale, W.J,,

Rawlings, C.D., Cook, C., Stone, I.J., Rixon, L.K.,

Enever, J.R., Walton, R.J., Litterbach, N., 1984. An investigation of the stress field and its effect upon mining conditions at Tahmoor Colliery, CSIRO Aust.,

N.S.W., Australia.

Div. Geomech., Geomech. of Coal Mining Rep. No.49,

72pp. 2.

NicholIs, B. and Stone, I., 1986. Development of Strata Control at Tahmoor Mine. The AusIMM Illawarra Branch, Ground Movement and Control related to Coal Mining Symposium, August, Wollongong, p. 109 - 117.

Walton, R.J., 1983.

A study of the behaviour of a bolted coal mine

roof subject to medium/high horizontal stress. Part 1: 2.1 metre long resin anchored bolts. CSIRO Aust., Div. Geomech., Geomech. of Coal Mining Rep. No. 46, 64 pp.

—136—

USE OF GEOPHYSICAL LOGS AS INPUT FOR MINE LAYOUT MODELLING

G.H. McNally Australian Coal Industry Research Laboratory Ltd-, (ACIRL) North Ryde, NSW

—137— 1.

INTRODUCTION

An investigation into geotechnlcal interpretation of downhole

geophysical

logs, and in particular the correlation of uniaxial compressive

strength

(UCS) with sonic log response, has recently been completed by ACtRL.

The

motivation for this project was the need for more cost-effective and realistic rock property input data for numerical and physical models of underground coal mining layouts.

Funding was provided by the Australian

Coal Association, and a more comprehensive account of the results obtained may be found in the End-of-Grant report to ACA (McNally,

1987).

Correlation of geomechanical parameters with sonic and neutron response has reached the stage where downhole logs are now used on a routine basis, along with conventional test results and experience, for specifying model input.

The numerical models are of three main types, in descending scale order,

Vertical sections through lougwall panels, to study caving

behaviour,

stress redistribution around mining voids, interaction between seam workings, and

subsidence;

Plan views of pillar and panel layouts, to select optimum face and pillar widths, estimate depths of yielding, and assess the need for rib support; and

Roadway cross sections, to evaluate optimum dimensions and roof support requirements.

Geophysical logs are principally used for defining strata units (Model Stratigraphy), and in estimating the bulk strength and stiffness of those units.

Geomechanical test results from short cored sections can in this

way be extrapolated

far above and below the mining excavation, the validity

of individual test values can be compared against a continuous record of overburden rock properties, and most importantly

the bulk or

rock mass 1

strength of layers several metres thick can be estimated from UCS specimens 0.1 to 0.2 m long.

GMcN-01

—138— During the course of the investigation approximately

1500 test results

(1000 UCS, 400 static elastic moduli, 100 tensile strengths) from 40 drillholes at eighteen sites were compared with sonic log response, and a lesser number with neutron log response.

The test sites were all at

operating or proposed coal mines, and covered a wide variety of sedimentary rocks, with strengths up to about 200 MPa.

Other geophysical logs

(gamma,

density, resistivity) were examined, but found to be less satisfactory for quantitative interpretation.

The principal findings of the study were

that,

Sonic log interval transit time (reciprocal velocity) provides a reliable, continuous record of rock strength in coal measures strata, and a relationship between UCS and transit time can be established by exponential regression analysis;

Similar empirical relations between UCS and neutron log response can be established, but they are more site specific.

There is also a marked

difference between the apparent neutron porosity of quartz-rich and clay-rich rocks of the same strength;

Static elastic modulus can be correlated with both sonic and neutron response, but the sonic correlation is again the more reliable, and in neither case is the correlation as good as that with UCS; and

indirect tensile strength can be estimated from sonic transit time, but the correlation is poor.

STRATA UNIT DEFINITION

Where a number of drillholes have been geophysically logged in the vicinity of the mine areas under investigation, the log suites obtained can provide an objective basis for sub-dividing overburden sequences into strata modelling units.

In this respect neutron logs, sometimes supplemented by

gamma, are more suitable than sonic, since layers can more easily be correlated

from drillhole to drillhole.

GMcN-02

—139—

Neutron logs are also better for highlighting strength differences within thick, massive sandstone and conglomerate units, such as the Great Northern Seam roof on the central coast of NSW, or in the Maules Creek Formation north of Gunnedah.

Without the log response, sub-division of these roof

sequences - necessary

for the model to adequately simulate caving - would

have to be arbitrary, due to the absence of obvious bedding planes. log response in these conglomerates

Sonic

is very uniform, regardless of

strength, due to the influence of the 60-80% of chert and volcanic pebbles generally present. finer-grained

These also import a higher sonic velocity

than that for

rocks of equal strength.

Where only one or two logged holes are available in the area of interest the normal situation - sonic logs plus core photographs are used for defining tops and bases of strata units. 1

blocking 1

One way of doing this is by

the log into a stepped profile made up of layers of more or less

constant sonic velocity

(actually constant sonic interval transit time, or

reciprocal velocity, since this Is the parameter presented on most logs, in units of microseconds per foot).

The thickness of each strata unit is

dictated by the geological variability, and by the modelling detail required - since layers close to the mine excavation are much more influenced by stress redistribution than those far above or below it.

STRENGTH ESTLMATION USING SONIC LOGS

The relationship between UCS and sonic log interval transit time (t) measured in microseconds per foot, the usual logging unit, was found to be exponential, and to take the approximate general form,

UCS

1000 e"" 0 , 0 3 3

MPa

Correlation curves from most test sites were fairly linear above 90 MPa, and the relationship is probably not valid above 120 MPa (where small increases in transit time correspond to large strength increments).

typical sonic/UCS correlation curve is illustrated by Figure 1.

GMcN-03

—140— For weaker rock, less than about 60 MPa, this reiatLonshLp

UCS

A00 e ~ 0 - 0 2 6

becomes,

MPa

Because of the steeper slope of the correlation curve (Figure 2) in this range, the sonic log presentation in terms of reciprocal velocity is actually more sensitive to variations in rock strength below 60 MPa than above it. would

Conversely, a log format in terms of metres per second

tend to emphasize high velocity

velocity

(hence high strength) layers.

For coal, the relationship between sonic transit time is once again exponential, and was found to be approximately,

UCS

250 000 e " 0 , 0 7 6

for 45 samples between 4 and 60 MPa.

UCS

MPa

In linear form this corresponds

150 - 1.04 t

to,

MPa

The exponential relationship flattens out above 30 MPa, indicating that stronger coals will have longer transit times (hence lower velocities) than rock of equal strength.

For coal in the most common strength range, 10-30

MPa, the linear form of the sonic/UCS relationship is sufficiently

accurate

for practical purposes.

Indirect tensile strength (ITS) did not correlate well with sonic transit time, but a composite curve based on 88 tests at five sites was of the form,

ITS

40 e ~ 0 - 0 2 8

MPa

The poor fit of this correlation curve is thought to be more due to the unreliability of Brazilian test results (relative to UCS), than to the inapplicability of sonic response to the estimation of tensile strength. For coal measures strata UCS is generally

10 to 20 times ITS, and fifteen

GMcN-04

—141— times is reasonable for estimating purposes; this ratio (UCS/iTS) is also known as the Brittleness Index, and may be a useful indicator of rock cuttability and cavability.

MODULUS ESTIMATION

Static modulus of elasticity correlated well with sonic log transit time (and also with neutron response in less stiff samples, below about 18 GPa). sonic/modulus relationship was found to be more site-specific sonic/UCS, highlighting the lithology-dependent also known as the Modulus Ratio*

The

than

nature of the ratio E/UCS,

In coal measures rocks modulus is usually

about 200 times UCS, and the multiplier is about 150 times UCS for coal. Modulus ratios greater than 500 indicate very brittle rock, while less than 100 is characteristic of stress-relieved or highly weathered

samples.

A composite sonic/modulus curve (Figure 3) based on 114 data pairs from three sites was of the form,

330 e - 0 ' 0 4 2

GPa

An alternative method of estimating modulus is by means of various empirical relationships between E, sonic velocity and formation density. These are much used in petroleum exploration, particularly where downhole logs can record both P and S wave velocity which only Vp was recorded).

(unlike the present study, in

An empirical relationship developed during

this investigation was of the form,

0.044 (p V p 2 ) 1 , 5 7

Where E is static modulus of elasticity

GPa

(not dynamic), p is rock density

(gm/cc or t/m ) and Vp is in kilometers per second (data from 6 boreholes at 2 sites, see Figure 4).

GMcN-05

—142— 5.

ROCK MASS STRENGTH ESTIMATION

Rock mass strength, as opposed to intact specimen strength, is the notional strength of a .joint bounded b l o c k , or of an element within a numerical model m e s h .

The minimum dimension of the element is 10 to

200 times that of the laboratory specimen, and the element is assumed to be weaker in proportion to the degree of fracturing

present.

The rock mass (in-situ) strength for a model element is estimated in a number of steps.

F i r s t , the mean transit time for a particular layer

is estimated from the sonic or simply by choosing a 'best fit T vertical line between highs and lows on the trace.

The average UCS

for that layer is then obtained from the transit time intersection with the sonic/UCS curve.

This is then considered as a single large

test specimen, using a scaling factor based on the following formula (modified from Hoek and Brown, 1981),

Strength Scaling Factor = (d.) 0 ' 1 8

where d and D are the UCS specimen diameter and the minimum element dimension respectively.

CONCLUSION

Geophysical logs, particularly multi-channel sonic, are an essential part of geotechnical investigations for underground coal mining. Uniaxial compressive strength, static modulus of elasticity and rock mass strength can all be estimated from sonic data and used for mine layout modelling input.

Neutron l o g s , while less useful for strength

and modulus estimation, can be helpful in defining model stratigraphy.

GMcN-06

—143— 7•

REFERENCES

HOEK, E. and BROWN, E.T. (1981) Underground Excavations in Rock. Institution of Mining and Metallurgy, London.

McNALLY, G.H. Geotechnical Applications and Interpretation of Downhole Geophysical Logs. End of Grant Report for ACIRL Project A21 to Australian Coal Association, July 1987.

J4,GeoLogs

GMcN-07

— 1 4 4 —

HOGERMAN CREEK FORMATION •

130-

GC6C.66

• GC 5 2 , 7 2 o G SO 1, 2

-120-

oo , 110I O OO o

£< 100 a:

UCS = H 5 0 e " Q

0 3 8 1

( n = 17 5 r » - 0 9 C I M P a

80 -

O2 70 -

6010

^O UNIAXIAL

—i— 100

110

C O M P R E S S I V E STRENGTH ( M P o I

120

130

Figure 1. Uniaxial Compressive Strength (UCS) versus Sonic Interval Time, German Creek Formation 150 K0 no M 5 120 Ui

*z 110 <

re 100

COMPOSITE CONRELAllOfJ CURVE. SAMPLES < 60 MPo 0761 UCS = 102 9e2 MPq (r 3 -0 81,. r = 0 70.n * K2) o Conglome»ole • Sandslone • Sandslone, silly • Mudslone. sill slone, shale, la mini le * Clayslone A Coal

?0 30 UNIAXIAL CUMPPCSSIVC SinfFKilll IMPal Figure 2.

UCS versus Sonic Interval Time for Rock less than 60 MPa

GMcN-08

E(GPo)

Figure 3 .

Figure 4 .

Static Elastic Modulus versus Sonic Interval Time

Static Elastic Modulus versus Dynamic Modulus

GMcN-09

—146—

4. ENVIRONMENTAL PLANNING

—148— ENVIRONMENTAL PLANNING AND PRACTICE IN OPEN-CUT COAL MINING

L . C . BELL Department of Agriculture University of Queensland St. Lucia.

4067

Abstract

Open-cut mining for coal leads to major disturbance of the landscape and

thus

creates

the

environmental insult. therefore, against

potential

for

major

on-site

and

off-site

The immediate objective of rehabilitation programs,

is to reduce this impact by stabilization of the mined surface

erosion;

a long term objective is to return the land to a form

suitable for other uses.

Consideration

rehabilitation

requirements

at an early

stage of

mine planning is the most cost-effective method of reducing environmental problems during mining and of achieving the desired post-mining land use. Achievement

of rehabilitation

objectives requires

the reconstructed

landscape

such

requirements

satisfied

and

that

are

that erosion, the

surface

that the morphology of drainage

and land use

landscape

this

capable of supporting vegetation.

The procedures necessary for the design of the recontoured landscape and

the

construction of

the new root zone for plants will

depend on the

nature of the pre-mining landscape and overburden characteristics.

Thus a

physical

inventory

non-coal

resources

essential

part

the

rehabilitation planning process.

Geologists can play a major role in this

process

through

their

pre-mine

surface

particularly

characteristics

and

through

participation

their

lands cape •

the

documentation and

design

subsurface of

overburden

hydrology

stable

and

post-mining

—149— 1.

INTRODUCTION Although

quarter

open-cut

the land

coal mining

disturbed

is responsible

by mining

for

in Australia,

approximately

one

the proportion of

the continent affected is probably less than 0.005% at this time ( 1 ) . local

environmental

however,

can

impact

of open-cut

significant.

The

coal mining on surrounding

Environmental

effects

include

land,

increased

sedimentation in streams through erosion of unstable land and reduction in surface and ground water quality toxic

levels

elements.

through turbidity,

Thus

most

State

salinity,

Governments

acidity and have

passed

legislation designed to reduce off-site effects from mined land through a requirement for progressive rehabilitation during the mining process. A

well

planned

rehabilitation

program

not

legislative requirements are met with a minimum of greater efficiency and productivity

only

ensures

that

cost but also promotes

in the overall operation ( 2 ) .

In this

paper, an overview of the aspects which need to be considered in coal mine rehabilitation w i l l

be given.

The role of geologists in a number of

the

phases of the rehabilitation planning process is essential, and particular attention w i l l be given to these phases.

REHABILITATION OBJECTIVES The immediate objective of any rehabilitation program is to create a

landscape which is and

rain

damage.

permanently

order

second

reduce

objective

stable against

the erosive forces of wind

both

and

on-site

should

to return

off-site

environmental

the mined

condition that allows it to be used in a productive manner.

land

Following the

temporary use of land for mining, a number of alternative land uses may be established,

viz.

animal

production,

recreational use, w i l d l i f e For

the rehabilitation objectives to be met,

must be satisfied uses

cropping,

involve

(2,3).

commercial

forestry,

conservation or urban development.

Firstly,

the establishment

since

two broad requirements

the majority of post-mining land

a vegetative

cover,

the surface of

the

reconstructed landscape must be capable of supporting plant growth in the long

term;

and soil.

the potential

substrates for

Secondly,

slopes and drainage

the

landscape must be compatible with

In certain instances,

these

achieving

alternative

strategies

may

the

overburden

and

spoil

the recontoured adequate

there may be l i t t l e f l e x i b i l i t y

rehabilitation

exist,

include

network of

the desired land use and with

erosion control. areas

plants

these

objectives; may

have

others,

economic

and/or

LCB-01

—150— ecological

advantages.

A systematic approach

to rehabilitation

planning

ensures that the optimum strategy is defined.

PRE-MINE ASSESSMENT OF ENVIRONMENTAL HAZARDS AND REHABILITATION OPTIONS. Many

factors

objectives. greatly

can

Ignorance

increase

the

influence

the

limitations

costs of

achievement

to plant

rehabilitation

growth on mined land can

once mining has

commenced.

As an example one can cite the situation where the overburden may contain one or more strata which may be toxic for the nature and extent of these strata involving

selective

developed.

and thus it

growth limitations this

these

Such a procedure may involve

machinery,

placement

information

vegetative

is important overburden

not

cover on spoil

plant

growth.

Delineation of

prior to mining enables a mine plan detrimental

materials

the use of additional

types of

to have information on the potential

spoil

prior

available,

the

to devising cost

the mine

trying

plan.

obtain

contaminated by toxic elements may far outweigh

the cost of the selective placement. Part physical

the rehabilitation

inventory

non-coal

influence

the

procedures

landscape

and

the

Preparation

required

process

involves

on the mine

base-line this w i l l

for

the new

root

zone

for

plants

the rehabilitation

plan

also

aided

(a)

(b)

glasshouse

materials 1

soil

growth

and

overburden,

limiting

factors

and

a as

the

construction

design

site

characterization these

planning

resources

(c)

the

recontoured (2,3).

laboratory

assessment

determination

techniques for establishment and maintenance of a vegetative cover at the mine site The

(4,5). base-line

inventory

topography, hydrology, These

factors

are

soils,

the mine

site

should

include

climate,

overburden, vegetation and current land use!

considered

detail

elsewhere

(2,3)

and w i l l

only be

considered briefly here. 3.1

Climate The amount,

distribution

and variability

of rainfall

evaporation and

temperature patterns have a marked effect

and success

a rehabilitation

program

through

their

together with on the

effect

nature erosion

and the growth of vegetation. 3.2

Topography Important components of topography are slope,

elevation and drainage

LCB-02

—151— density. from

An assessment

which

can

achieved with

the

pre-mine the

topography

elevation

effects of

disturbance more

overburden w i l l

elevated

pre-mining surface.

and

which

Knowledge

piles.

result

steeper

baseline

which

can

The

swell

in a landform

slopes

than

the

Examination of the slopes both within and surrounding

the mine site w i l l

which

slopes

generally

contains

indicate

the degree of recontouring needed to attain a

visual blending of spoil piles with the surrounding

relative

provides

and

subsequent reshaping of overburden spoil

which

calculated

the

stability

pre-mine

drainage

over geological

time,

terrain.

density,

which

is a useful

to design a new landscape which w i l l

has

achieved

starting point from

be stable

to erosive

forces.

Generally, however, a greater post-mining drainage density may be required to achieve stability because of the generally higher elevation and steeper slopes of the recontoured landscape. 3.3

Hydrology Open-cut

subsurface

coal

mining

hydrology.

can

drastically

alter

the

The importance of characterising

surface

and

the surface water

flows prior to mining in rehabilitation planning has already been referred to.

Underground water

may

affect

not

only

users.

The

floors;

in the USA,

flows may be cut by the mining

post-mining

possibility

land

these

use

effects

options

process,

but

greatest

also

and

this

downstream

in alluvial

valley

mining in such areas is prohibited unless it

can be

demonstrated that the aquifers can be reconstructed following mining. 3.4

Soils The

include

potential

overburden

substrates spoil

and

for

plant

growth

soil.

Soil

is defined here as the

open-cut

coal

mantle (1-2 m) of pedogenic material consisting of an organically surface

(A)

matter

and

horizon

underlain

interfacing with

rock (C horizon).

the

subsoil

overburden

(B)

horizon

through

document

rehabilitation site

during

conducted soils with

the

programme,

the

early

organic

weathered

soil.

Soils covering a typical mine lease are likely order

a zone

enriched

The overburden can be considered to be all the material

lying between the coal and the surface

low

mines

surface

early enough,

soil

resources which may

essential

planning it

has

problem engineering

stages

to be quite diverse. in

the

to prepare a soils map of

the

available

project.

the additional

advantage

features

the

of view

construction, building foundations,

from

point

this

highlighting of road

dam construction and so on.

LCB-03

-152The properties of soils vary both laterally and vertically. apparently

uniform

soils,

material

lower

the

profile

Even in

may

differ

markedly in its plant growth characteristics from that at the surface. Australian

soils,

increases

depth are common. the

chemical

horizons

Laboratory

the

levels

salinity

properties

each

major

the

characterization

chemical

factors

such

limiting

plant

growth

physical

limiting

tests

sodicity

In with

In devising a rehabilitation plan, it is important that

and physical

soil

the various types

are

salinity

and

the

samples w i l l

factors

available

and

ease

(6). for

acidity of

surface

the

mine

testing

and

subsoil

measured.

preliminary

the likelihood of other of

physical

amelioration

A range

and be

provide

assessment of the availability of plant nutrients,

non-nutrient

and

factors

nutrient

laboratory

chemical

and

these have

been

soils,

and

summarized by Bell (1981)

( 5 ) , and Hannan (1984)

(3).

Ideally,

glasshouse

trials

types

should

with

the

horizons

each

the

soil

the

laboratory analyses to confirm the plant response predicted from the soil tests unless this has been previously

conducted for

the range of soils in

the particular locality in connection with neighbouring mining operations. 3.5

Overburden As

material

indicated lying

previously,

between

the

overburden

soil

and

the

can be

considered

coal layer;

between two coal

seams is commonly called inter bur den,

the deepest

coal

seam is called underburden.

underburden

commonly

addition

unconsolidated

consist

surficial

alluvium

all

the

lying

and the rock below

Overburden,

sandstones,

the material

interburden and

mudstones

and

shales

but may

also

include

development

upper layer of basalt or other volcanic rock. Characterization delineate

those

detrimental

to,

rehabilitation properties

strata

overburden

prior

which

are

likely

revegetation

planning.

can allow

the likely

this

and

planning,

part

effect

Geologists

of rehabilition

beneficial

knowledge

and ground water quality after mining. aspect

mine

essential

Additionally,

prediction of

this

of of

for,

to or

pre-mine overburden

of spoil on surface

can play a key role in

topic

discussed more

fully in Section 4 . 3.6

Flora and Fauna In those

is desired,

cases where a return to native

plant

a base-line vegetation survey prior

species after mining

to mining w i l l assist

defining those species which are climatically suited to the area and which

LCB-04

—153— may be useful

components of

increasingly

being

the post-mining vegetation ( 7 ) .

realised

self-sustaining

ecosystems,

mining

provide

and

yard-stick

fauna

surveys

against

post-mining ecosystem stability 3.7

that

play

faunal

which

the

can be measured

Additionally,

an important

role

populations

prior

success

achieving

(8).

Pre-mining Land Capability and Use The capability of land is a measure of its capacity to be used for a

given

purpose

indefinitely.

The

land

use

prior

to mining

may

not

appropriate to its capability with the result that land degradation may be occurring.

Documentation of both land capability and use provides a guide

as to what can sensibly be achieved in the post-mining landscape.

OVERBURDEN SAMPLING AND ANALYSIS After

the exploratory

undertaken

drilling

phase,

to delineate more accurately

pattern drilling is

the quantity

generally

and quality

the

coal reserves.

Geologists commonly use the overburden samples obtained in

this

operation

development, of

e.g.

potentially

stability

however,

that

measurement Essential

gain

hazardous

spoil

(9).

overburden

relevant

future

mine

the occurrence

blasting

requirements

and

still

universally

recognized,

samples

not

should

also

future

retained

to the future rehabilitation

for

program.

information which can be derived from these samples includes

major

water

quality

of various

toxic materials such as pyrite which

problems

strata

during

to support

and

plant

after

growth.

necessary for early decisions to be made regarding toxic

the It

properties relevant

the presence of potentially

ability

information

consequences of mining,

faults,

piles

these

additional

the hydrological

strata

required.

and

whether

Unfortunately

soil

retention

and

mining

(a)

can lead

and

(b)

the

Such information

selective placement of

replacement

spoil

there are a number of examples in Australia where

failure to fully document the properties of overburden early in the mining process has led to prohibitive rehabilitation costs at a later 4.1

stage.

Sampling Intensity Like

soil,

and vertically,

overburden varies

measure of this v a r i a b i l i t y . guidelines et a l .

characteristics

both

by Sobek et a l .

(11) and Sendlein et a l .

programs

conducted

laterally

to obtain a

Comprehensive coverage of sampling intensity

is given in publications

(1980)

research

in its

and the sampling intensity must be sufficient

the

(1983)

(12).

University

(1978)

(10),

Barrett

In the rehabilitation Queensland

for

coal

LCB-05

—154— mines,

samples

are

collected

across

the

proposed mine

possible

subsequently

analyzed

varied

m intervals path.

from

many

The number

1 m increments

depending

stratigraphy

holes

and

initial

reconnaissance analyses. 4.2

Analyses The properties

of overburden material

below

the zone of weathering

may change significantly when exposed to the atmosphere following mining. Physical changes such as slaking and dispersion are common with solid rock being reduced to fine-sized material

in a few years.

Important

chemical

changes include the leaching of soluble salts and the oxidation of pyritic minerals to produce acid.

It is important,

therefore,

weathering

are

taken

into

account

when

unweathered

overburden

samples

are

being

assessed

properties

future properties of overburden spoil.

that the effects of the

analyses

the

light

Simple laboratory weathering

on the

tests

can provide valuable information on the nature of the future spoil. The

overburden

planning have Australia made,

analyses

been well

(3,5,14)

however,

and will

about

considered most

useful

documented elsewhere for

important

rehabilitation (10,11,13)

and

Brief

comment will

not be detailed here.

the most

for

the USA

factors requiring

assessment

coal mine overburden in Australia. 4.2.1

Active and potential

acidity

Most

growth.

plants

require

pH value

between 5 . 5

and 8 . 5

for

adequate

While pyrite - containing overburden may have a pH value in this

range in the unweathered state, its pH may rapidly decrease to less than 3 with the oxidation of the pyrite. sterilized such

for

the growth of

overburden

concentrations quality

and

capacity hazard

can

thus make

Additionally,

extremely

acid

difficult

for

is effectively

surface runoff

and

have

companies

from

high

metal

to meet

water

regulations.

Measurement amount

spoil

At this pH, the material

plants.

pyritic

the

sulphur

total and

potential

of other overburden minerals

associated

with

each

acidity

comparison with

assessment

the inherent

allows a determination of

strata.

Where

such

the

neutralizing

acid-base

the acid

accounting

predicts an excessive net acidity,

the pyritic overburden should be either

selectively

placed

depth

subsequently

non-pyritic

overburden

spoil

or or

soil

covered

the

with

post-mining

layer

landscape.

Pyritic overburden is found in a number of mines in the Bowen and Sydney Coal Basins and in the Collie f i e l d in Western Australia.

LCB-06

—155— 4.2.2

Salinity High

levels

soluble

plant

growth

runoff

from

the mine

Many mines

the

site

Bowen and

several

permeable,

wet

seasons,

spoil

being

high in salt to significantly relatively

salts

the resultant

overburden

and also

unsuitable

Sydney

Coal

can inhibit

lead for

stock

Basins have

affect plant growth.

however,

salt

but

process

the

to the

reduce water

and

agriculture.

spoil

sufficiently

Where the substrate is

w i l l leach out is

surface

extremely

the root

slow

zone

relatively

impermeable high clay material. 4.2.3

Sodicity Toxic

levels

salinity

sedimentary

associated with high levels of sodium which following

the reduction in electrical

leaching.

Exposure

spoil,

this

and

sodic

one of

strata

the major

materials

conductivity can

are

can result in clay

thus

at the surface

cause

limitations

severe

often

dispersion through

crusting

to the establishment

vegetation on spoil at many coal mines in Queensland and New South Wales. 4 . 2 . 4 Nutrient

availability

Overburden and

less

copper

spoil

commonly

and zinc.

addition.

These

Less

commonly

potassium

deficient

and

deficiencies

easily

some of

nitrogen and

are easily rectified

rectified

phosphorus

the micronutrients

the

dearth

by of

such

fertilizer beneficial

micro-organisms which increase availability of various nutrients to plants and which

are

normally

Such organisms w i l l layer

fresh

present

slowly

soil

but

absent

colonize vegetated spoil, spoil

ensures

their

in overburden

spoil.

but replacement of a

rapid

return

the

new

landscape. 4.2.5

Physical

properties

The physical ameliorate, the

properties of a medium are often the most difficult to

and

thus often

dictate

the yield

easily

altered

chemical

and

potential

for

microbiological

plants

after

properties

are

changed. A suitable root zone for

plants

should have a good available water

capacity but be sufficiently well drained so as not to affect root growth through

lack

mechanical

aeration;

impedance

additionally the

properties are a function of its

stability.

expanding

the

medium

root

should

system.

not

All

provide these

the pore size distribution in the medium and

This distribution,

in turn,

is determined by the primary

particle size distribution and the aggregation of these

particles.

LCB-07

—156— spoil

fine-grained

Overburden

sediments;

some instances,

dominate

produce

sandstone

capacity

but moderate

the Bowen and Sydney

heterogeneous

spoil

Basins,

mixture

and such

which

to high hydraulic Coal

coarse-

coarse-grained materials has

low water

conductivity.

the overburden

tost

holding

commonly

contains a

significant

proportion of fine-grained material such as shales and mudstones to give a spoil

which

has

conductivity.

moderate

to promote surface rainfall,

high

available

Lack of aggregation of crusting

water

particles

through particle

but

poor

in weathered spoil

capacity

tends

sorting under

the action of

and this can be aggravated by dispersion of the colloid fraction

which is commonly sodic.

CONSTRUCTION OF THE PLANT GROWTH MEDIUM In

constructing

suitable

medium

for

plant

growth

order

achieve a particular land use goal, a decision must be made as to whether the root zone is to consist of soil alone, Characterization

the properties

soil plus spoil or spoil alone.

soil

and spoil

(Sections 3 and

allows a rational judgment to be made on this matter. Soil should be conserved and used in the rehabilitation program when overburden material

cannot

after

treatments

costing

soil

applied.

and

ameliorative replacing

retention are of

the

are

the

Generally,

multiplicity

however,

achieved more quickly with cropping land, where

the

the desired post-mining land use up to

most

soil

that

involved

Cost-benefit

complicated by the difficulty

effects

growth.

support

factors

retention.

which

show

Indeed for

return

the

native

soil

use

plant

goals

re-instatement

soil retention and replacement is essential.

intention

the magnitude

influence

land

even

conserving

analyses

in quantifying

analyses

vegetation

are of

Additionally, after

mining,

replacement of fresh surface soil is the most economical and reliable way of

ensuring

exist

the re-establishment

native

selection of

ecosystems.

soil horizons,

When

the wide soil

diversity conserved,

species which

decisions

the

optimum depth of replaced soil and the effect

of stockpiling all have to be addressed

(2).

Selective handling of overburden layers may be required material which is not suitable for land use goals by contaminating

plant

(a) to bury

growth or which may also affect

surface and groundwater

( b ) to retain

desirable root zone material. As

indicated

previously,

adverse

properties

overburden

strata

LCB-08

—157— which

may

limit

plant

growth

Where

the

major

portion

the

include

undertaken

selective

handling

should

suitable

strata

the

recontoured

adverse

strata

make

procedures

should

small

be directed

acidity,

overburden

proportion

ensuring

those

the

sodicity. properties,

placement

In of

and

undesirable

to ensure

surface.

towards

salinity

has

the most

cases where

overburden,

the material

the

handling

does not

end

up at the surface. Even when a decision has spoil,

the

total

depth

been made

soil

plus

to replace

spoil which

a layer of

should

root growth should be at least 1 m and preferably 2 m depth. will

ensure adequate water reserves for

a buffer

against

occur from underlying of

spoil

LANDSCAPE DESIGN

toxic

spoil.

final

landscape

over for

Such a depth

seasons and provide

and/or

The mechanics

is discussed by Hannan ( 1 9 8 4 )

The

plants in dry

the upward movement of acidity

soil

suitable

salinity which may

selective

placement

(3).

should

compatible

with

the

desired

post-mining land use, adequate erosion control and drainage requirements. 6.1

Land Use and Erosion Requirements The

greatest

re-instatement fertile close

soil to

with

its

rotational 14% with

constraint cropping low

slopes

original

contours,

basis with appropriate

in landscape land.

pastures erosion

design

associated with

Prime

agricultural

6%) would

be expected

but

can

land which

established

control

measures

such

land to

cropped

slopes

the

which

has

reinstated on a

up to about

contour

banks

and

waterways. The establishment of pastures poses fewer limitations on recontoured landscape, with associated proper

slopes up to 18 to 20% being possible as long as they are

with

appropriate

management

soil/spoil

benching

ensure

surface.

The

pasture

use

reduce

vigour

conventional

slope and

length

and

with

protection

the

agricultural

machinery

also limited on slopes > 20%. Where the intention is to establish the

native

appropriate rainfall advocated.

vegetation

after

mining,

benching

reduce

slope

intensities

are

high,

however,

Such

limitation

commercial forestry or to return

slopes

length. and

20% In

slopes

particularly

vegetation establishment where retention of a soil

may

northern less

important

used

with

Australia,

than with

20%

are

native

layer may be necessary

LCB-09

—158— to

provide

source

propagules;

the

replaced

soil

particularly

vulnerable to erosion on slopes > 2 0 % . 6.2

Drainage

Requirements

The f i n a l

landscape must be such that runoff

the mine area to adjacent density r e q u i r e d w i l l , for

the

particular considered

given by Hannan Other assessing

Detailed

drainage

input

from

flow

into

the depression

(15).

water

The drainage

the management

discussion

requirements

for

the

units

factors

mined

land

essential

are

void and the s t a b i l i t y

geologists the f i n a l

CONCLUSIONS A major factor

desired.

the size of

(3).

where

likely

the faces l i n i n g

use

can be conducted w i t h i n

excessive erosion.

influence

assessing

(1984)

areas

the

in turn,

land in

streams without

preliminary

i n achieving

assessment

successful r e h a b i l i t a t i o n

landscape

post-mining

limitations

overburden

characterization

are

planning.

Because

their

expertise

major

ensuring

role

play

plant

growth.

thus

that

o b j e c t i v e s and with a minimum of

stability

and

of mined the

land

potential

Landform,

hydrology

important

facets

rehabilitation

areas,

geologists

these

companies meet

their

and

have a

rehabilitation

cost.

REFERENCES 1.

Bell,

L.C.

(1986).

Australian Press, 2.

Hannan,

J.C.

G.H.

Mining.

Soils

Brisbane,

pp.

and B e l l ,

L.C.

Martin

In J . S .

- The Human

Russell and R. F. Impact.

Hannan, J . C .

(1986).

Coal Mining

Bell,

L.C.

Practice.

Parkville.

Mine R e h a b i l i t a t i o n .

Mining Industry.

pp.

P.C.

(1975).

Bell,

L.C.

(1981).

R e h a b i l i t a t i o n of mine wastes

Mildura,

A systematic approach

fertilizer

requirements

Australian

Mining

September,

198 1,

for

Industry

Canberra,

the

pp.

Third

National

T26-T28 .

to the assessment of

rehabilitation

Council pp.

the Coal

Sydney.

in Northern A u s t r a l i a - problems and approaches. Chemical Engineering Conference,

Australian

233-252.

A Handbook for

New South Wales Coal A s s o c i a t i o n ,

and Whiteman,

(Eds.). Queensland

Surface mine reclamation.

( E d . ) Australasian

(1984).

444-466.

I n s t i t u t e of Mining and Metallurgy, 3.

Isbell

University

Environmental

mine

wastes.

Workshop

21-24

15-36.

LCB-10

—159— 6.

Bell, L . C .

(1985).

Opportunities and constraints in achieving land

use goals.

Proceedings of Seminar on Coal Mining as a Land Use.

8-9 August,

1985, Brisbane.

Australian Coal Association,

Sydney,

Re-establishing A u s t r a l i a ' s

flora

pp. 240-255. 7.

Ford, R. and Langkamp, P. on

mined

(1987).

areas.

Environment.

_In_ Mining

and

the

Return

Australian Mining Industry Council,

the

Living

Canberra.

pp.

10-14. 8.

Allen,

N.T.

and

(1987).

the

Welcoming back the native w i l d l i f e .

Return

the

Living

Industry Council, Canberra, 9.

Merritt, R . D . and

(1983).

Premine

Environment.

pp.

Mining

15-19.

Coal Overburden.

Planning.

In Mining

Australian

Geological

Noyes Data

Characterization

Corporation,

Park Ridge,

New

Jersey. 10. Sobek, A. A . , Field

Schuller, W.A. , Freeman, J . R . and

Laboratory

Minesoils.

Environmental

EPA-600/2-78-054, Cincinnati, 11. Barrett, J . , R.D. ,

McWhorter,

Mines.

U.S.

and Smith, R.M.

(1978).

Applicable

Protection

Technology

Environmental

Overburdens

and

Series

Protection

Agency,

Ohio.

Deutsch,

Recommended

Methods

for

General

P.C.,

Ethridge,

D.B.

and

F.G.,

Youngberg,

Franklin, W . T . ,

Heil,

A.C.

(1980).

Procedures

Overburden

and

Hydrologic

Studies

Surface

Technical

Report

INT-71,

Forest

Service,

U.S.

Ogden, Utah. 12. Sendlein,

L.V.A.,

(Eds.)

Vazicigil,

(198 3 ) .

H. , Carlson,

Surface

Reclamation Handbook.

chemical

western

analysis

United

States.

C.L.

and Russell, H. K.

Environmental

Monitoring

and

Elsevier, Amsterdam.

13. Sandoval, F.M. and Power, J . F . for

Mining

(1978).

Laboratory methods recommended

mined-land

spoils

Agriculture

Handbook

and

overburden

No.

525,

USDA,

Washington D.C. 14. Anon ( 1 9 8 3 ) .

Chemical Characteristics of Overburden from Surface Coal

Mines in the Upper Hunter Valley.

Hunter Valley Technical Report

No.3, State Pollution Control Commission, 15. Seedsman, R.W.

and Williams, D . J .

rehabilitated

strip

Professional

Approach.

(1987).

coal mines.

20-21 July,

Institute of Mining and Metallurgy,

Sydney.

Long term stability of Mining and Environment - A 1987, Brisbane, Parkville.

pp.

Australasian

27-32.

LCB-11

—160—

ENVIRONMENTAL IMPLICATIONS ARISING PROM THE UNDERGROUND EXTRACTION OP BITUMINOUS COAL

R.A. Hughson Floor 29 Legal and General House Department of Mineral Resources 8-18 Bent Street SYDNEY NSW 2000 Telephone:

240 4535

—161—

INTRODUCTION

1.1.

WHY IMPLICATIONS The very nature of undertaking extraction results in change to the natural order.

Undertaking extraction in an

underground or covered environment, magnifies the degree of effect on this natural order change, particularly as depth increases. The reason for using the word implication in the title is to extend the suggestion of involvement towards an intertwining of many factors which result in environmental change, some of a temporary nature and some geologically permanent. From the time when early man sought flint and later metallic ores, clay and building stone, homo sapiens has modified the natural order.

The industrial revolution

period was when man began to question his industrial activities.

Gordon Bottomley born in 1874 wrote in his

poem - "To Iron-Pounders and Others". "When the old hollowed earth is cracked, And when, to grasp more power and feasts, Its ores are emptied, wasted, lacked, The middens of your burning beasts. Shall be racked over till they yield, Last priceless slags for fashionings high, Ploughs to make grass in every field, Chisels men's hands to magnify. Already towards the end of the 19th Century man was expressing his concern about an unfavourable environment, developing due to industrial expansion RH-01

—162—

Todays attitudes are an extension of this concern which has been expressing itself for over 100 years. When the implications of our historical past and present day needs are recognised, the difficulty of securing an idealistic solution can be gauged. Much of the past is viewed from a subjective and emotional standpoint, especially by those who have lived with it, accepted it for what it was, why it was there and had their own vested interest to protect; owner, manager and worker alike. This intertwining of vested interests has been under challenge for many years both internally and externally, a struggle between enlightenment and entrenched attitudes, oblivious to the strengthening wind of attitudinal change as the levels of community expectation heightened. No longer can we ignore the expressed views of the community, supported by a level of expertise equal to our own, capable of erudite expression and often in fundamental opposition to our own professional preferences. It is to meet this challenge, a result partly of our own past inadequacies, that all extraction and particularly underground extraction within or close to populous areas, has to achieve a level of environmental purity, acceptable to the more aware and hopefully responsible members of our society.

Failure can only increase antagonism and

encourage those of the opposing viewpoint, who seek to place ever increasing levels of constraint and compliance on the extractive industries. RH-02

—163—

The Environmental Issues.

2.1.

The Visible Surface. Community awareness of mining as practiced underground is limited to what can be seen on the surface and what is fed to them through the media, - usually highly sensationalised due to tragedy or industrial disputation, - and through visits to operations and museums, akin to biased sampling because of the obvious limitations of such exposure. From a geologists 1 point of view there are specific matters of surface interest where his or her professional expertise is necessary, to ensure minimum surface disturbance and the prevention of unnecessary vandalism. Other aspects are just as likely to be of importance to a geologist as to other community members.

Let us first

consider the more general aspects of environmental intrusion likely to result in temporary or permanent change. An underground mine compared to an open cut mine has substantially less visual impact and in many cases is only known to a small proportion of the local inhabitants. What can be seen are pit head buildings, including a washery if one is required, power lines, pondage for water storage, product coal disposal facilities and stockpiles, of which reject will become one of the few permanent reminders of a colliery 1 s existence.

A well constructed

and rehabilitated reject stockpile can result in enhancement and the provision of additional useful land. RH-03

These facilities are mostly capable of removal at the cessation of operations or of being constructed to blend in with the surroundings, ensuring permanent acceptance when finally rehabilitated. Modern colliery surface structures are designed to blend in with their surroundings. The most recently constructed colliery surface sites are a credit to the companies and to the industry. As examples one can nominate Cordeaux and Tower Collieries, placed in environmentally sensitive areas and both blending in to their environment in an unobtrusive manner and meeting with the stringent lease conditions imposed. Prior to any mining taking place we are required to prove up the resource through drilling, to obtain samples for assessment purposes and gain structural knowledge. This is usually the first stage likely to suggest environmental interference, through surveying especially in timbered country and establishing access roads and drill sites. The more difficult the terrain, usually the greater the environmental interference. Once the deposit is shown to be worthy of more detailed examination, larger samples are usually sought, thus requiring the excavation of both waste and product materials with such activities beginning the process of permanent change. Provided that all of the activities are carried out in a responsible manner, the extent of permanent change should be minimal. Drill sites should be cleaned up before leaving, roads can be scarified to promote new growth, and drill holes, surface drifts and shafts plugged according to imposed conditions. Given natures ability to rehabilitate itself, lasting damage should be totally avoidable. RH-04

—165—

Even where an underground colliery has existed for years, its location, once the site has been cleared, may leave little more than lasting memories and to illustrate this point, how many people here could walk to the site of Balmain colliery.

The most likely lasting effects from underground extraction and the one likely to have the greatest impact during the extraction phase is subsidence.

Unless extraction is

limited to first workings, which is simply excavating roadways to form pillars of coal, the degree of subsidence is likely to result in significant topographical change. First workings are unlikely to be economically viable and are certainly going to result in an unsatisfactory level of resource recovery.

Surface movement resulting from underground extraction will vary from the accepted minimum of 20mm, to a measured maximum for New South Wales which is of the order of 65$ of seam working thickness, which for a 3.0m seam working height^ results in lowering the ground surface by some 1.9m. Where a number of seams are extracted in the same area the effect is cumulative. The low end of the scale is reached at the extremities of the angle of draw and the most pronounced movement occurs towards the centre of the total extraction panels, of super critical widths.

Subsidence must not be viewed only as movement resulting in topographical change.

It is a mechanism producing ground

strains during the active phase which open up cracks, push structures laterally and cause tilt which is usually self correcting.

The results of subsidence can be significantly

magnified by changes in the terrain and the proximity of outcropping or near surface strata and conversely, RH-05

—166-

ameliorated by unconsolidated sediments and sediment type. Our understanding of subsidence can allow us to design a system of extraction which can meet all conceivable needs. The problem is often having an economically viable system of extraction.

Anything less than total extraction accepts

some degree of sterilisation. When considering what criteria should be addressed, the range includes seam depth, extraction thickness, extraction panel width, angle of draw, cover rock types, seam floor and roof characteristics, geological structural features, ground water regimes and surface topography. Subsidence creates conditions which when controlled, has no marked long lasting effect on both natural and man made structures. When control is not fully exercised, unacceptable levels of damage can occur.

Even.where terrain changes occur,

correction can be undertaken at a price.

The aim is to

minimise the cost and disruption caused; any assessment should take into account the loss of resource likely to occur due to sterilisation and the cost of introducing modern technological developments to prevent both disruption and sterilisation. 2.2.

Underground and out of sight. The saying, "What you see, you get" does not apply to underground coal extraction. The whole sub-terrainean environment is altered due to both removal and displacement.

Coal mining takes place over

substantial areas due to its lateral development.

RH-06

—167—

The least disruption in a geological sense occurs in the strata closest to the surface and the greatest disruption at the point of extraction.

In between, strata beds fail and distort breaking into discrete blocks within the caved goaf area, above which the forces of strain cause fissuring and bed separation occurs. Depending upon the depth of mining, the fissuring becomes less pronounced as the surface is approached.

Differing

rock types also control the degree of fissuring and cushioning, exerted above the main zone of failure.

Drill

holes as you known show the ability of plastic type rocks to displace and close voids, so yielding rock beds can influence the effects of strata displacement, deformation and failure.

The resources existing within sedimentary strata extend beyond coal to water and petroleum, both of which are likely to be affected by strata dislocation.

The degree of

protection to be given will dictate the extent to which coal can be removed.

The principles of subsidence apply to

the intervening strata between the seam and the surface and our N.E.R.D.D.C. borehole research project is establishing the behaviour characteristics of this twilight zone, covering vertical movement, permeability and bulk density change.

An improved understanding of this zone's characteristics will improve our ability to relate the potential of subsurface strata to predictable levels of dislocation.

This

is particularly important as the upper beds integrity controls the downward flow of surface waters, such as exist above coastal mines and support impounded waters which sterilise substantial areas in the Southern Coalfield.

RH-07

—168—

Similarly migration of gas and fluid from both above and below a worked seam can be detrimental and exercising control is necessary if those resources are to be managed responsibly. Finally, we come to disposal of waste material both solid, liquid and gaseous which result from mining activities. Waste rock and wash plant reject are generally disposed of by surface emplacement, water is utilised after being pumped for coal treatment and if being discharged requires treatment itself.

Hydrocarbon gas is generally removed

from underground as a component of the exhausted ventilation air current.

At two mines in N.S.W. the gas is

being recovered through "in seam" gas drainage technology to fuel an electrical power generating system.

At two

others the gas after drainage is exhausted to the atmosphere. Pan ventilation though outside the area of geological interest is acknowledged as an environmental problem, to which engineering solutions exist to ensure compliance levels can be attained. 3.

Conclusions. This presentation aims to show how geological knowledge and its application integrates with the environmental problems associated with underground coal mining. In many cases, only through a thorough understanding of the geological setting can the solutions to some of our environmental problems be found because of the problems occurring in this geological setting. The few departures into the unrelated areas of engineering which have no geological significance, are included to RH-08

—169—

complete the picture of environmental implications, of which we must all remain aware irrespective of academic discipline.

RH-09

—170—

5. COAL TECHNOLOGY

-172-

OOAL PREPARATION AND POSSIBLE FUTURE DEVELOPMENTS

Author

Mr. N.W. Gibson School of Applied Science - Division of Materials Technology Wollongong College of T.A.F.E. P.O. Box 1223 North Wollongong N.S.W. 2500

Abstract The preparation of coals for coke in the metallurgical industries has been aligned with the technological progress in the ironmaking blast furnace. Coke consumption in the blast furnace has been reduced considerably through a series of operational and technical changes, in which coal preparation and coal blending has contributed to improved chemical and mechanical properties of coke. The techniques of coal preparation are reviewed and the limitations of coal cleaning units are presented as they relate to

the coke making process. Enhancements to present ironmaking

practice and new developments that involve non-coking type coals for smelting reduction, are discussed in the context of cost considerations for steelmaking

routes. Coal preparation is anticipated

to follow the

teclinological requirements of the ironmaking process, with attention to the lowering of ash content, sulphur content and moisture content. Microwave technology, involving coals, is emerging with an alternative process for metal smelting.

—173— Carbon based

fuels

have

long

enjoyed

unique position

in the

metallurgical industries, serving the functions of energy

source, either

directly

through power

in the

form

of heat energy or

indirectly

generation, or as a reducing agent, in the form of solid carbon or carbon monoxide gas.

Ironmaking The mainstream of development in the preparation of coals, coke or carbon monoxide

fuels and

reducing agents,

technological progress

in the

has been

through years

ironmaking blast furnace.

In turn the

ironmaking developments have been structured by new demands in subsequent steelmaking practices. In large steelplants, based on blast basic oxygen steelmaking

furnaces and

units, the concept of strict raw material and

liquid iron composition has been practiced

to optimise the heat balance

and reactions in the steelmaking process. The preparation of coal as a raw material for coke in the blast furnace is critical to the furnace operation. Coke has four major functions: (a) It

the principal

source of heat energy arising

from

the

combustion of carbon at the tuyere region of the blast furnace. 2C + 0 2 = 200

AH298K = -222,000 kJ kgmol-*

(b) Hie carbon monoxide gas produced, provides reducing

conditions for

the metallies in the furnace charge. Fe203 + 300 = 2Fe + 3002 (c) The coke

layers, shown

retained through

in figure

the cohesive

1, formed during charging, are

zone where the metallics undergo

softening and melting and restrict the permeability gas. The well defined

coke slits

in this

of the reducing

zone permit both

permeability and efficient gas distribution.

NG-01

—174— (d) Coke in the lower zones of the furnace supplies the carbon for combustion and ore and coke layers

supports the raw material burden in this high temperature zone. Hence the high

temperature

mechanical

and

chemical properties of the coke are essential.

Figure 1 Schematic of blast furnace zonesJ Raw materials The developments in the technology of preparation of raw materials for the blast furnace and operational efficiencies has significantly reduced the consumption of coke2-6 as shown in figure 2. The review period suggests a lowering of coke rate from

1200 kg/t of iron produced in

1924 to the

current rate of around 460 kg/t with the the gains of the 1960/80 period being quite pronounced.

It is suggested that no single factor has

contributed to the reduction in coke rate as many operational features, such as furnace size, charging methods, raw material preparation and blast temperature, have been improved.

However within the

raw material

preparation area the technology has been centered on the blending of coals to achieve the optimum properties for coke making7. 1400' -

1200"

£< 1000AC

ULi 800 O o 600 -

nn n n

400 -

t> o o

in 00 05

YEAR

Figure 2 Blast furnace coke rate The factors that have been considered as optimising the properties for the blending of coals include ash content, swelling

index, reflectance,

fluidity, volatile matter and composition. Where indigenous prime coking coals are available for the ironmaking unit, attention has been centered

NG-02

—175— on providing consistent ash content and swelling/fluidity characteristics. Blending, for producers without indigenous coking coal a number

of supply

sources, is

blend properties would

generally very

be consistent

resources and with

extensive, although

the

with comparable specifications

for

ash content, reflectance and fluidity.

Coal preparation The

selection

coal

cleaning

equipment

satisfy

specifications requires a knowledge of the nature of an

assessment

profitability. Considering

the

equipment

the technical

efficiencies of

and

coal

cleaning equipment

the

product

economic

measures

only, the

performance

context are

the

the coal, the design

influenced by

four

major

factors: (a) The density/mass distribution of the feed coal. (b) The size distribution of the feed coal c (c) The relative density of separation of the coal. (d) The design characteristics of the equipment.

The objective of coal cleaning equipment is to at the chosen

relative density.

achieve a sharp separation

Figure 3 presents

an assessment

relative efficiencies of the principal coal cleaning units® coal by

density and

examined. A

mass

prediction

indicies) has been

distribution is

of the

yields

established using

used for

and

each

ash contents

typical design

of the

A conmon feed of the

units

(presented

partition data

for

each of the units. RD

ASH

1.8

INDEX

Figure 3 Coal cleaning efficiencies

NG-03

—176— The dense medium units are essentially independent of separating density and show high efficiency over the full range of separating densities. This feature is a characteristic of dense medium coal cleaning units in achieving a sharp separation independent of the

density and mass

distribution of the incoming coal feed. In practice, and with close control of operating parameters, dense medium units are able to accomodate around 25% of "near density" material within io.l

of the separating

density. The sharpness of separation for the dense medium units is dependent upon size distribution, dense medium baths generally have a lower size limit of 6 ran below which there is a pronounced deterioration in efficiency. Dense medium cyclones have high efficiency for a wide size range from 40 nun down to 1 mm, with a marginal decrease in efficiency down to 0.5 ran and pronounced decrease in efficiency for sizes below below 0.3 ran. Jigs, Tables, Spirals and Water Cyclones show the greatest variation in separating efficiencies. Within the separating mechanism of these units is the function of particle settling velocity, which is proportional to both particle density and size, hence the efficiency of separation will be maximised by maintaining a uniform feed size distribution. Jigs however, are capable of handling a wide size range, with reasonable efficiency and when separating at relative densities above 1.60. The jig is particularly susceptible to misplacing material when the amount of near density material exceeds 7-10%. With a uniform feed size range and high separating density, jig efficiencies will approach that of the dense medium units. Tables exhibit high separating efficiency when the feed contains a controlled narrow size range and less than 7% near density material, but are limited by low productivity. Multiple start spirals are dependent on the density/size distribution and appear effective when near density material is less than 7% and size is in the range -2 mm to +0.1ran.Water cyclones are relatively inefficient separating units. As the normally brittle reactive class macerals predominately report to the fine sizes, the recovery of the fine sizes is important in optimising the bonding properties of the coke. As flotation of coal fines is a process independent of relative density, relying on the surface properties

NG-04

—177— of the particles, surface oxidation will markedly reduce

floatability.

Efficient flotation is dependent on particle size, efficiency

increases

below 0.5 mm, but decreases with ultrafines below 50 jm where strict control of pulp density will minimise

the drop in efficiency. Flotation

processes continue to suffer from the inability of the reagents to be sufficiently selective in separating particles of say, 10% ash content, from those of 15% ash content. In addition, certain coals, when associated with active clays,

inhibit the action of reagents in flotation due to

slime coating of the particles.

Hie dependence of most coal cleaning processes

on sizing results

typical coal preparation circuit being divided

in the

into coarse sizing

(-150+20 mm), intermediate sizing (-20 +0.5 mm) and fine sizing (-0.5 mm). There are few instances of size reduction units being employed exclusively for the liberation of clean coal

from associated

refuse, although

the

initial size reduction that limits the maximum size to the coarse coal cleaning

unit

achieves some

particle

liberation. Size

principally involves the application of rotary breakers and

reduction

toothed roll

crushers with hammer mills typically preparing the cleaned coal ( to -3 ram) for charging to the coke ovens. Screens, normally used in open circuit, achieve size selection for the respective coal cleaning -0.5 mm

and within this range may serve the

units down to

important

function of

dewatering. Water classifying cyclones, where appropriate, may be utilised for finer size selection in spirals, tables and flotation circuits and serve the function of dewatering/thickening units.

The requirement of low moisture contents for coke oven feed material and the environmental constraints on discarding of refuse, completes the coal cleaning circuit with dewatering drainage on vibrating

techniques. Coarse coal is dewatered by

screens, intermediate

sizes are dewatered by the

combination of sieve bends/vibrating screens/centrifuges

and fine sizes

generally require pre-thickening prior to vacuum filtration * Dewatering of refuse varies considerably according

to the discard constraints and

involves screen dewatering of coarse and intermediate sizes, thickening of fine sizes

followed by emplacement

in tailings dams.

In the more

constrained situation the fine sizes will require thickening, centrifuging or mechanical/vacuum filtration before waste site emplacement. In general, water circuits require critical controls on plant recirculated water and on water released from the plant, tailing dams or other emplacement sites.

NG-05

—178— Developments It is anticipated

that the developments in coal preparation will be

closely aligned with

technological

changes in the

ironmaking and

steelmaking process. Long term commitment to iron and steel making requires a substantial investment cost and a recent European study8 of the specific investment costs for crude steel production is presented in Figure 4.

From

this

study

the viability

the

traditional

cokemaking/blast furnace/basic oxygen steelmaking route is only apparent for large scale operations in excess of 2x106t/a. Projected plants and mini-steelworks requiring capacities less than the large scale operation, are more likely to involve the alternate direct reduction/electric arc furnace steelmaking process. x

1000

H C0

O o 800

coking plant-blast furnace-b.o.s direct reduction-e.a.f

Ui 5

>UJ

600

o o UJ Q. 400 CO

0.5

1.0

1.5

2.0

2.5

CRUDE STEEL PRODUCTION, 10 6 t/a

Figure 4 Steelplant specific investment costs The continued development of the blast furnace route will occur in large scale plants through enhancement of the operating process • Hie direction of enhancement will

involve process technologies that are aimed at more

efficiently and economically utilising the energy components. Currently, techniques of injection of natural gas, coke oven gas, pulverised coal with recirculation of the blast furnace gas and plasma gasification are expected to lower coke rates. The relative accent placed on any of these techniques will be dependent upon the cost efficiency of the process. Coke will continue to be prepared to similar specifications through conventional coal preparation plants. When coal is used as a blast furnace injectant or in the alternative direct reduction units, a wide range of coal qualities may be utilised making some previously NG-06

—179— now suitable.

As European studies9

show

represents 40% of

the capital costs

of the

ironmaking

new

unsatisfactory indigenous coals that the cost

of coke ovens

conventional blast

furnace

non-coking coals are of potential established direct reduction steel in electric

metal bath to

processes

units producing sponge

arc furnaces, the

smelting reduction, where

plant,

coal and

produce liquid

utilising

interest. Whilst there are a iron and

newer developments oxygen are

iron and

number of melting to

will incorporate

injected directly into

the evolved

carbon monoxide

gas

pre-reduces the counter-flowing fine iron ores.

The present technologies

in coal

undergo enhancements in line

preparation processes

with blast furnace and

processes. However the optimising of the energy

are expected

alternate ironmaking

components for ironmaking

practice using some form of coal injection is highly sensitive to moisture ash content10. Enhancement of

content but less sensitive to preparation

units,

especially

continue to improve

through

on-line

the operating efficiencies

for lower

ash contents

contents,

especially

and in

desirably lower

the

fine

coals,

improvements. A potential new technology

present coal

instrumentation,

of the units

sulphur contents. will

require

Moisture

technological

is the microwave drying

coals11 which operates by the absorbtion of microwave

will

and provide

of fine

energy by moisture,

without excessively raising the temperature of the coal.

Microwave technology is further offering a new approach to the smelting of metals. In

applying

examining a number of

this

technology,

program

alternate smelting techniques

investigation

involving composites

of various types of coal with metallic ores12. References 1 D.R. JENKINS: 'Gas flow distribution in the ironmaking blast furnace* f Proc. Australia Japan Extractive Metallurgy Symposium, Sydney, July 1980, A.I.M.M.: 257 2 H.E. MbGANNON et al ed.: 'The making shaping and treating of steel *, USS Steel Corp., 5th Ed., 1941: 280, 288-289 3 ibid: 7th Ed., 1957: 254 4 ibid: 8th Ed., 1964: 420 5 ibid: 10th Ed., 1985: 580 6 K. KANBARA and A. SHIGEMI: 'Blast furnace dissection', Nippon Steel Report Overseas No. 10, November 1977. 7 K. CHARLTON: Metals and Materials, 1986, 2,(12): 755 8 E. STEINMETZ et al.: Metallurgical Plant and Technology, 1986, 14,(3): 24 9 R.B. SMITH and M.J. CORBETT: Ironmaking Steelmaking, 1987, 14,(2),: 50 10 ibid: 62-63 11 N.P. CHIRONIS ed.: Coal Age, 1986, 91,(12): 64-65 12 H.K. WORNER: Private communication. NG-07

-ISO-

IMPLICATIONS OF COAL PROPERTIES FOR THERMAL COAL EXPORTS

Allan G. Waters Senior Research Officer BHP CENTRAL RESEARCH LABORATORIES P.O. Box 188 WALLSEND. N.S.W. 2287

ABSTRACT The computerised mathematical model described in the paper can be used to determine the equivalent value of a range of possible coal feeds to a particular pulverised coal fired power station. The model considers the technical and economic effects of the various coal properties on station operation, e.g. required pulveriser capacity, ash collection efficiency, FGD, waste disposal etc. The information generated by the model can be used for marketing of energy coal and optimising the return from a particular coal reserve.

AGW-01

—181—

INTRODUCTION

Coal quality not only varies between supplying

countries,

but can be extremely different from the sources w i t h i n a country. Apart from coal quality, factors affecting

the

delivered coal cost include transport costs between the producer and the consumer, w h i c h will be dependent on mine and port location and will differ for each situation.

addition, each consumer operates under a particular set of regulations and cost factors, and so the problem of assigning a value on different coals for a nominated consumer

arises.

It is the purpose of this paper to describe the development and operation of a computerised mathematical model of a coal fired power station that enables the relative economic value of energy coals to be estimated. The model incorporates

both

technical and economic aspects of the generation of electricity by the combustion of pulverised coal. It can be used for calculations based on a "green field" or existing power utility. 2.

MODEL DESCRIPTION

Figure 1 shows a block diagram of the model. The first operation is to input power station parameters for the unit being considered. These include capacity (MW),

location,

standard transport charges, pulverising mill data

(number,

capacity, power consumption), emission standards, and type of ash collecting device. The coal or blend of coals to be burnt by the utility is selected and the information is transferred from data files. The type of information stored in these data files is shown in the appropriate block in Figure 1. The program then calls the BOILER subroutine w h i c h carries out a heat and mass balance. Such an operation is the AGW-02

—182—

PROGRAM E C U f E C VERSION 1

DATA FILES COAL PROPERTIES MOISTURE PROXIMATE ANALYSIS ULTIMATE ANALYSIS ASH ANALYSIS HARDGROVE GRINDABILITY INDEX ABRASION INDEX ASH FUSION TEMPERATURES SUBROUTINE BOILER HEAT AND MASS BALANCE CALL FGD BOILER EFFICIENCY NET HEAT TO STEAM FLUE GAS COMPOSITION FLY ASH COMPOSITION/CONCENTRATION COAL TONNAGE REQUIRED

CALL BOILE R.

CALL TRANS STOCKPILE MILL

SUB, TRANS SHIPPING UNLOADING OVERLAND TRANS,

SUB, STOCKP STOCKPILE & HANDLING

SUB, MILL PULV, CAP FACT PULV, LOADING PULV, POWER CON

CALL B A G F U. OR ESP SUB, ESP ELEC, PRECIP, AREA POWER COSTS CALL FGD DISPOS PERIPH JL SUB, PERIPH NET POWER OUTPUT OF UNIT

SUB, FGD % FG SCRUBBED COSTS POWER

SUB. DISPOS DISPOSAL OF ASH & FGD SLUDGE

CALL OPCOST

PRINT COAL TPA BOILER CALCS EQUIP PERFORMANCE COSTS PENALTIES/SAVINGS

Figure 1

1 SUBROUTINE OPCOST OPERATING COSTS CAPITAL CHARGES MAINTENANCE COSTS POWER COST AS FUNCTION OF COAL PRICE

Block Flow Diagram for Computer Model of Pulverised Coal Power Station Operation.

AGW-03

—183— starting point for the evaluation of all operations. The basis of this calculation is one tonne of "as received"

coal,

i.e., material made up of coal, mineral matter and water. The stoichiometric quantity of air required for combustion is determined from the ultimate analysis of the coal and the total air requirement is calculated using the specified excess air level, usually in the range of 15 to 20%. The quantity of the gases (carbon dioxide, water vapour, nitrogen, oxygen and sulphur dioxide) resulting from combustion is determined. An additional 20 to 25% of "air leakage" is allowed. The net heat available for steam production per tonne of "as received" coal is calculated by considering: heat released by coal combustion, heat required to convert water into water vapour, the sensible heat loss in the flue gases heat needed in reheating flue gases that have passed through a flue gas desulphurisation device, and losses due to radiation and unburnt carbon. By using the net heat available, net turbine heat rate* and the unit capacity (MW), the hourly coal tonnage

requirement

can be calculated, as well as gas flow rates, fly ash production rates and air requirements. Annual coal requirements are computed using the specified capacity factor (i 0 e., the proportion of time that the unit is operating). This information is transferred back into the main program for use in the other

subroutines.

Net turbine heat rate is the amount of heat needed to be

transferred into superheated steam to produce 1 kWh of electricity and is determined by the thermodynamics of the steam cycle.

AGW-04

—184— The TRANS subroutine uses the annual tonnage of coal to calculate a shipping cost and an unloading/handling cost. There is also provision for an overland transport cost to a utility. The costs of handling and storing the coal at the power utility are evaluated by subroutine STOCKP.

Pulveriser performance is evaluated in subroutine MILL. The Hardgrove Grindability Index and the required fineness of grind are used to calculate the mill capacity factor which modifies the nominal capacity to an actual capacity. The nominal capacity is the tonnes per hour of coal with a Hardgrove Grindability Index of 50 that a pulveriser will grind to 70% minus 0.075 mm (75 microns). By considering the hourly coal rate and the actual capacity of the pulverisers, the number required on line and their power consumption can be determined.

The collection of fly ash is carried out by an electrostatic precipitator or a bag filter. If the modelled power station is not yet operational, i.e., the performance characteristics of the ash collection device is unknown, subroutine ESP takes the emission standard and likely maximum current density and calculates the minimum precipitator efficiency required, the minimum specific collection area needed, the power consumption and the quantity of fly ash collected. Subroutine BAGFIL carries out similar calculations with regard to collection by a bag filter.

In many countries, a flue gas desulphurisation device is required to meet emission regulations for sulphur dioxide. The performance of such a device is evaluated in subroutine FGD. The percentage of the flue gas to be cleaned is calculated by a consideration of the sulphur dioxide level in the flue gases and the emission standard. The scrubber system used is a turbulent limestone contacting type where finely ground limestone in slurry form is mixed with the flue gases.

AGW-05

—185— Sulphur dioxide is removed by chemical reaction with calcium carbonate in the limestone. The flue gas that has been cleaned may need to be reheated to restore buoyancy before mixing with the uncleaned portion and put to the stack. The flue gas composition is then recalculated. The amount of limestone needed and the amount of FGD reject solids produced are determined. Power consumption, labour costs, maintenance charges and capital charges are also computed for this section of the power utility.

The disposal of fly ash, bottom ash and FGD wastes is a major cost item for many potential coal consumers, and the costs of this system are calculated in subroutine DISPOS. Subroutine PERIPH sums the power consumption of all the equipment used by the utility. The net quantity of saleable power can then be determined by subtracting the power consumption of peripherial devices from the stated capacity of the unit. The operating and capital charges for the power station are computed by subroutine OPCOST* Data are taken from other subroutines as well as being calculated in this section of the program. The power cost at the busbar, i.e., excluding cost of coal, is then determined. A range of coal prices is then included to give the power production cost as a function of free on board (FOB) coal price. Such information then allows the comparative values of different coals to be assessed.

The method used to assign an economic value to a coal is to select a base case or yardstick coal to which a coal price (FOB) can be ascribed. The equivalent value of another coal is the price (FOB) which gives the same power production cost as the base coal.

AGW-06

—186—

Ash Content (% db)

Moisture Content (%as received)

F i g u r e 2. Effect of Coal A s h content on coal v a l u e .

Figure 3. Effect of M o i s t u r e content on coal v a l u e .

Table 2.

content

Effect

of coal sulphur

Coal Q u a l i t y

on coal

Coal V a l u e ($US/tonne

value

FOB)

Ash

Total Sulphur

Annual

Tonnage

5 10 15 20

0.5 0.5 0.5 0.5

907700 960000 1018700 1087500

37.45 33 .70 30.00 26.15

15 15 15 15

0.5 1.0 1.5 2.5

1018700 1025900 1028600 1031000

30.00 24.00 19 .70 13 .70

Base case coal

value

AGW-07

—187— 3.

OPERATION OF THE MODEL

The capabilities of the model can be demonstrated byconsidering the effect of some coal properties on the equivalent coal value. To do this, the use of coal in a 500 MW power station is considered. It is envisaged that the power station is based on common design parameters and is located in a region which requires moderate transport costs and stringent emission control. The base coal was a 15% ash Hunter Valley coal. The effect of ash variation was determined by holding other properties constant

(except

specific energy and volatile matter which varied proportionally with ash) and allowing ash to vary from 5% to 20%. The results given in Fig. 2 indicate that coal price (FOB) is very dependent upon ash with a 5% ash coal worth approximately $US7„45 more per tonne than a 15% ash coal.

The effect of moisture on equivalent coal value is similar to ash but not as marked

(Fig. 3). Table 2 shows the

dramatic effect of coal sulphur content on coal value.

It can be seen that a 0.5 percentage point increase in sulphur content has a much greater effect on coal value than a 5.0 percentage point increase in ash content. This is because the capital and operating costs associated with Flue Gas Desulphurisation

(FGD) to meet air quality

requirements

are very high.

It must be emphasised that the prices given in this paper are indicative only and specific to the coal properties and power station factors used.

AGW-08

—188—

IMPORTANT COAL PROPERTIES FOR PCMER GENERATION

DR A LCWE ENGINEER/COMBUSTION INVESTIGATION

ELECTRICITY COMMISSION OF NEW SOUTH WALES PO BOX 5257 GPO SYDNEY NSW AUSTRALIA 2001

—189— 1.

INTRODUCTION Power stations are very capital

intensive,

the cost of building

modern power station is of the same order as the present which that station would burn over its entire l i f e . cost

penalties

because

the

whether

the station

out of

service,

ciency,

low rank u n i t s .

costs

is operating

then

additional

operating efficiency.

Coal

capital

or not.

are

Also,

Downtime incurs severe incurred

irrespective

high ranking

generation must be carried affects

both

power

worth of the coal

station

Care must therefore be exercised

units

by lower

of are

effi-

availability

and

in selecting

appro-

analysis

which

priate coal for a particular power station. In general

power

stations

have

performance guarantees are to be met. range of the most important coal

a design

coal

properties.

Within

this range

pected that full load may be carried, although at reduced

range in coal properties.

by the design coal

ex-

efficiencies.

The selection of the type and capacity of the various in the power station will be effected

with

It is also usual to have an allowable

plant

and the

items

allowable

Coals with properties outside the allowable range

may be expected to affect the unit performance, usually but not always in a detrimental manner.

the

alty/bonus should be applied when applied

to the delivered

the coal its intrinsic It the

items.

affected

then

a cost

This

penalty/bonus

price of the coal on a unit heat basis

pen-

gives

value.

is stressed

interaction

plant operation

to the coal to compensate.

between

that the intrinsic

value

the

individual

coal

and

the

of coal

a function

power

station

plant

It is therefore not possible to properly price a coal without consi-

dering the power station in which it is to be burnt. Ihis paper reviews the major coal

plant

items in

station and considers those coal properties which affect their

a power

performance.

Where possible within the confines of this short paper, an indication of the magnitude of given.

the effect of a particular

The list of properties discussed

property on a plant

item

will

in the paper is not exhaustive but

the most important properties and effects are included. 2.

COAL HANDLING PLANT This equipment

includes

the

system

for delivery

of coal

into

the

power station, the coal stockyard and the system for conveying coal from the stockyard

to the pulverizing

the power station rail

transport

site

are

mills.

is usually via

used.

Australia

a conveyor

For export c o a l ,

sea

the delivery of coal system

although

transport

road

to and

important. AL-01

—190— Stockyard

equipment

South Wales

most

variable.

uses both automatic

Ihe

b u l l d o z e r s with slot reclaim hoppers. chutes are common to all

Electricity

stacker/reclaimers

Conveyors, b i n s ,

areas o f coal handling

Commission

and mobile

New

plant such as

feeders and

transfer

plant.

The prime requirement o f the coal plant i s to maintain a continuous release o f heat of all

in the b o i l e r s .

types i s mass flow in

a permanent restricted

inability

The basic

design

tonnes per hour.

to supply s u f f i c i e n t

mass flow throughput

heat

but design mass

flow.

shortfalls

energy throughput due to plant

and maintenance costs due to d u s t ,

Ideally

Table

1 lists

a study

would

identify

which coal

coal

properties

energy in

f a i l u r e or

which

affect

out on

property will

cause

problems

numerical value o f that property.

For

plant is

either by a

the coal

that

include

temporary

increased

operating

etc.

be carried

importance e x i s t s .

for coal

the b o i l e r ,

Other problems will

Further,

o f that property may be evaluated. plant

or by i n s u f f i c i e n t

i s transported in

parameter

Clearly the worst problem

handling

plant

the

item

plant

plant.

basis

and

what

the cost e f f e c t o f a degradation

should

example,

coal

individual

be noted

a transfer

chute

that in

a hierarchy the coal

stock-

yard vhich does not d i r e c t l y e f f e c t coal

flow to the b o i l e r s will be o f l e s s

importance

the

than

transfer

chute

therefore have a l e s s l i b e r a l It empirical

important

measurements

e f f e c t i s indicated shear c e l l coal

distribution

note

are

used

directly

power

station

system

and

may

fact

design. t h a t many c o a l

inter-related

1 under contributing to determine affected

properties

with

coal

other

properties. flow

by moisture,

are

properties. For

properties. mineral

matter

This

example, However, or

size

changes.

1. COAL PROPERTIES EFFECTING COAL HANDLING PLANT

Property Specific

Coal

Table

tests may be

flowability

TABLE

and

within

Energy

Flow

Properties

Contributing Properties Moisture Ash Ultimate

Analysis

Moisture Mineral Matter Analysis Size Distribution

Effect 1? decrease in s p e c i f i c energy increases required mass throughput o f coal by ] % . As flow properties degrade coal throughput remains r e l a t i v e l y constant until catastrophic blockages occur at a c r i t i c a l flow property v a l u e . I b i s c r i t i c a l flow property value is highly site s p e c i f i c .

AL-02

—191— TABLE 1, COAL PROPERTIES EFFECTING COAL HANDLING (CONTD.)

Contributing Properties

Property

Effect

Dustiness

Moisture Mineral Matter Analysis Size Distribution

Increased operating and maintenance costs with dusty coals. Potential for increased loss of a v a i l a b i l i t y .

Combustibility (Spontaneous Combustion)

Coal Rank Size Distribution Moisture Sulphur

Specific energy of stockpiled coal decreases due to spontaneous combustion. Plant layout and procedures are dictated by spontaneous combustibility.

MILLS The pulverizing mills reduce the coal

from a top size typically of

20 to 40 mm to a fine powder with approximately 70% by weight passing a 200 mesh screen (74 um screen opening). direct on line throughput

For power generation, mills are usually

with no storage between

restriction

unless a spare mill

therefore

the mills

results

available.

and

immediately

For this

the boiler. in

reason

a loss

A mill

usual

output

mill capacity so that at least one mill is spare at unit full load.

select Despite

this spare capacity mills typically contribute to significant loss of availa b i l i t y in coal fired power plants. There are

three

types of mill

commonly used

for bituminous

coal.

These may be categorised by their rotating speed into low speed

(horizontal

ball m i l l ) , medium speed

(hammer or

attriter type m i l l ) .

(vertical

spindle mill)

and high speed

Each of these mills have different characteristics and

are affected differently by the various coal properties.

The low and medium

speed mills are most commonly used for bituninous coals and this discussion refers principally to these two types of m i l l . Mills are selected example index

the

mill

(HGI)

screen.

and

on a nominal maximum throughput.

throughput with a

tonnes/hr

product

fineness

This may for

Hardgrove 70%

Table 2 indicates the effect on throughput

grindability

passing

while

200 mesh

assuming

a con-

stant product fineness as the coal properties change by a small amount about the nominal value (large deviations may cause a non linear It is probable that the mill

response).

will physically process

substantially

greater quantities of coal than the nominal throughput, this will result in a degradation

in product fineness.

The lower

the effect on fineness of changing mill

section of

Table

2 indicates

throughput.

AL-03

—192— TABLE 2. COAL PROPERTIES EFFECTING MILLING PLANT TONNAGE THROUGHPUT

Mill

Type

Property

Total

Medium Speed

- 3% throughput for 1% moisture increase

- 1.5% throughput for 1% moisture increase above approx 1256 moisture

- 1 % throughput for 1 unit reduction in HGI

- 1% throughput for 1 u n i t reduction in HGI

- 3% throughput for 5 mm increase in top s i z e

No loss in throughput 60 mm top s i z e

Reduction in fraction passing 200 mesh screen by 0 . 3 5 ? for 1% increase in throughput

Reduction in fraction passing 200 mesh screen by 0.9% for 1% increase in throughput

Moisture

Hardgrove Gr ind ab il ity Raw Coal

Mill

Low Speed

Ind ex

Top Size

Throughput

below

BOILER The function of the b o i l e r

is to i g n i t e

and burn the coal

and completely and to ensure that the heat content o f the coal i s l y transferred

to the steam.

properly

proportioned

steam

order

that

design

peratures

cause

loss

cause loss o f

bituninous

production

steam

unit

evaporating, conditions

efficiency

important

superheating be

while

that and

the

heat

reheating

the

Low steam

tem-

temperatures

may

attained. high

steam

ignition

availability.

Table 3 of

In addition

between

reliably

efficient-

lists

coal.

properties The major

unduly high

importance

problems

quantities

encountered

unburnt

carbon

and

combustion

combustion and

flame

are

stab-

ility. In

drun

type

radiant heat transfer by

convective

heat

boilers

the

evaporation

transfer.

The

absorption

controlled by the radiating

and

slagging

transfer

the

properties.

effected

in the furnace while the superheating

problems

Table 4 l i s t s these

heat

principally

and reheating

the

properties o f the c o a l .

boiler

are

therefore

furnace

Most heat to

these

properties.

AL-04

—193— TAI3LE3iCOALPROPERTIE^

Contributing Property

Property Volatile Content

Effect Special burner design for flame stabilization required below a dry ash free volatile content of 2555

Ultimate Analysis

Carbon Hydrogen Nitrogen Sul phur Oxygen

Air requirements are affected by ultimate analysis, unit increase of carbon/hydrogen ratio increases air requirements per unit heat release by 0.8%

Fuel Ratio

Fixed Carbon Volatile Content

0.06% increase in efficiency loss due to unburnt carbon for 10? increase in fuel ratio at fuel ratio of 1c6.

Moisture

A ]% increase in moisture decreases boiler efficiency by 0.25% requiring a proportional increase in firing rate

™*

TABLE 4. COAL PROPERTIES EFFECTING HEAT TRANSFER

Contributing Property

Property

Effect

Slagging Propensity

Ash Elemental Analysis Ash Fusion Tem peratures Coal Particle Mineral Analysis

Slagging propensity generally ranked as low, intermediate, high or severe. Response to slagging propensity is function of unit thermal rating.

Furnace Wall Emissivity

Ash Elemental Analysis Wall deposit physical state

Furnace wall emissivity is typically 0*8, a decrease of ]% will increase furnace outlet gas temperature by 1.6 deg. C.

Foul ing Propensity

Ash Elemental Analysis "Active" Alkalies (Sodium and Potassium) Ash Fusion Tem peratures

Fouling propensity ranked low to severe. Response to slagging propensity is highly unit specific.

ASH AND DUST PLANT The

disposed o f .

solid

residue

from

combustion

coal

must

collected

and

This solid residue includes mainly ash but also has a fraction AL-05

—194— of

unburnt

which

fall

coal.

Eelow

large

pieces

furnace

ash

slurry.

Furnace ash may constitute

the c o a l .

Most o f the remainder

again

usually disposed

mediate

hoppers

between

s i z e between the

properties a f f e c t i n g the dust

from the

and

material

and

extracted

the

disposal

air

there

hopper,

crushed

collected

slurry.

the furnace ash heater

water

from

the

and

the

small

the d u s t ,

economiser.

Dust c o l l e c t i o n

This

total

as a

ash

plant

fraction,

inter-

Table

equipment

into

collecting

collected

the performance o f the ash collection

equipment.

hopper walls.

to disposal

10? of the

the dust

A further and

filled

furnace

pumped

t y p i c a l l y 5 to

of as

clinker

grit

lists

coal

and disposal

is discussed

and

else-

where . TABLE 5.

COAL PROPERTIES AFFECTING ASH AND DUST PLANT

Contributing Property

Property

Effect

Ash Quantity Per Unit Heat Release

Ash Level S p e c i f i c Energy

A 1% increase in ash quantity per unit heat release increases the ash and dust plant duty by 1 ? .

SIagging Propensity

Ash Elemental Analysis Ash Fusion Temperatures Coal Particle Mineral Analysis

High slagging propensity increases duty on ash extraction p l a n t . Formation of large c l i n k e r s may cause blockages in hopper doors and contribute to ash crusher problems.

Ash S o l u b i l i t y

For wet hopper systems with recirculated water, formation o f scale in p i p e l i n e s may cause problems.

Erosiveness

Ash Elemental Analysis Ash Mineral Composition

Increased erosiveness will increase in p i p e l i n e s and sluiceways.

Clinker Reactivity

wear

Some coals produce clinker in the furnace which is prone to explosive release o f energy on quenching in the ash hopper.

DUST COLLECTING PLANT The fine dust exiting

gases are exhausted ers (bag house)

the boiler must be collected

to atmosphere.

or by e l e c t r o s t a t i c

This i s

achieved

precipitators.

before

the

either by fabric In general

flue filt-

electrostatic

AL-06

—195— precipitators dominate this field

on a world wide basis although a substan-

tial committment to fabric filters has been made by ECNSW. Ihe major problem found with precipitators is excessive dust emissions.

Coal

properties

given in Table 6 ( A ) . ential

pressure,

replace

defective

effecting

dust

emissions

from

precipitator

are

The major problem with fabric filters is high differ-

excessive

emissions

are

bags.

Properties

affecting

fabric filters are listed

most

likely

due

differential

failure

pressure

in Table 6 ( B ) .

TABLE 6, COAL PROPERTIES AFFECTING DUST COLLECTING PLANT (A)

PRECIPITATORS

Contributing Property

Property

Effect A 1% increase in dust burden will increase emissions by 1?.

Dust Burden (Ash Per Unit Gas Volume)

Ash Specific Ultimate

Gas Flow Per Unit Heat Release

Carbon/Hyd rogen Ratio Moisture Level

A 1? increase in gas flow per unit heat release will increase emissions by 1 . 5 * .

Ash Resistivity

A resistivity change of 1 order of magnitude would suggest an increase in emissions by a factor of 2 .

Energy Analysis

Sulphur

(B)

General trend for reducing vity as sulphur increases, one order of magnitude per change. Below ]% sulphur, vity is dominated by other

resistipossibly sulphur resistifactors.

FABRIC FILTERS

Property

Contributing Property

Effect

Dust Burden

Ash Specific Energy Ultimate Analysis

Differential Pressure will with dust burden.

Gas Flow Per Unit Heat Release

Carbon/ Hyd rogen Ratio Moisture Level

A 1? increase in gas flow per unit heat release will increase differential pressure over the filter bags by 1%.

increase

AL-07

—196— 8.

CONCLUSIONS A wide variety of coal properties are of importance in determining

the

impact of a particular coal on a specific

properties operations.

are

intrinsic

In optimising

product should be selected

the coal,

others

a mine product,

should

also

power station on the coal.

these

by the

mining

affected

it must be recognised

recognised

that

the

accordingly.

that

there

influence

Operating methods and procedures will

the performance of a coal in a specific plant. include operational

Some of

are

to the specific requirements of the target power

station and the product quality determined It

power plant.

considerations

in the

the

influence

It is therefore necessary to

evaluation of

the most

economic

product description for a particular customer.

AL-08

6. HYDROGEOLOGY

+ 198 EXPLORATION FOR GROUNDWATER SOURCES AND HAZARDS IN COAL MINING William H. Morton

ABSTRACT The occurrence of groundwater can have a significant affect on coal mine design and operating costs. The value of this affect can and should be assessed at a preliminary level during resource evaluation since drillhole analysis is required which can be achieved at little extra cost to an existing drilling program. However; the coal exploration geologist must develop an interest in and some knowledge of the testing procedures required in order to ensure responsible data is collected. This paper discusses the importance of groundwater resources to coal mining and outlines the data collection which can be achieved by the coal geologist as an integral part of his normal duties provided minimal additional time is allowed by management.

* Principal Consultant, Australian Groundwater Consultants Pty Limited, 273 Alfred St., North Sydney

199

EXPLORATION FOR GROUNDWATER SOURCES AND HAZARDS IN COAL MINING

WATER AND MINING

Coal mines in common with other mining operations, have the necessity of a water supply and occasionally the problem of managing excess water within the workings. Water supply is required for dust suppression, coal washing, fire fighting, domestic and plant usage and in some cases, in very large quantities for power station cooling and boiler feed. At the same time, excess mine water may have to be removed from mine workings if it creates or aggravates geotechnical problems or causes special maintenance on mine machinery or time delays in mine travelling. In addition, mine water may need to be treated to be acceptable for disposal. These aspects are important factors in the economic feasibility of a mine and it is desirable therefore, that investigations into water resources and, in particular, into groundwater occurrence should proceed in parallel with coal reserve delineation. Total water resource utilisation concepts then can be employed in planning the mining and, if appropriate, the power generation operations. 2.

WATER SUPPLY REQUIREMENTS

Water supply requirements vary with climate, with various chemical standards of water usage and with the location of the mine. Table 1 sets out the major water requirement ranges as derived from experience, and the predominant controls over these requirements which can vary markedly through the year and through the life of the mine. TABLE 1: WATER REQUIREMENTS AND CONTROLS ML/d

Per Unit

Coal Washing

0-0.2

1000 tonne

Controls

Town Water Supply

0.2-0.6

1000 people

Standards

Dust Suppression

0-0.5

hectare

Salinity

Power Station Cooling

80-160

1000 MW

Water Temp/ Salinity

Fire fighting/suppression

0.1-0.3

hectare

Separate storage

Lithology

WM 1

200

3. MINE WATER PROBLEMS Apart from the simple cost of pumping water out of the mine, four broad categories of problems created by mine waters are recognised. These are increased mining cost, hazardous mining or geotechnical conditions, disposal problems for contaminated waters, and environmental impact on surrounding land. 3.1 Mining Cost Wet conditions cause mining costs to rise because of (i) increased maintenance on mining machinery (15-30%), (ii) exclusion of the use of dry explosives (200-1600%), (iii) provision of sump pumps with protection against aggressive water and high suspended solids content (waters entering mine excavations are often acidic and commonly brackish so as to be aggressive to most mine machinery), (iv) dangerous working areas which reduce productivity, (v) flooded roadways which increase travel time around mine, (vi) flow to sumps causing erosion and deposition of silt and sand on coal benches and faces, resulting in an increased need for washing or an increased as-mined coal "ash" content. 3.2 Hazardous Mining and Geotechnical Conditions Unrelieved hydrostatic pressure is a common cause of slope failure and floor, roof or bench heave in mines. To offset these conditions, suitable slope angles must be adopted and/or provision made to leave material of strength in the floors and roofs or as pillars to avoid failures. These requirements can represent major costs and frequently much coal is sterilised by such designs. Excess mine water requires efficient mine drainage and sump storage. Because mine designs generally require a downdip progress, this allows inflows to accumulate at the working face with consequent slowing down of production. Further, travel roads must be kept dry to reduce the time, and to maintain the safety, of movement of personnel. A problem occasionally encountered in coal mines is that influent water may also release gases to the mine workings. These gases may include methane and hydrogen sulphide and if not drained in advance of development can create further hazards and expense in the maintenance of properly ventilated conditions. WM 2

201

It is evident that dewatering and depressurisation of groundwaters in advance of the mining face is desirable and can alleviate some of the need for expensive geotechnical control; however, the hazards need to be well evaluated in advance and remedies incorporated into mine design. Often it is possible to vary mine design to avoid areas of extreme water hazard or inflow without resorting to dewatering or depressurisation. 3.3

Disposal Problems

Water entering mines, in addition to having a natural dissolved salt load, may become acid and take into solution hazardous metals or become turbid, coloured and sediment laden once in the mine.

Disposal of such waters in significant quantities to local streams is

normally not acceptable without settlement, filtration or chemical treatment.

Re-use

around the mine is an option in many cases, but it is then necessary to accurately predict inflows to allow an efficient mine water management plan. 3.4

Environmental Impacts

Mine excavations become the new base level for groundwater flow around them and, thus,generate a variously large or small area of water level depression which can interfere with local town water supply wells, cause saline intrusion or water quality changes, dry out wetlands and swamps as well as cause land subsidence and base level stream flow decline. The degree to which these effects are a problem depends on the nature of groundwater occurrences and the sensitivity of the environment and the other pressures present upon it. Failure to evaluate these possibilities and to identify in advance suitable management strategies can result in the development of serious problems and of high costs. 4.

INVESTIGATION TECHNIQUES

From the foregoing it is apparent that adequate groundwater studies must be undertaken in advance of mining to allow for proper project planning. 4.1

Drilling Data

The exploratory drilling program to assess coal reserves provide an excellent opportunity to gather preliminary groundwater data at very little cost. It is necessary only for the driller to be asked to record certain observations on his daily drilling sheets; this could be included as part of the specifications in his contract. However, it does require dedication and understanding on behalf of the coal geologist to make sure sensible records are being recorded and to pick up any opportunity to obtain further test data when significant water resources are encountered. WM 3

202

A list of useful data to be recorded by the contractor with assistance from the geologist as required is as follows:Penetration rate, or time to drill each 3 metre section.

Recorded with

information on the drilling technique and weight on the bit during drilling. Loss circulation zones. Caving zones. Artesian flow rates and shut-in hydrostatic pressures. Airlifted water yield rates for each successive 6 metre section as drilled. Electrical conductivity of drilling fluid or formation water at each 6 metre interval. Static water level or mud level measured before start of each shift and more frequently if possible, particularly at each coal seam.

Measured inside

drillrods. O n completion of drilling if the airlifted yield is less than 1 L/sec, the hole should be topped up to the surface with water and the water level decline measured at one minute intervals for 10 minutes and thereafter at 5 minute intervals for one hour. The original standing water level should be measured prior to testing. O n completion of drilling if the airlifted yield is greater than 1 L/sec, the hole should be airlifted for 10 minutes whilst measuring the total flow, then water level recovery measured inside the drillrods at minute intervals for 10 minutes and thereafter at 5 minute intervals for one hour. 4.2

Hydrogeological Data

The geological data collected during exploratory drilling programs can provide useful qualitative information on the occurrence, distribution and boundary conditions of underground reservoirs.

A detailed fracture pattern analysis may show up weaker rock

zones either under stress or of open cracks, and a regional overview may show up crustal tension cracks as topographic lineaments.

This data plus stratigraphic descriptions can

provide a first order assessment of the degree of anisotrophy of fracture permeability. These concepts are essential in understanding the groundwater flow system and in preparing accurate predictive models.

Downhole geophysical logs of boreholes, including resistivity, self potential, gamma, neutron-neutron or density, will generally identify aquifer zones and may indicate porosity and formation water salinity distribution. Drill cores can be logged to give orientation and frequency of fractures, and a description given of the fracture surface regarding the probable passage of underground water. WM 4

203

Geological profiles across the lease area and beyond can be prepared to include the position of the static water level surface and any faults, splits, pinch-outs, etc.. The depth of weathering is significant to groundwater interpretations as is the location and reduced level of any springs, rivers and lakes in relation to the outcrop or subcrop of coal seams and aquifers. 4.3

Monitoring Data

When exploration licences stipulate that all exploratory holes should be grouted on completion, there should be some consideration given to the value of utilising the holes as permanent geotechnical and hydrological observation points. Over any proposed mining area it is important to know the slope of the surface of the groundwater in the aquifer ie., the water table, and how this surface fluctuates with time, rainfall and runoff. A spread of exploratory holes should be retained as piezometers, with separate and cemented isolations to each aquifer, and the water level in these should be measured weekly or bi-weekly for one climatic year. At the same time records of rainfall and catchment(s) runoff should be collected. This data will be utilised in determining the amount of seasonal recharge or infiltration to the underground reservoir, and the likely variation of hydrostatic pressures for slope stability designs. 4.4

Hydraulic Parameters

Having established the location, shape and boundary conditions of any groundwater flow zones, it is then necessary to determine the parameters controlling

groundwater

movement. The size and vertical distribution of the hydraulic conductivity (permeability) is important, as is the bulk transmissivity and storativity of the total strata sequence as measured under long term pumping stress. A variety of testing methods are available and the final choice will depend upon site specific conditions and objectives. This is a critical design level of investigation and it is advisable at this stage to have the assistance of a specialist in groundwater hydrology. 5.

APPLICATION OF FIELD DATA TO MINE PLANNING AND DESIGN

The field data and observations are first to be used to develop a conceptual model of how the groundwater system operates in a dynamic state. This requires the use of the water table contours to determine hydraulic gradients and flow paths, and utilising the hydrogeological and hydrochemical data to determine flow boundaries and their nature.

WM 5

- 204 -

For a hardrock anisotropic aquifer, a groundwater hazard map can be prepared to show potential zones of higher flow and likely ground instability to be avoided in mine design. Finally, for any rock type a mathematical model can be designed to simulate the current groundwater reservoir behaviour, and to predict changes due to various mining methods, or to predict inflows at different times in the mine design life. 6. CONCLUSION Mine planning and construction requires a sound knowledge of the local and regional hydrogeological conditions. This knowledge can be used to reduce problems of inflow drainage and disposal, as well as for the provision of a water supply. Much of the data required for preliminary design assessment can be obtained during the resource evaluation drilling programs. The level of value of data depends upon the interest of the supervising coal geologist who should seek the responsibility for useful data collection and refer to a specialist hydrogeologist when groundwater is seen to be of significance in mine designs. 7. REFERENCE Australian Groundwater Consultants Pty Limited, 1984. Effects of Coal Mining on Groundwater Resources in the Upper Hunter Valley. New South Wales Coal Association Publication.

WM 6

—205—

AN OVERVIEW OF SOME GEOLOGICAL AND HYDROLOGICAL STUDIES CONDUCTED BY THE SECV FOR OPEN CUT BROWN COAL MINING IN THE LATROBE VALLEY, VICTORIA

J C BRUMLEY SENIOR ENGINEERING GEOLOGIST COAL RESOURCES DIVISION FUEL DEPARTMENT STATE ELECTRICITY COMMISSION OF VICTORIA SWANN HOUSE, 22 WILLIAM STREET MELBOURNE

3000

—206—

INTRODUCTION

This extended abstract presents an overview of a selection of geological investigations, with an emphasis on the hydrogeological studies which are conducted by the State Electricity Commission of Victoria (SECV) for the planning and operation of brown coal open cut mines in the Latrobe Valley, Victoria, One of the world's largest brown coal deposits is mined by the SECV for the generation of approximately 80% of Victoria's power requirements.

The

total coal resource in the Latrobe Valley is estimated at some 82 000 Mt of which some 47 000 Mt is classified as available from conceptual open cuts (VBCC, 1983).

Annual production is currently approximately 41 Mt, derived

from open cut mines at Yallourn, Morwell and Loy Yang (Figure 1).

A total

of some 900 Mt has been mined since the deposits were first worked in 1888.

FIGURE 1 : Latrobe Valley The major coal seams occur in the Latrobe Valley Depression (LVD) which lies within the on-shore portion of the Gippsland Basin as shown in Figure 1.

The LVD contains up to about 770 m of Tertiary sediments

comprising thick brown coal seams separated by clays and sands and some basalt flows along the western and southern margins of the Depression. illustrative N-S geological section through the LVD is shown in Figure 2 (Holdgate, 1987).

JCB-01

TRARALGON

SYNCUNE

YALLOURN

OPEN

CUT"

<s> <D

-J

C-.

CO 1

FIGURE 2 : N-S Geological Section, Latrobe Valley (from Holdgate 1987)

—208—

The economic importance of the coal deposits is due mainly to their proximity to Melbourne (approx 140 km), the low overburden to coal ratio in the mining areas (usually less than 1:5), the thick and extensive nature of the coal seams, the remarkably low ash content (1-3%, dry basis) and the nature of the coal, overburden and interseam sediments, which allow mining by bucket wheel excavators without the need for blasting. These attributes are offset to some degree by a relatively high moisture content usually in the range of 53 to 67%, with a correspondingly low net wet specific energy (NWSE) of about 6.5 to 10.2 MJ/kg. There is also a need to relieve high artesian pressures which would otherwise cause instability problems in the deeper mines. The coal quality factors restrict the utilisation of the resource primarily to thermal power generation at the mine site, although a high potential exists for economic conversion to oil, the production of solar dried brown coal and for use as a feed stock of relatively pure carbon for industry. Other uses include the production of char and briquettes. The variable properties of the low rank coal and the associated stratigraphic complexities, present an interesting variety of geological and hydrogeological challenges to the mining and power generation activities. These challenges are effectively met by a responsive coal resource service at a cost amounting to less than 1% of the overall direct annual expenditure on open cut development and coal production. The major geological and hydrogeological inputs to this service involve stratigraphic, coal quality and groundwater resource evaluation studies. Geotechnical studies are not covered directly in this presentation, but are documented for instance by Washusen and Fraser (1982) and Ballard et al (1987). The organisational structure and methodology by which these studies are achieved is described by Barton (1982, 1985, 1987) and Gaulton (1982). Comprehensive documentation of the coal geology is given by Gloe (1976, 1984), Gloe and Holdgate (1987) and Holdgate (1987). Data management practises are summarised by Gill (1982) and Barton (1987), and drilling and sampling procedures by Gloe arid Badenhope (1976) and Barton (1987).

JCB-03

—209—

GEOLOGICAL STUDIES

2.1

Stratigraphy

Stratigraphic studies of the LVD form the basis for all resource evaluation and groundwater studies for planning and operating the open cut mines. They are the key to optimal resource utilisation. A typical example of the application of stratigraphy to regional coal seam and aquifer delineation is shown in Figure 2. This geological section was prepared by Holdgate (1987) on the basis of strategically placed investigation bores drilled between Yallourn and Churchhill. The new and existing borehole data, including geophysical logs, were interpreted to provide a significantly enhanced depiction of the strata in this area. The application of stratigraphy within the open cut areas is well illustrated by Holdgate's (1985) stratigraphic interpretation of the complex distribution of aquifer sands at Morwell Open Cut (MOC). Accurate delineation of the distribution and continuity of the sands is necessary to achieve effective placement of bores required to relieve the artesian pressures and to monitor the drawdown to ensure stability of the mine. The detailed stratigraphy also provides the basis for predictive mathematical model studies of the artesian dewatering requirements at the mine. 2.2

Coal Properties and Quality

Applied research into the chemical, physical and petrological properties of brown coal is carried out to achieve optimal utilisation of the coal resource. As described in the introduction, the coal is characterised by high moisture content, relatively low NWSE and low ash. Petrology studies incorporating physical, chemical and petrographic analyses of the range of brown coal lithotypes in Latrobe Valley coals have been documented by George (1975, 1982), and Allardice et al (1978). The studies have demonstrated a correlation of lithotypes with maceral composition (particularly liptinite), volatile matter, hydrogen content and specific energy (George 1982). For example, the relationship between total liptinite and lithotype for various coal seams is shown in Figure 3 (pers JCB-04

—210—

comm P Bolger).

Mapping of lithotypes in the coal face and in large

diameter bores has therefore become an important input to coal utilisation studies.

Lithotype

FIGURE 3 : Correlation of % total liptinite with lithotype for various coal seams. The nature of the ash forming constituents in the brown coal warrant special attention because, even though the ash content is low, severe fouling problems occur in the power station boilers.

Different ash

minerals can be formed from soluble salts and discrete mineral particles in the coal according to the combustion conditions (Kiss and King 1977).

The

ash is formed primarily from sodium, magnesium, calcium, aluminium, sulphur, iron and sodium chloride. Barton (1987) describes the methods used to analyse coal quality data.

One

of the major advances described is the use of geostatistics to enhance power station performance by predicting NWSE, ash, moisture, sodium and sodium in ash for specific blocks of coal.

Webster (1986, 1987) shows that

good correlation is obtained between the geostatistical predictions and the power station samples.

JCB-05

—211—

Interesting studies into the origin of ash in the Latrobe Valley brown coal have been carried out by Kiss et al (1985) and Brockway and Borsaru (1985). On the basis of the hydrogeological model of the LVD (Brumley et al 1981), evidence has been documented which suggests post depositional migration of inorganics through the coal seams under the influence of upwards leakage and surface discharge from confined aquifers in the LVD.

The studies

suggest that the distribution of inorganics results mainly from this seepage through the coal rather than from the inorganic constituents in the original plant material.

Brumley and Holdgate (1983) also suggest that

slow discharge of artesian groundwater played a significant role in maintaining the LVD coal forming environment in the absence of the periodic influx of water and sediment commonly associated with the accumulation of massive coal deposits. 2.3

Artesian Groundwater

Artesian water pressures at LYOC and MOC need to be controlled to ensure stability of the open cuts.

The SECV is also accountable for the regional

effect on the groundwater resources and the consequential regional subsidence. Artesian water is extracted from the aquifers and the reduction of aquifer pressures is monitored in a comprehensive network of bores around the open cuts.

Aquifer pressures have been reduced for up to 25 to 50 km east of

MOC due to the extraction since 1960 of some 530 GL which is equivalent to approximately two tonnes of water for every tonne of coal mined. Dewatering at LYOC commenced in 1984.

Up to some 18 GL per year will be

extracted, totalling approximately 200 GL by the year 2010.

Artesian

dewatering at LYOC will be more difficult than at MOC because of the complex stratigraphy of the Loy Yang Dome, the low permeability of the aquifers and the intersection of the aquifer sequence by the mine.

LYOC

dewatering will interact with that from MOC and will also contribute to the regional subsidence which now extends some 25 km from MOC. The amount of aquifer water extracted is strictly limited to that required for open cut safety.

The water is of excellent quality and is incorporated

into the water supply for the power station and mine works.

JCB-06

—212—

Mathematical modelling of the groundwater system was commenced by Fraser (1980) and further developed by Evans (1986) and Bolger (1987).

The recent

work has refined the prediction of the rate of dewatering from MOC and LYOC according to revised open cut development plans.

In particular, there is a

reduced requirement for dewatering at LYOC, which necessitates a comparable increase in dewatering at MOC.

Likewise, reduced dewatering at LYOC

indicates that regional potentiometric surface contours will not be lowered as much as previously estimated by Evans (1986). The major challenge in the groundwater studies is the development of a realistic hydrogeological model on which to base the predictive mathematical model studies of future open cut dewatering requirements and the regional effects thereof. Two basic components exist in any hydrogeological model. stratigraphy.

The first is the

The detailed delineation of the strata and their lithology

enables discrete aquifers and aquifer systems to be selected.

The Tertiary

sands, silts and gravels comprise the aquifers together with the jointed coal seams in the stress relieved areas around the open cuts.

Elsewhere,

the coal seams function as aquitards along with the clay strata.

The

categorisation of the strata into aquifers and aquitards is on a relative permeability basis and is somewhat subjective because a full spectrum of permeabilities actually occurs within the Tertiary sequence.

The

designation of the coal seams as regional aquitards contrasts with the major Australian black coal Basins in which the fractured coal seams invariably form the aquifers (Brumley 1985).

The Lower Cretaceous and

Palaeozoic rocks are also being studied to determine if they make a significant contribution as fractured rock aquifers to the groundwater system in the Tertiary strata. The second basic component of the hydrogeological model is the water balance.

To achieve realistic modelling of a groundwater system under

stress from major extraction, it is necessary to know the relative proportion of water yield derived from recharge, from the aquifers and from the aquitards.

By attributing approximately 90% of the volume of regional

subsidence to the yield from consolidation of the aquitards, Brumley and Reid (1982) demonstrated that aquitard yield represents approximately 30% of the total volume extracted from MOC.

The work was extended by Evans

(1986) to show a linear relationship between subsidence and pumpage with

JCB-07

—213— time (Figure 5).

The contribution from recharge was estimated by Brumley

and Reid (1982) and refined by Evans (1986) by calculating a "potentiometric yield" from the aquifer sequence and deducting this volume together with the aquitard yield from the total volume extracted from the LVD.

The balance of approximately 45% of the water extracted was

attributed to recharge.

The estimate was substantiated by field

identification of recharge areas (Brumley, 1981). 420

I40i— 130

•

120 MO

£ l O O

Pumpag®

330

- 300 ^

§90 j*

390

- 3 6 0

• Subsidence

270

:u " 240> -j 210 • O

7 0 -

180 o

150

3 , 4 0 W

3 0

120

o o

10 -

60 30 >

I 9 6 0 61

62 63

° 64 65

79 80

T I M E (Year)

FIGURE 4 : Cumulative volumes of subsidence and pumpage with time (from Evans 1986). This synthesis of stratigraphy, hydrogeology and monitoring of regional water levels and subsidence, forms the basic hydrogeological model upon which the predictive mathematical modelling for open cut dewatering is based.

Detailed strati graphic work in the complex Loy Yang area is

currently being carried out to improve the modelling for LYOC.

CONCLUSION

A selection of studies has been presented to provide an insight into the geological and hydrogeological work performed for planning and operating the open cut brown coal mines in the Latrobe Valley, Victoria.

ACKNOWLEDGEMENTS

The permission of the SECV to address this conference is appreciated.

The

contributions made by colleagues in providing material for this abstract is acknowledged.

JCB-08

—214— REFERENCES AT lard ice, D J; George, A M; King, T N W; and Kiss, L T, 1978. The Properties of Brown Coals from the Latrobe Valley - Results of Analyses and Tests. SECV, Research and Development Department, Rept No 357 (unpubl). Ballard, D K; Waghorne, E P; and Gleeson, H B, 1987. Brown Coal Supply for Electricity Generation Initial Operation at Loy Yang Open Cut. Coal Power '87, Aus IMM Annual Conference, Newcastle. Barton, C M, 1982. A Review of Brown Coal Geology by the State Electricity Commission of Victoria. Aust Coal Geology, Vol 4, Pt 2. Barton, C M, 1985. Factors Governing the Effective Planning and Conduct of Exploration and Geological Evaluation of Lignite Deposits. Proc. Conference on Lignite Mining in Thailand, EGAT Bangkok. Barton, C M, 1987. An update of Brown Coal Geology Practice in the State Electricity Commission of Victoria. Australian Coal Industry Research Laboratories Journal (In Press). Bolger, P F, 1987. Regional Groundwater Investigation of the Latrobe Valley, Stage III Mathematical Modelling Studies. SECV, Geological Report No 41 (unpubl). Brockway, D J; and Borsaru, R M, 1985. Ion Concentration of Profiles in Victorian Brown Coals. Proc. Int. Conf. on Coal Science, Sydney, NSW. Brumley, J C, 1981. General Hydrogeology of the Latrobe Valley Depression and the Effects of Aquifer Dewatering at Morwell Open Cut, Victoria. Proc. Seminar on Groundwater, April 8, Aust Geomechanics Society, Vic Group. Brumley, J C, 1985. Hydrogeological Aspects of Representative Major Black Coal Deposits in Australia. Advanced Surface Mining Technology Project Report. Warren Centre for Advanced Engineering, University of Sydney. Brumley, J C; Barton, C M; Holdgate, G R; and Reid, M A, 1981. Regional Groundwater Investigation of the Latrobe Valley 1976-1981. Joint SECV/DMEV Rept (unpubl). Brumley, J C; and Reid, M A, 1982. An Investigation of the Groundwater Resources of the Latrobe Valley, Victoria. Aust Coal Geology Vol 4 Pt (2). Brumley, J C; and Holdgate, G R, 1983. An Assessment of the Confined Groundwater of the Moe Swamp Basin, Victoria. Int. Conf. on Groundwater and Man, Sydney, Vol 1. Evans, R S, 1986. A Regional Groundwater Model for Open Cut Coal Winning in the Latrobe Valley, Victoria. AWRC Conf. Groundwater Systems w Under Stress, Brisbane. Fraser, C J, 1980. Latrobe Valley Regional Groundwater Investigation. Initial Mathematical Modelling Studies. SECV, Development Div Report (unpubl).

JCB-09

—215— Gaul ton, R J, 1982. The Role of the Mine Geologist in Brown Coal Winning. Aust Coal Geology, Vol 4, Pt 2. George, A M, 1975. Brown Coal Lithotypes in the Latrobe Valley Deposits. SECV, Exploration and Geological Div Penological Report No 17 (unpubl). George, A M, 1982. Latrobe Valley Brown Coal - Lithotypes: Materials: Coal Properties. Aust Coal Geology, Vol 4, Pt (1). Gill, B F, 1982. Data Processing of Bore Records. Vol 4, Pt 2.

Aust Coal Geology,

Gloe, C S, 1984. The Geology Discovery and Assessment of the Brown Coal A Huge Fortune in Chancery. Monograph II, Aust IMM. Gloe, C S; and Badenhope, F A, 1976. Drilling Investigations for the Loy Yang Project, Victoria, Australia. National Seminar on Open Cast Mining, Neyveli, India. Gloe, C S; and Holdgate, G R, 1987. Geology and Resources Monograph on Brown Coal. Coal Corporation of Victoria, Traralgon, Victoria (In Press). Holdgate, G R, 1985. Stratigraphy of the Morwell 2 Aquifer Sands in Sectors 3 arid 4 of the Morwell Open Cut. SECV, Exploration and Geological Div Report No 31 (unpubl). Holdgate, G R, 1987. Further Latrobe Valley Regional Groundwater Bore Cross-sections. Geological Report No 42, Coal Resources Division, Fuel Department, SECV. Kiss, L T; and King, T N, 1977. The Expression of Results of Coal Analysis: The Case for Brown Coals. Fuel, Vol 56. Kiss, L T; Brockway, D J; George, A M; and Stacy, W 0, 1985. The Distribution of Minerals, Inorganics, and Sulphur in Brown Coal. Proc. Int. Conf on Coal Science, Sydney, NSW. Victorian Brown Coal Council, 1983 (VBCC). Resource Development Study, Volume 3. The Brown Coal Resource. Washusen, J A; and Fraser, C J, 1982. Stability Control and Monitoring in Deep Latrobe Valley Open Cuts. Proc. Aus IMM Annual Conference, Melbourne. Webster, R, 1986. Preliminary Assessment of Geostatistical Coal Quality Predictions of Net Wet Specific Energy, Sodium and Soluble Aluminium for Loy Yang Power Station. SECV Coal Resources Div, Technical Note No 139 (unpubl). Webster, R, 1987. Preliminary Assessment of Geostatistical Coal Quality Predictions for Morwell Open Cut. SECV Coal Resources Div, Technical Note No 152 (unpubl).

JCB-10

—216—

7. GEOTECHNICAL STUDIES

—218—

DEVELOPMENT OF AN AUSTRALIAN COLLIERY ROADWAY SUPPORT NOMOGRAM USING ARTIFICIAL INTELLIGENCE COMPUTING

SYSTEMS

J, Shepherd, K.P. Walton and Z. Nemes-Nemeth

Australian Coal Industry Research Laboratories Ltd. P.O. Box 83, North Ryde, NSW 2113, Australia.

ABSTRACT A

system

for

gathering

observational

geotechnical data for

Australian colliery roadways has been devised.

Using an expert

system

computing technique and carrying out a pilot study on a

subset

predicted

roadway

data,

58%

relationship joint

a between

properties.

collieries

the

likely

accuracy,

roof conditions can be

using

only

,the

angular

compressive stress and joints, and other Further

analysis

the

expected to identify the critical

factors required in roof support analysis.

data

from 20

observational

—219— 1.

Background

Overseas

research

nomograms

for the prediction of roadway conditions and

has

resulted in the development of several

requirements, on the basis of the mining conditions. best

known

Tunneling

support

The three

systems are the Rock Mass Rating (RMR) 1 , the 2 Quality Index (Q) , and the National Coal Board such

Roadway

Nomogram 3 .

However

none

Australian

coal

these

systems

mining

industry.

geotechnical

framework

requirements

was

and

have

data

been In

base

adopted

order

the

provide a

for examining

support

decided to develop a computerised

Expert

System to form the basis of support design.

The

expert

capable on

the

predicting support needs in Australian

history.

Australian

mine,

should

It is based on observational data gathered coal

capable

conditions,

and

provide

assessing

mines and, when applied to a particular

"learning"

presented

from

examples of the local

adapting itself to suit these conditions.

for

relatively

conditions

prior

geotechnical analysis.

rules,

collieries,

basis of the existing roof conditions and the previous

support from

system aims to provide a set of intelligent

quick to

and

cheap

proceeding

method with

further

Preliminary results and methodology

are

here.

Collection Of Roof Control Data

Statistics

roof control were

collected

from 20 collieries in New South Wales and Queensland.

The data

collected location

effecting

divisible

roof conditions, floor condition, roof

structure,

roof

geotechnical Figure

prevailing

stress

field.

roadway,

exhibiting

the

they

into various aspects as follows: site

and mining method, roof support details, roof support

deformation, roof

factors

type

information shows

lithology, and

the

an hypothetical

of factors of interest and how

interact.

JS-KW-ZN-01

—220—

F i g u r e 1. T y p i c a l m i n e r o a d w a y in e x c e s s l a t e r a l s o m e o f the f a c t o r s a f f e c t i n g r o o f c o n t r o l . 6. is compressive stress.

stress conditions, showing the m a x i m u m p r i n c i p a l

The data was collected in a systematic fashion, using an observational Using

this

techniques

method

involving

roadway

assessment

form.

form a database can be derived for analysis. are

also

valuable

preparation

for

any

Such strata

mechanics or support evaluation work.

For

the

purposes

support analysis it is imperative to be

able to compare the nature of the roof with support deformation and

behavi our ,

assessment

and

whether

able

support

to make an objective visual is functioning adequately and

providing

safe

roadway

necessary

devise

for

assessing

conditions

and

support, to ensure data consistency.

A system

was devised, based on quantitative measurements of

ratings

sag, rating

guttering

height,

conditions. a

set

support

was

guidelines

deflection

found

and

so on.

The

system used for roof conditions is provided in Table 1.

The example roadway shown in previous Figure 1 would be given a "4" roof rating.

JS-KW-ZN-02

—221—

Table 1.

Roof condition ratings in order of increasing

Index No.

severity

Roof Condition Description

Good, tight, no cracking/some

Dilated cracks/scaliness up to 0.3 metres in height

Heavy to very heavy*/gutter up to 0.5 metres in height /falls* up to 1.0 metre in height

Gutters greater than 0.5 metres in height/very heavy with falls up to 1.0 metres in height or gutters up to 0.5 metres in height

cracking

Falls greater than 1.0 metre in height *

Very heavy roof has sag estimated as greater than 0.2 metres. A fall is an opening higher than 0.3 metres.

Data collection was carried out with two specific aims:

to examine areas of similar roof lithological

types

but with different support installed, and

ii.

examine

different roof condition types with or

without contrasting support types.

was

sets. was

considered that this approach would yield optimum data Unfortunately the stand-up time for the roadways

studied

generally unknown and therefore, the data does not account

for the time

2.1

factor.

Preliminary Data Analysis

Initial analysis of the data revealed that increasing the angle between

the

direction condition. varied been

maximum

(stress"RAD)

horizontal had

stress

and the roadway axis

debilitating effect on roadway

Also, at a constant stress'RAD, roadway

depending presented

roof bolt anchorage.

conditions

These results have

more fully elsewhere 4 , and are consistent with

those of other workers, which show a strong correlation

between

roadway

Southern

conditions

and

Coalfield, New South Wales

stress^RAD .

correlation may be identified

within

the

The present study shows that this statewide.

JS-KW-ZN-03

—222—

However,

traditional

additional

results.

statistical

analysis

yielded

few

Critical factors appeared to be disguised

the lack of time-dependent information and by the amount of

erroneous

noisy data.

It was apparent that information of

this nature required expert interpretation, preferably with the assistance

expert

system

to cope with the large data

sets. 3.

Expert System Development

3.1

The Principle Of Rule

Expert

systems

designed

are

Induction

a new, 5th generation computer

manipulate

databases.

Their

aim

concepts is

decision-making,

supplying

available

a human expert.

the

from

expert

system

can

rather

than

duplicate

answers

technique, numbers

human reasoning in

of a quality usually only

Like their human counterpart,

justify

answers

through

a chain of

reasoning.

Expert systems can be used as 'smart assistants', handling

capable

experts

to interpret the more difficult problems.

routine tasks efficiently, freeing human As they are

capable

be used to

justifying

decisions

they

can

deliver knowledge in a readily usable form. becoming

increasingly

also

Expert systems are

common in mining industry

applications,

such as methane risk prediction and shearer fault d i a g n o s i s 6 . The

structure

inference may

produced

expert.

The

expert

decisions

dealing

with

if-then-else The

expert

system

usually

consists of an

engine, capable of manipulating general rules, which from

rule

examples

induction

create

similar

of decisions provided by the process uses these examples of

a general decision procedure

circumstances,

often

the

for

form of

rules.

expert system building tool chosen for rule-generation was

Rulemaster 7 , generates

which

uses

the

ID3 algorithm 8 .

This algorithm

a tree of questions which will most efficiently

find

JS-KW-ZN-04

—223— a solution, based on the examples provided. agrees

with

all

decisions

reached

The generated tree the example set, and

generalises

space.

very useful side effect of this is that it becomes

reach decisions for other parts of the example

readily

apparent

which

criteria

are significant in reaching

conclusions, and which are superfluous or rarely used. The ID3 algorithm always selects the statistically most powerful indicator first, and therefore frequently occurring factors often appeared to overshadow features which might be real causes. It was necessary to structure the data set by removing such factors. Additionally, only one goal could be determined in each rule. It was therefore decided to divide the data into subsets, each of which could be used to establish a sub-goal. These sub-rules could then be integrated to form a more general rule, in this case predicting the support requi rements.

3.2

Prediction Of Roof Conditions From Joint Properties

The

effect

of joint properties on roof condition is used here

example

(Figure

the

rule-making

process.

shows

the

role

collected joint data in

the

The flowchart

determining the overall support requirements of a roadway.

The

SUPPORT REQUIREMENTS

ROADWAY

EXPECTED ROOF

REQUIREMENTS

PERFORMANCE

ROCK

STRESS

JOINT

ROOF

STRENGTH

CONDITIONS

SEVERITY

TYPE

STRUCTURE

JOINT

LENGTH

TYPE

SPACING

TREND

Figuro 2.

PREVIOUS SUPPORT

CONDITION

JOINT"RAD

JOINT'STRESS

JOINT"FAILURE

AMOUNT OF JOINT CONTROL IN FAILURE

Flowchart showing role of j o i n t dat. in p r e d i c t i n g support r e q u i r . - n t s .

JS-KW-ZN-05

—224—

roof

conditions

geotechnical

suggested

data

are

used

joint

geometry

and

other

with factors such as the roadway

properties

and the previous support history, to provide a rule

governing

support

requirements.

The sub-rule predicting

roof

condition from joint and stress geometry is given in Figure 3.

This

rule

where

the

THEN

underlined

equivalent

statements

a series of IF THEN are

denoted

statements, arrow.

The

section of the decision tree would be intepreted as

follows:

and

joint*RAD<46

and

s tress ~RAD<72

rating

This tree was generated from a

condition small

is joint controlled and if stress"RAD>22

will

then

the

roof

test data set of 65 examples of jointed roof, taken from

the Southern

failure

assist

Coalfield.

intepreting

the

tree,

rose

diagrams

were

constructed,

showing the angular relationships between

roadway

and

stress that were identified as significant by the

decision

making process.

associated

roof

(FAILUREJOINT CONTROLLED]

condition

joints,

Each geometrical relationship has an rating,

; [STRESS"RAD1 <22 : t J O I N T T Y P E j ZONE : - > (4, g o a l ) RECURRENT : - > (5, g o a l ) INDIVIDUAL : - > ( p r i n t s "II given II", >22 : [JOINT'RAD]

predicted

the

rule.

no e x a m p l e goal)

<^6

STRESS"RAP] <72 : -> T 3 , g o a l ) >-72 : [STRESS*RADT^ <291 : -> (4, g o a l ) >=291 : -> (2, g o a l ) >46 : [ J O I N T T R E N D ] <43 : ( S T R E S S " R A D ] <275 : -> (3, g o a l ) >-275 : -> (5, g o a l ) >43 : -> (3, g o a l ) : [JOINT"RAD] <56 : I J O I N T " R A D ] <5 : -> (1, g o a l ) >-5 : I J O I N T T R E N D J <41 : ( S T R E S S " R A D ] <293 : -> (3, g o a l ) >-293 : - > (1, g o a l ) >-41 : - > (3, g o a l ) >56 : [ S P A C I N G ] <650 : => (3, g o a l ) >-650 : ( J O I N T T Y P E ] ZONE : «> (4, g o a l ) RECURRENT : -> (3 f g o a l ) INDIVIDUAL : J STRESS"RAD] <67 : => (4, g o a l ) >-67 : (JOINT"RAD] <79 : => (3, g o a l ) >-79 : -> (4, g o a l )

F i g u r e 3. Induced s u b - r u l e , p r e d i c t i n g roof c o n d i t i o n rating g e o m e t r y , w h e r e the c o n d i t i o n r a t i n g r a n g e s f r o m 1 to 5.

Figure

Figure Figure

4c 4a

1 . ^Figure

Figure Figure

from

4d 4e

joint

JS-KW-ZN-06

—225— Selected

rose

diagrams

are

provided

Figure 4, with the

branches to which they refer indicated in Figure 3.

can

seen

stress

field

opinion

suggests

from Figure 4 that the joints and prevailing

form

stress'RAD

has

(Figures

4a,b,c).

minor

role

interactive

failure

debilitating

system.

joint

Even when expert

controlled,

increasing

effect on the roof conditions

Conversely, where joints appear to play a

in the failure mechanism, roof condition may still

deteriorate principal

an that

the

angle

compressive

between the joints and the maximum

stress decreases (Figures 4d,e).

Where

there is no evidence of excess lateral stress the presence of a joint zone may have a worse effect on the roof condition than a recurrent

joint

set

significant mechanistic

Examination that

the

Southern

similar spacing.

These results have

implications.

of the decision tree in previous Figure 3 suggests joint

trend may also be a significant factor in the

Coalfield.

Magnetic

bearings

43°

and

41°

JOINT CONTROLLED FAILURE MECHANISM

RAO

RAD

KEY

c. coof cating-2

a. coof cating-3 FAILURE MECHANISM NOT JOINT CONTROLLED

RAD

d. coof cating-3

RAO

e. coof cating-4

RAD

£. coof eating4 foe a joint zone 3 foe cecuccent joints

Figuce 4. Rose diagrams ;showing the relationships between joint, stress and roadway direction (RAD), <and roof condition rating, identified by the decision are the maximum and minimum lateral stresses. tree in Figure 3. 6. and1

JS-KW-ZN-07

2 2 6

r e s p e c t i v e l y , were identified as significant cut-off points for both j o i n t - c o n t r o l l e d and n o n - j o i n t - c o n t r o l l e d

failure.

of less than 4 3 ° were indicators of generally worse A

Trends

conditions.

balloon diagram of trend frequency (Figure 5) shows that two

of the three sets identified have trends less than 43°. The

decision

tree

shown

Figure 3 was used to generate a

small expert system capable of predicting the roof condition on the

basis

of the induced rules.

A section of a roof analysis

session is given in Figure 6, with user responses in u n d e r l i n e d type.

The rest of the text is produced by the expert

3.3

Implications For R e l i a b i l i t y

was

found

predicted were

that

with

predicted

conditions

sample

analyses roof conditions were

an accuracy of 58%. wrongly,

system.

was

H o w e v e r , where

conditions

found that 60% of the wrong

erred by only one rating, and most of these

were

in the "2" to "4" range.

seen

that

this

could represent a difference of less than 100

PftOJtCI Nit 013* J.I.I U,rnU

LOCALITY No. 1 BALLOON

ratings

From previous Table 1 it can be

JO-JUl-l w

Mmifiifluam 1

DENSITY

joint controlled [y,n,xnal n joint"RAD 58 average spacing 300 Roof rating is 3 Enter 's' for stop or RETURN to continue : RETURN joint controlled [y,n,xna] n joint"RAD 78 average spacing 5000 Joint type is: zone recur rent individual NA [ a , b, c , d 1 c stress'RAD 500 joint'RAD 7_8 Roof rating is 3 Enter

's' for stop or RETURN to continue : RETURN

N = 65 ALPHA= 0 . 5 0 RAW D E N S I T I E S

joint controlled [y,n,xna] n joint"RAD 18 joint'RAD 18 joint trend 18 stress"RAD 85 average spacing 300 Roof rating is 3

ACIRL MINING RESEARCH

Enter 's' for stop or RETURN to continue : s

Figure 5 . Balloon density diagram of joint trends identified in the Southern C o a l f i e l d .

Figure 6 . Sample roof analysis session based on rules in decision tree in Figure 4. Angles in degrees and all other units in millimetres.

JS-KW-ZN-08

—227— millimetres inaccuracy system.

failure

the

height.

data

Therefore this could

collection

reflect

rather than in the

expert

It would suggest that with refinement the system would

certainly

be able to distinguish between the three broad

cases

interest

"2""),

most

poor

roof

72%

the operator; good

5").

the "3" conditions were predicted accurately, the

fact

that

65%

consisted

"3"

conditions.

example

set

was

loaded

should

the

initial

possibly

example

set

This would suggest that as the

equally

conditions, accuracy in predicting

with

the

these would also

extreme

improve.

be noted however, that it is difficult to determine

whether

answers

common

ones.

anomalous

have

been

modelled

unusual examples or

Further structuring of the data set to eliminate

examples should yield increased

be stressed

reliabilty.

It must

that the reliabilty of such a system depends on the

veracity and scope of the

(a "1 or

(a "3" or "4") and roof likely to fall

reflecting

roof

examples.

Conclusions

Observational

geotechnical data gathered

roadways

essential

investigation

into support

Geotechnical

data

for

underground

precursor

for

gathering must be

systematically

performed and this can be best achieved using the assessment

expert

system

assessment.

was

may

technique

be capable of providing a rapid, for

initial

roadway

support

The method could be tailored to suit

conditions.

consisting

roadway

form.

reliable

colliery

any

requirements.

Using

small

joint and compressive

possible

acccuracy of 58%.

predict

roof

pilot data

stress

local set,

information,

conditions

with an

Results suggest that reliability

could

JS-KW-ZN-09

—228—

substantially

i m p r o v e d by the u s e of f u r t h e r

s e t s and a d d i t i o n a l roof The

i n t e r a c t i o n of the n u m e r o u s v a r i a b l e s a f f e c t i n g

performance

factors

may

presence

ratio,

variables,

mechanistic

roof

amongst

Typically,

and

may

implications.

rock p r o p e r t i e s ,

joints

and

the

others.

Future work

using

therefore By

have

leading

significant

clearer effective

design.

Acknowledgements are g r a t e f u l for f u n d s from the D e p a r t m e n t of N a t i o n a l and

Energy

Development

and

Demonstration

(Reference 82/2111).

under

the

National

Program

Energy

Contract

Energy

Research No.

M i n e m a n a g e m e n t s a r e a l s o t h a n k e d for

g e n e r o u s h e l p a n d s u p p o r t in g a t h e r i n g d a t a for t h i s

525 their

study.

References

B I E N I A W S K I , Z . T . (1974) Geomechanics classification of rock masses and a p p l i c a t i o n in t u n n e l l i n g . A d v a n c e s In R o c k M e c h a n i c s , P r o c e e d i n g s Of The T h i r d Congress, Denver, vol.11(a). p.27-32.

the

roadway

of the roof f a i l u r e m e c h a n i s m , it is h o p e d

Development

roof

important

the e x p e r t s y s t e m w i l l c o n t r i b u t e to m o r e

support

stress^RAD, absence

understanding

complex system.

e x p e r t s y s t e m can e x a m i n e the r e l a t i v e i m p o r t a n c e

these

that

width/height the

example

information.

its ISRM

B A R T O N , N . , L I O N , R . , AND LUNDE, J . (1974) Engineering classification of rock m a s s e s for d e s i g n tunnel support. ROCK M E C H A N I C S , v o l . 6 , p.189-236. NATIONAL COAL BOARD WORKING PARTY REPORT (1972) D e s i g n of m i n e l a y o u t s . Working Party Report, National Coal Board Department. p.13-18.

Mining

JS-KW-ZN-10

—229— 4.

SHEPHERD, J., RIXON, L.K., WALTON, K.P., HUMPHREYS, D.R. AND O'BEIRNE, T. (1987) Geotechnical methods for the evaluation of underground roadway conditions and support requirements, Australian Coal Industry Research Laboratories Ltd, Published Report 87-2,

GALE, W.J. AND BLACKWOOD, R.L. (1987) Stress distributions and rock failure around coal mine roadways. INT. J. ROCK. MECH. MIN. SCI. & GEOMECH. ABSTR. vol.24 (3). p.165-173.

COAL AGE (1987 ) Expert systems coming to mining. COAL AGE, vol.92 (.3). p. 54.

MICHIE, D. , MUGGLETON, S., RIESE, C. AND ZUBRIL, S. ( 1984 ) Rulemaster: a second generation knowledge engineering facility. Proc. IEEE First Conference On Artificial Intelligence Applications, Denver, Colorado, December, pp. 591-596.

QUINLAN f J.R. ( 1983) Learning efficient classification procedures and their application to chess end games. In: R.S. Michalski, J.G. Carbonell and T.M. Mitchell (eds.), "Machine Learning", Tioga, Palo Alto, CA. pp. 463-482 .

JS-KW-ZN-11

- 230 -

ASSESSMENT OF GEOTECHNICAL CONDITIONS AHEAD OF SURFACE COAL MINING - CASE HISTORIES FROM THE BOWEN BASIN. NIGEL H.H. GODFREY, GEOLOGIST, GOONYELLA MINE, BHP/UTAH COAL LIMITED, P.M.B., MORANBAH, QLD. 4744.

Abstract: Many years of successful operation of several very large and deep, opencut, strip coal mines, by BHP/Utah Coal Limited (and their antecedents), have seen the remedying of many problems related to pitwall instability and blasting/dragline digging conditions. This has largely been achieved by a small, in-house geotechnical team studying the geological background to the problems concerned - resulting in the production of suitable operational remedial measures in collaboration with the relevant mine staff. Many of the geological features which have been identified as adversely affecting operating conditions, particularly with regard to overburden excavation and pitwall instability, will be of general application at other mines. Some of these features are briefly outlined and further illustrated by way of case histories.

- 231 ASSESSMENT OF GEOTECHNICAL CONDITIONS AHEAD OF SURFACE COAL MINING - CASE HISTORIES FROM THE BOWEN BASIN

1.0.

INTRODUCTION

BHP/Utah Coal Limited have been operating a number of very large, open

cut strip coal mines in the Bowen Basin of Queensland for

deep,

periods

varying from 8 to 20 years (see Fig. 1).

Over

this time,

pitwall

stability

and

overburden

digging

have

been

recurring problems at certain of the mines (Boyd & Saunders '79,

Mallet

& Gerrard '80, Philp, Boyd,Eckersley * 81, Godfrey

'83),

and

studies

have

been

made to find the

and

hence

the

problems have

concerned.

been

done

Limited

Group,

the

These studies BHP-Utah

Coal

Services

either

collaboration

C.S.I.R.O.

causes

for

Technical

Geotechnical or,

remedies

with

alone, the

Division of Geomechanics

and/or consultants.

The

studies

which,

alone

established

a series of basic pre-conditions

or in combination,

contribute significantly

factors

pitwall

instability or poor digging conditions.

These

pre-conditions

geology

are

largely concerned with

and the weathering involved.

the

Accordingly,

nature

the

they also hold for

many other open cut coal areas in Australia.

The

factors pertaining have been amply covered elsewhere

(e.g.

'83), but some of the major ones are briefly shown here in diagram

Davies form

as background for the case histories which follow (see Fig. 2 and 9).

NG-01

- 232 2.0.

CASE HISTORIES

2.1.

Blasting

2.1.1.

Blasting - General

Blasting

geological

area

information

considerations

where appreciable cost savings can is properly used during

blast

made

design.

Major

(often ignored) are very briefly listed below and

shown

under in diagram form (Fig. 2). W A T C H FOR P A T C H E S OF SECONDARY CARBONATE C E M E N T A T I O N AND ESP FOR ZONE OF S U C H C E M E N T A T I O N IN B A N D 1 TO 3M T H I C K AT SS B A S E (POOR B A S E B L A S T LEAVES NEARLY UNDIGGABLE HARD SS)

H A P P I N G OF J O I N T S AND O T H E R D I S C O N T I N U I T I E S R E Q U I R E D , T O S H O W F R E Q U E N C Y AND C H A R A C T E R E . G . P E R S I S T E N C E , F I L L I N G , O P E N OR C L O S E D N A T U R E , F R E Q U E N C Y A N D O R I E N T A T I O N OF J O I N T S TO H I G H W A L L F A C E ; A L L C O N T R I B U T E T O B L A S T D E S I G N E F F I C I E N C Y .

INFERRED INHERE^ MAXIMUM STRESS DIRECTION FOUND BY S T E R E O N E T JOINT ANALYSIS OPTIMIZE BLAST D I R E C T I O N FOR DESIRED BLAST EFFECT WITH SMALLER POWDER FACTORS

(i) M A S S I V E S A N O S T O N E W I T H INFREQUENT BUT PERSISTENT J O I N T I N G T A K I N G U P M O S T OF O V E R B U R D E N D O W N TO OR 'WITHIN 1 TO 2M OF C O A L . H A R D B L A S T I N G EXCEPT WHEN W E A K .

( i i i ) 2 to 15M (AND O C C A S I O N A L L Y T H I C K E R ) U N I T S T H I N B E D D E D AND WELL JOINTED MUDSTONES/SILTSTONES; O F T E N W I T H C A R B O N A C E O U S PLANT R E M A I N S , O V E R L I E S C O A L , MAY NOT REQUIRE BLASTING. ( S A V E S COAL T O P W A S T A G E . ( B L A S T IS L O A D E D FOR O V E R L Y I N G SS AND MS IS 0 V E R 9 L A S T E D A L O N G W I T H COAL TOP))

(ii) BEDDED S A N D S T O N E S . CLEARLY B E D D E D , MAY HAVE S P A R S E , T H I N , SILTSTONE INTERBEDS. SOME B E D S M A Y BE H A R D E R C A R B O N A T E C E M E N TED (HARD D R I L L I N G ) . EASIER BLASTING BREAK U P . M A Y O C C U P Y O N L Y P A R T OF O V E R B U R D E N , OR BE P A R T I C U L A R P R O M I N E N T B E D D I N G UNIT WITHIN MASSIVE SANDSTONE PILE.

REPRESENTATIVE CALIBRATED DOWNHOLE SONIC-VELOCITY-LOGGING-INDICATED ROCK S T R E N G T H S U S E D TO A S S E S S O V E R A L L P R E S E N C E OF W E A K O R H A R D M A S S I V E S A N D S T O N E A N D B E D D E D S A N D S T O N E S FOR B L A S T P L A N N I N G . P R O D U C T I O N B L A S T H O L E C H I P P I L E S A L S O M O N I T O R E D FOR F U R T H E R I N D I C A T I O N OF R E L A T I V E D I S T R I B U T I O N OF R O C K T Y P E S P R E S E N T A N O F I N E R B L A S T D E S I G N C O N T R O L (DRILL P E N E T R A T I O N R A T E S t / O R R O T A R Y T O R Q U E V A R I A T I O N S P L U S V I S U A L I N S P E C T I O N AND N O T A T I O N ) .

MASSIVE CAPPING SANDSTONE L E F T „ INTACT

' E X T R E M E L Y OR STRONGLY^ • WEATHERED

S U B S I D I A R Y C O A L AND/ OR W E A K F I S S I L E MS HOSTS HORIZONTAL V E N T I N G OF EXPLOSIVE GASE

~ ^ THIN BEDDED .SILTSTONES „

. STRONGLY WEATHERED

MODERATELY/MILDLY WEATHERED

THIN LAYERED SILT/ M U D S T O N E S MAY N E E D M I N I M A L OR NO BLASTING M S / S S T MAY BE INCR CARBONATE CEMENTED DOWNDIP & NEED GREATER POWDER FACTOR

P O W D E R F A C T O R S R E D U C E D FOR O V E R B U R D E N M O S T L Y OF T H I N N E R BEDDED SILTSTONES.

•) EXPLOSIVE GAS VENT ALONG WEAK UNDERLYING BED/S LEAVES O V E R L Y I N G , M A S S I V E , S T R O N G SS INTACT. REQUIRES PRECISE DECKING, b ) 2 - 5 & 10M T H I C K L A Y E R E D SILTSTONES ABOVE COAL MAY NOT N E E D B L A S T I N G IF W E L L JOINTED. C) W A T C H FOR S I L T S T O N E S AS IN b ) A B O V E , B E C O M I N G M O R E CARBONATE CEMENTED AND HARDER DOWNDIP MAY N E E D I N C R E A S E D P O W D E R FACTOR.

F"RESH

J-1

t "TRESFL

(v) R O C K S W I T H H I G H D E G R E E S OF W E A T H E R I N G IN O V E R - B U R D E N S T A C K MAY BE DUG W I T H O U T B L A S T I N G . IF UNDERLYING ONLY MODERATELY W E A T H E R E D TO F R E S H R O C K S A R E T H I N B E D D E D L U T I T E S , C A N A L S O O F T E N BE DUG W I T H M I N I M A L OR NO B L A S T I N G

FIG.2.

r'T'ff

(vi) L A C U S T R I N E AND M I R E S E D I M E N T S B E T W E E N COAL SEAMS O F T E N R E Q U I R E M I N I M A L OR NO B L A S T I N G

C O M P O S I T E C O N C E P T U A L I Z E D H I G H W A L L S K E T C H D I A G R A M S S H O W I N G SOME OF V A R I O U S G E O L O G I C A L F A C T O R S B E A R I N G U P O N B L A S T E F F I C I E N C Y AND DRAGLINE DIGGING.

NG-02

Overburden

blasted

- 233

which

could

removed

solely

and

rock

dragline (see Fig. 2). b)

design

must take

account of

discontinuity

Blast

patterns

(joints,

inherent

bedding,

stresses

faults) and styles

promoting optimum throw and fragmentation for least blast cost. (Hagan '80) c)

Blast

design

must

take account of rock types

strengths

force of column charge is wasted via sub horizontal

gas

sequences blasted (Hagan & Morphett d)

Explosive venting

and

86)

along minor coal seams and/or other weak strata.

Use

properly decked charges is important, (see Fig. 2). d)

Water in blast holes to

5m and 6m above coal reduces effectiveness

and/or leads to blast procedure complications. Some

case

histories involving blasting at BHP/Utah Coal Limited

strip

mines are: 2.1.2.

Goonyella

A two seam,

single pass coal extraction will begin in the north of

the

mine in 1990 over a pit length of some 2 kms. The

Bucketwheel

Excavator has previously removed the top 15 to 35m

overburden for the next 6 strips downdip.

A truck and shovel

is to remove overburden down to 5 to 10m above the Middle dragline, is

operation

Goonyella.

to remove the remainder down to the Middle Goonyella seam,

plus the interburden down to the Lower Goonyella seam (see Fig. 3). Prior to the latter,

however,

had to be made in hard, difficulties to

an initial 120m x 700m

with fragmentation,

optimum

excavation

massive sandstones (see Fig. 4).

has

After initial

improvement has been made by changing

blast direction derived from an

analysis

inferred

maximum stress directions within the sandstone (Godfrey 8 7 ) .

For this,

following

joint

procedures inferred

analysis using stereonet projections was outlined

made

Cohen C 7 9 ) and Coles Phillips

stress directions derived,

and ('71)

again using stereonet

maximum

projections

(see Fig. 5) and a preferred blasting direction recommended (see Fig. 6). NG-03

- 234 Increased fragmentation and decreased vertical lift has resulted and the method will be used wherever feasible in future (however, often, the set nature

blasting

the

highwall strike direction limits

direction

and

"best-fit-attainable"

use

suffice).

FIG. A.

the

philosophy

GOONYEILA MINE. CLEANSKIN PIT TWO SEAH OPERATION. SCHEMA FOR ORIGINAL START UP EXCAVATION.

NG-04

optimum has

- 235 -

/ Blast direction fi®. 6 : Stress directions and blast directions; c f . to batter escalation directions

stereonet plot -Major conjugate joint p l a n e set -stress analysis

2.1.3, Blackwater Engineering design input into blasting at Blackwater has been undertaken since

1983

(Price & Thomson

*83) using geological data

derived

from

whole of mine highwall mapping and downdip geological predictive studies '83 and f84).

(Godfrey has

resulted

An average 27% overall decrease in blast costs

so far and the application of geological

information is

seen as being signal to this success.

Application of many of the parameters so far mentioned, or shown in Fig, 2,has played a large part in this exercise. 2.1.4. At

Peak Downs Mine

this

mine

25 km long mine, water in blastholes in pits in the

half

blasting Godfrey

resulted

costs.

in 5% of the mine length

absorbing

southern

20% of the

Studies were undertaken in '84 and '85 (Davies

'85,

86) to determine the origins of this water for assessment of

remedial measures.

Sources for the water are shown in Fig. 7, below.

Coal

seam and unconsolidated Cainozoic material was permeability tested

using

falling

measurement

head tests in stand pipes in both cases

and

by direct

seepage in bench and water storage sump faces

Cainozoic. NG-05

for the

- 236 This

was

combined with surface mapping to define any

areas

and

geological

mapping

of bench and

potentially more permeable strata. be

supplying most of the water,

done

sump

likely

walls

recharge identify

When Cainozoic material was found to

a close spaced drilling programme

define the Cainozoic/Permian unconformity surface and

was

outline

underground drainage paths for water leakage onto the bench. Study findings and remedial measures are depicted in Fig.8.

1 . W A T E R IN B L A S T H O L E S VIA C O A L S E A M S COAL S E A M S ARE LOW P E R M E A B I L I T Y R E G I O N A L A Q U I F E R S C O N F I N E D BY E F F E C T I V E L Y I M P E R M E A B L E S A N D S T O N E O V E R B U R D E N . IF PIT IS K E P T D R A I N E D A F T E R C O A L E X T R A C T I O N S , E X P O S E D S E A M D R A I N S BY S E E P A G E A N D IS D E W A T E R E D TO B E L O W O V E R B U R D E N . SHOULD PITWATER SUBSEQUENTLY POND ABOVE EXPOSED SEAM L E V E L , PIEZOMETRIC SURFACE LEVEL REACHES E L E V A T E D P O S I T I O N IN O V E R B U R D E N . W A T E R IN B L A S T H O L E S M A Y R E S U L T , P A R T I C U L A R L Y IN B A C K R O W S .

W A T E R P O N D A G E IN SPOIL PERCOLATES TO S P O I L B A S E A N D COAL FLOOR

R A I N RUN OFF P L U S BASE I COAL FLOOR P O N D S IN P I T

WATER TRAVELS DOWN SPOIL B A S E A N D C O A L FLOOR ""

WATER

REGIONAL PIEZOMETRIC S U R F A C E FROM C O N F I N E D EAM A Q U I F E R

SPOIL WATER

FLOODED PITS PREVENT DEWATERING OF C O A L S E A M A N D T H I S M A I N T A I N S E L E V A T E D P I E Z O M E T R I C S U R F A C E IN OVERBURDEN. BLAST HOLES WHICH B R O A C H C O A L A R E F L O O D E D TO LEVEL OF T H I S P I E Z O M E T R I C S U R F A C E IN B L A S T H O L E S VIA C A I N O Z O I C U N C O N S O L I D A T E D S E D I M E N T S

CAINOZOIC PERMIAN UNCONFORMIT

LOW PERMEABILITY COAL SEAMS (4 t o 5 X 10 - 5 C M / S E C ) ( D Y S A R T U P P E R 1 I 2 ) ( B O T H FLOOR AND S E A M S ( D Y S A R T L O W E R 1 I 2 ) ARE H Y D R A U L I C A L L Y CONNECTED)

W A T E R S E E P S O U T OF C A I N O Z O I C B A S A L C L A Y E Y ;ANDS A N D G R A V E L S O V E R 1 2 - 1 8 M O N T H S , O N T O B E N C H iURFACE A B O V E U N C O N F O R M I T Y .

P I E Z O M E T R I C " S U R F A C E IN C A I N O Z O I C IS R E G I O N A L L Y H I G H (3 TO 6M B E L O W GR0Utt£) P R O B A B L Y DUE E N H A N C E D R E C H A R G E FROM R E C Y C L E D PITWATER

WATER ENTERS BLAST HOLES INDIRECTLY V I A SS J O I N T I N G A N D D I R E C T L Y FROM B E N C H S U R F A C E RUN OFF

W A T E R 2 TO 6M H I G H IN B L A S T H O L E S

P E R M I A N COAL MEASURES SEDIMENTS (LARGELY MASSIVE LITHIC SANDSTONES) D Y S A R T COAL S E A M SERIES

FIG. 7.

PEAK D O W N S - W A T E R

W A T E R IN PIT D E R I V E D SEEPAGE

FROM R U N OFF

BLASTHOLES.

NG-06

PITWATER AVOIDS RECHARGE REGION - DISCHARGE TO CREEK NEAR SCML BOUNDARY

NEW DRAINAGE CHANNEL

V\\\

9M THICK SAND BED HERE IS PERCHED WATER PONDAGE. BED THINS TO SOUTH AND RESTRICTS DRAINAGE ^

^ P I T W A T E R DISCHARGED V INT0 CREEK S ^ ^ * SHALLOW FLOOD^BASlfl -

PITWATER SEEPS THROUGH CREEK BASE TO OLD~ PALAEOCHANNEL

OLD DISCHARGE (SLOW) TO NEARBY BOOMERANG CREEK CROSSING LEASE

PITWATER PUMPED UP FROM PIT

M U)

BASAL SANDS AND CLAYEY SANDS HARBOUR PERSISTENT DISCHARGE FROM PALAEO VALLEYS

2J 0 1 O

FIG. 8.

PEAK DOWNS - YURA PIT WATER RECYCLING PATHS

- 238 2.2

Pitvall Instability geological features which contribute to this are shown in

Some series

generalized

composite

sketches

Fig.9,

below.

the Two

illustrative case histories follow.

MAJOR SPOIL FAILURE - TWO WEDGE MODE (ADAPTED FROM P H I L P - B O Y D AND E C K E R S L E Y ' 8 1 )

BOXCUT LOWWALL FAILURES. ( N . B . B 2 CAN A L S O O C C U R IN HIGHWALLS WITH PITWARDS DIPPING UNCONFORMITY SURFACE ( P O T E N T I A L FOR V E R Y L A R G E F A I L U R E S ) )

REMNANT LOW ALL^COAL WEDGE PASSIVE WEDGE SLIDES ALONG S H E A R Z O N E IN SATURATED SPOIL AT S P O I L B A S E

POROUS SATURATED BOTTOM SANDS OPEN \ T O WATER FLOW FROM U N C O N F I N E D REGIONAL ^AQUIFER

B O X C U T LI LOW WALL

ISPSSiPfV^

IRCULAR SLIP FAILURES WEAK CAINOZOIC

\ ALONG PRESHEARED BEDDING P L A N E IN C O A L FLOOR

1 . MAJOR FAILURES ALONG PRE-SHEARED BEDDING PLANES OR B E D D I N G H O R I Z O N SHEAR Z O N E S N E A R C O A L T O P OR H I G H E R IN L O W W A L L . H A Y H A V E L O W SHEAR RESISTANCE DUE SATURATION FROM O V E R L Y I N G P E R C H E D W A T E R P O N D A G E IN U N C O N S O L I D A T E D CAINOZOIC. FAILURE POTENTIAL E X A C E R B A T E D BY C L E F T P R E S S U R E S F R O M W A T E R IN J O I N T S B E H I N D LOWWALL OPENED BY OVERBURDEN DIFFERENTIAL STRESS RELIEF . MOVEMENT INTO P I T .

HIGHWALL FAILURES-LARGE POTENTIAL. <i> I N T A C T R U P T U R E IN C O H E S I O N P O O R UNCONSOLIDATED CAINOZOIC O R , HIGHLY WEATHERED PERMIAN (ii)SLIDE-OUT ALONG EXTENSIVE,PITWARDS DIPPING BASAL FAILURE PLANE (USUALLY, P R E - S H E A R E D B E D D I N G H O R I Z O N S IN SILTSTONES/CLAYSTONES)

FAILURE MASS SLIDES ^ - - E R O D E D PERMIAN ALONG SATURATED AND HIGHLY C 0 A L MEASURES WEATHERED UNCONFORMITY SURFACE DIPPING PITWARDS

MINOR TO MAJOR FAILURES ALONG SATURATED PITWARDS DIPPING UNCONFORMITY SURFACE AS BASAL FAILURE PLANE: INTACT RUPTURE CIRCULAR SLIP FAILURES T H R O U G H W E A K , S A T U R A T E D C A I N O Z O I C IN PERCHED PONDAGE ZONES

HIGHWALL FAILURES - LIMITED POTENTIAL. (i) S I M P L E S L U M P I N G ANO S H E E T S L I D I N G IN C O H E S I O N P O O R M A T E R I A L AS © (i), BUT OUTCROP AVAILABLE LIMITS EXTENT ( i i ) B L O C K F A I L U R E S , S L I D E O U T OF J O I N T A N D / O R F A U L T L I M I T E D B L O C K S ON B A S A L O U T F A C E R P L A N E / S . ( C O M M O N IN P A R A L L E L F A U L T E D OR S T R O N G L Y J O I N T E D Z O N E S IN C O M P E T E N T R O C K )

EAR L I M I T SET Y JOINTING AND/OR

- WEAK PRE-SHEARED S I L T S T O N E OR C L A Y S T O N E BEDDING HORIZON DIPS PITWARDS AT 3* TO 2 0 * . IF E X T E N S I V E , PROMOTES VERY LARGE HIGHWALL FAILURES

(i)

A B O V E , IS O F T E N S T A R T E D BY I N I T I A L S L I D I N G ON W E A K , P I T W A R D S D I P P I N G , BASAL FAILURE PLANE

(C & D A B O V E , A D A P T E D A F T E R (ADAPTED A F T E R G Q N A N O ' 8 0 )

FIG. 9.

LOW COHESION SLUMPING I SHEET SLIDING' C O M P E T E N T ROCK! F A I L A S SEJ OF B L O C K

W i UNCONFORMITY ( - O R , C A N BE L I M I T OF H I G H L Y WEATHERED ROCK)

(ii) B E D D I N G P L A N E , J O I N T OR F A U L T P L A N E DIP I N T O PIT

DAVIES'83

C O M P O S I T E C R O S S S E C T I O N A L S K E T C H E S OF C O M M O N FAILURE MODES

PITWALL

NG-08

- 239 -

2.2.1. Perched Water Pondages and Boxcut Instability A

new 3 km long Isaacs Pit area at Goonyella mine was recently

Previous of

studies (Smith

several incised

down

the

83 and Ellery 185) established the existence

Quaternary to

lateral migration

opened.

Recent

palaeochannels.

of the nearby Isaac River,

the

Caused

palaeochannels

cut

Permian unconformity at up to 25m depths below current

ground levels. Three

these

southern half. Fig.

palaeochannels traverse the boxcut, with two

in the

They lie both west and east (downdip) of the boxcut (see

10) and contain perched water pondages of some magnitude.

saturated

10 -1cm/sec highwall

With

lower sand and gravel permeabilities of the order of 10 -4 to the

contained

stability

water

constitutes both a

lowwall

hazard for up to 300m lengths in the

and a

southern pit

half, by way of interaction with overburden characteristics as : (and see Fig. 9). 1)

Extensive high and lowwall failure via intact rupture of saturated, unconsolidated,

Quaternary

fluvial

sediments

in perched water

pondage zones. 2)

Highwall

failure

on wetted

Cainozoic/Tertiary

unconformity

surfaces dipping westwards into the pit in palaeovalleys. 3)

Very

extensive lowwall failure along a remarkably persistent (4.35

kms) thin, pre-sheared, bedding plane horizon in rhythmic laminated claystones lying 20 to 30cm above the Middle Goonyella seam. These rhythmites around

have

3 to

similar dips to the coal seam and

5 degrees into the pit in the

hence

dip at

lowwall with

local

increases to 10 and 15 degrees (Godfrey f87) (see Fig. 9 & 10) Methods of study included terrain evaluation and resistivity surveys and drilling, integrated with compilation of all previous pertinent data. An

attempt

pondage wide

has

been made to reduce the effect of

the

perched water

on boxcut lowwall stability by the dragline digging of

a 65m

drain 80m west of the boxcut and parallel to the southern half of

Isaac Pit (see Fig.

10). Whilst this has largely succeeded in draining

the

there

perched water,

has been severe slumpage of

unconsolidated Cainozoic in the drain walls.

NG-09

the

saturated

240

D R A I N A G E OF P E R C H E D W A T E R PONDAGE FROM SATURATED CAINOZOIC M A T E R I A L INTO DRAIN

E X T E N S I V E S L U M P A G E OF S A T U R A T E D ONSOLIDATED CAINOZOIC MATERIAL

ISAAC

PIT B O X C U T

LOWWALL

UNCONSOLIDATED SATURATED CAINOZOIC SEDIMENTS WET UP UNDERLYING PERMIAN CLAYSTONES. IF C A I N O Z O I C NOT DRAINED,MASSIVE LOWWALL F A I L U R E S L I D I N G O U T ON SATURATED PRE-SHEARED LAYER JUST ABOVE C O A L , S T E R I L I Z E S COAL

E M P L A C E M E N T OF A G R I C U L T U R A L P I P E FOR PERMANENT DRAINAGE H I N D E R E D BY D R A I N B A T T E R FAILURE PERSI STENT,EXTENSIVE THIN B E D D I N G P L A N E S H E A R 20 to 3 0 C M A B C V E C O A L . (VERY L O W S H E A R R E S I S T A N C E WHEN W E T ) . T O 10* R E G I O N A L D I P T O PERMIAN OVERCOAL LAMINATED RHYTHMITE AND SUB R H Y T H M I T E M U O S T O N E S (PITWARDS BOXCUT LOWWALL)

1,000m

FIG.10.

G O O N Y E L L A M I N E , I S A A C P I T . S K E T C H P L A N A N D P E R S P E C T I V E D I A G R A M S H O W I N G D E T A I L S OF WATER PONDAGE AND GEOLOGY AND THEIR I N F L U E N C E ON B O X C U T L O W W A L L S T A B I L I T Y .

2.2.2.

Reverse Dip and Pitvall Instability

Isopach

and

PERCHED

structure contour maps of the principal

seam

mined

(the

Dysart seam) in Bauhinia pit, about two-thirds north along the 23 km long Saraji

Mine,

showed dip reversals

westwards

into the pit for about 1

km, variously, for strips 13 to 17 (see Fig. 11). Previous

studies

thick

bedding

shear zones in laminated mudstones between 1 and 3m above

(Godfrey

'83) had shown widespread

coal

and, again, at between 8 to 10m above coal. planes

also

run

coal.

When

exposed in the highwalls,

Thin, shear split, bedding

for at least 600m pit lengths around ^m to 10m these seep water in places

generally overhang some 2 to 5cm.

NG-10

above and

- 241 The

shear

altered,

zones

often

swelling

lie in strongly weathered

clays.

zones

have

shown

them to have direct shear stress tested

order

= 0 to 10.5 kPa + 0 = 4.5° to 15° at 15 to

and

contain

Past studies of such bedding plane strengths

shears in the

20% moisture

contents (Godfrey '77, Toh and Kajewski f79).

At these strengths, even low angles of pitwards dip may promote highwall failure.

Accordingly,

drill holes was according

confirmation confirmed with

the

and

conducted

two-stage programme of 59 close spaced chip along

isopachs

delineation

derived

series

from

earlier

of the reverse

traverses, selected

dip.

drilling, Reverse

for

dip was

but no evidence of significant faulting (Godfrey '86).(Though

the opening of Strips 14 and 15 the reverse dip has been shown to

be combined with a normal fault of 3m westerly downthrow).

Extensive

highwall failure has occurred but foreknowledge of conditions

has enabled mine staff to plan coal extraction with minimal losses.

NORMAL FAULT PLUS REVERSE DIP

FIG. 11.

SARAJI MINE. BAUHINIA PIT. SKETCH OF ISOPACH MAP SHOWING REVERSE DIP AS INDICATED BY CLOSE DRILLING, PLUS, SECTION OF ONE DRILL TRAVERSE SHOWING REVERSE DIP INDICATED BY DRILLING,AND FAULT PLUS REVERSE DIP (AS WAS THE ACTUAL CASE)

NG-11

- 242 -

ACKNOWLEDGEMENTS Thanks are due to BHP/Utah Coal Limited for permission to prepare and publish the paper; Alan Davies, Bruce Ellery and Kim Stahl for helpful comment and Mrs. Ray Currey for typing. The opinions expressed are not necessarily those of the company, BOYD, G.L. & SAUNDERS, B.J. 1979: "Geological and geotechnical conditions influencing pitwall stability, Acacia and Bauhinia Pits, Saraji Mine", UDC Tech. Serv. Dept. Report S1839 (unpubl.) COHEN, C.J., 1979:"Geological Joint Survey, An Analysis for Optimum Quarry Bench Design to Maximise Fragmentation"; Quarry Mine & Pit, Vol 18, No.4. COLES-PHILLIPS, F., 1971: "The Use of Stereographies Projections in Structural Geology", pp 25-26, 3rd. Edn. Edward Arnold Ltd., London. DAVIES, A.L., 1983: "Geotechnical Conditions affecting Strip Coal Mining in the Bowen Basin". Proc Symp on Permian Geology of Qld. G.S.A. Qld. Div.". DAVIES, A.L., 1985: "Interim Report on Peack Downs Regional Ground Water Investigation 1984". UDC Tech.Serv. Report P2396 (unpubl.) ELLERY, B.I., 1985: "Geotechnical Study of Isaac Pit Boxcut Goonyella Mine". UDC Tech.Serv.Dept. Report G2454 (unpubl.) GODFREY, N.H.H., 1977: "Direct Shear Tests on Goonyella Overclay Shear Planes" UDC Tech.Serv.Dept. Report G1817 (unpubl.) GODFREY, N.H.H., 1982: "Downdip Overburden Study, Acacia and Bauhinia Pits, Saraji Mine". UDC Tech.Serv.Dept. Report S2196 (unpubl.) GODFREY, N.H.H., 1983: "Accompanying Notes to 1983 Highwall Maps & Downdip Fence Diagrams-Burngrove, Airstrip, Besgrove, Stewarton, Emu Creek and Tannyfoil Pits". UDC Tech.Serv.Dept. Report B2296 (Unpubl.) GODFREY, N.H.H., 1984: "1983 Downdip Overburden Investigation. Tolmie Deep Creek and Taurus Pits, Blackwater Mine". UDC Tech.Serv. Dept. Report B2355 (unpubl.) GODFREY, N.H.H., 1986: "Peak Downs Regional Investigation-Cainozoic Study - 1985". UDC (unpubl.)

Ground Water Geotech. Group

GODFREY, N.H.H., 1986: "Bauhinia Pit Overburden Investigation 1985/86, Saraji Mine". UDC Tech. Serv. Dept. Report S2500 (unpubl.) GODFREY, N.H.H., 1987: "Cleanskin Pit Two Seam Operation - Initial Ramp 10 Centre Excavation - Rock Stress Analysis for Fragmentation

- 243 -

Improvement11. UDC Goonyella Mine Internal Memo. GODFREY, N.H.H., 1987: "Low Wall Shear Plane - Ramp 1 South". Goonyella Mine Internal Memo.

UDC

HAGAN, T.N., 1980: "The Effects of Some Structural Properties of Rock on the Design and Results of Blasting". Proc. 3rd Aust-NZ Conf. on Geomechanics, Wellington,NZ, May 1980. HAGAN, T.N. and MORPHETT, R.J., "Design and Implementation of Bench Blasts in Complex Rock Formations". Proc. Int. Symp on Energy in Complex Rock Formations, Beijing, China, Nov f86. MALLET

C.W. and GERRARD, C.M., 1980: "Highwall & Spoil Pile Stability in Open Strip Mining, Bowen Basin, Queensland". C.S.I.R.O. Aust.Div. Appl. Geomech., Lecture 51.

PHILP, M.G., BOYD, G.L. & ECKERSLEY, J.D. 1981: "Stabilisation of Large Scale Spoil Piles in Deep Open Strip Coal Mines"; in Proceedings of the AIMM Symposium on Strip Mining - 45 metres and Beyond. Aust. Inst. Min.Metall., Melbourne, 131-144. PRICE, A.J. & THOMPSON, S.D. "Improvements in Drilling and Blasting Practice at Utah Development Company's Blackwater Mine, Queensland, Australia". Trans. Inst. Min. Metall. London. No.94 (Sect.A). Oct '85. SMITH, I.

"Goonyella Mine, Groundwater Study for Isaac Pit" Feb f83. Golder Assoc. Report No. 8263 - 0035 to UDCL Tech. Serv* Dept. (unpubl.)

T0H, C.T., & KAJEWSKI, R., 1979: "Direct Shear Test on Clay Filled Discontinuities from Goonyella". (CSIR0 Aust. Div. Appl. Geomech, Project Report 47 (unpubl.)

—244—

8. GEOPHYSICS

—246-

SURFACE SEISMIC TECHNIQUES by Derecke Palmer

Senior Geophysicist (Seismic) Geological Survey of N.S.W. Department of Mineral Resources G.P.O. Box 5288 SYDNEY 2001

—247— Current Usage Modern high resolution seismic for the National Coal Board HRS methods was recognised

(HRS) reflection methods were developed

in the UK in the 1970's. by A.M. Clarke

The need to develop

(1976) as a means of maximising

the benefits of mechanisation.

In Australia, there has not been widespread use of HRS methods, apart from the Broken Hill Proprietary Company Limited (BHP). major

Not only was BHP a

sponsor of the first HRS crew to come to Australia, but

it has also

contracted most of the surveys in this country to date.

Data Acquisition and Processing The quality of data obtained cases

higher

than,

that

been

achieved

with

small

base of the weathering. National

Coal

Fairbain

et.

conditions

anywhere

explosive

is as high as, and

else.

charges

The best

placed

in many

resolution

has

holes

the

in drill

These techniques were originally developed for the

Board al.,

in Australia

obtained

(Zidkowski

1986) and

and

Lerwill,

have

been

they

1979;

modified

(Harman, 1981, 1983, 1984; Palmer, 1987).

Ziolkowski, to

1979;

suit

Australian

Dominant

frequencies

are commonly between 100 Hz and 160 Hz.

Quite with

good

Elf Aquitaine King

resolution

the Mini - SOSIE

and

has

technique

been

Australia

(Registered Trade Mark, Societe

achieved

in many

areas

Nationale

(Production), which uses a surface energy source (King, 1979;

Greenhalgh,

1981;

Pinchin

et.

al.,

1983).

Dominant

frequencies

are commonly less than about 100 Hz.

Interpretation The detection HRS methods. claimed

faults was the major

In Australia

(Harman, 1984).

as the magnitude

resolution

impetus for the development

down to 3 m displacements

has

been

However the nature of the faulting is as important

the displacement.

Harman

(1981, p. 8) describes

the

benefits of knowing the direction of movement for the design of workings to cross the fault. Furthermore,

Harman

(1984) and

Palmer

(1987) describe

stratigraphic

features which can be very important in mine planning.

DP-2

—248— The major part of this presentation will demonstrate the resolution of structural

and

stratigraphic

which

not

readily

are

features

resolved

with

existing

other

HRS

geological

techniques, or

but

geophysical

methods.

Future Usage The numerous publications demonstrate that HRS methods are efficaceous in the

detection

and

resolution

structural

which influence coal mine planning. resolution

with

high

frequency

and

stratigraphic

features

Proposals for further improvements in

seismic

vibratory

energy

source

and

three

location,

field

dimensional methods are under consideration. Why then are HRS methods not used more extensively? Cost

often

given

reason.

Depending

upon

conditions, target depth, etc., costs can range from $5,000 to $10,000 per kilometre.

However even for a very detailed

survey, this amounts to less

than a few cents per mined tonne of coal, and so it is insignificant when compared with other costs of mining. It

would

appear

that

the

represents the major difficulty.

interpretation

the

seismic

sections

The interpretation of the high resolution

seismic sections requires both a thorough knowledge of the local geology as well

section.

appreciation

Currently

the

processes

which

produced

the

seismic

few earth scientists working in the coal industry have

both of these requirements. Accordingly depend

as much

more upon

use

HRS

the development

widespread

seismic

methods

the

interpretation

future

will

skills and

the adaptation of seismic stratigraphy to coal field geology, as well as to technical advances in resolution.

DP-3

—249— References

Clarke, A.M., and

much

1976, Why modern

exploration has

mining;

with

little

Colliery

to do with

Guardian

geology

Annual

Review,

vol. 224, p. 323-336.

Fairbairn, C.M., Holt, J.M., and Padget, N.J., 1986, Case histories of the use

surface

seismics

the

Geophysics,

ed . D.J.

Buchanan

series

No.

Society

and

coal

L.J.

Exploration

mining

industry

Jackson,

Geophysics

Geophysicists,

jLri

Tulsa,

Coal

reprint Oklahoma,

466 p.

Harman, P.G., coal

1981,

industry;

High

resolution

Australian

Coal

seismic

Geology,

reflection techniques

in the

Journal

Geology

the

Coal

Group of the Geological Society of Australia, vol. 3, p. 3-14.

Harman,

P.G.,

Australia

1983,

High

with

special

resolution

Queensland _in G.O.

Argall

Coal

Symposium,

Exploration

San Francisco.

Harman,

P.G.,

seismic

reference ed.

Papers Sydney,

reflection the

from

techniques

Blackwater the Fourth

Coalfield, International

Australia.

Miller

Freeman,

reflection

techniques

applied

30 p.

1984, High

in coalfields:

resolution

Cook Colliery

seismic

Queensland

- A case history;

Exploration

Geophysics, Bulletin, Aust. Soc. Explor. Geophys. vol. 15 p. 220-228.

King, D.W.,

1979, Min-SOSIE

seismic

profiling

in the Gloucester

Basin of

N.S.W.; Bulletin Aust. Soc. Explor. Geophys. vol. 10, p. 156-163.

King,

D.W.,

and

coal

deposits

Coal

Geology

Greenhalgh,

S.A.,

in Australia; Group

the

1981,

Mini-SOSIE

Australian Geological

Coal

seismic

Geology,

Society

profiling

Journal

the

Australia,

vol.

p. 15-20.

Palmer,

D.,

1987,

High

resolution

seismic

reflection

surveys

for

coal;

Geoexploration, vol. 24.

DP-4

—250— Pinchin,

J. , Goleby,

reflection

survey

B.R.,

and

at W a m b o

Ryan,

D. , 1983,

Colliery,

Hunter

experimental

Valley, N.S.W.;

seismic Bulletin,

A u s t . Soc. Explor. Geophys. vol. 14, p. 36-42.

Ziolkowski, (ed.).

A.,

1979,

Seismic

Developments

profiling

Geophysical

Science Publishers Ltd, London, pp.

Ziolkowski,

A.,

resolution

and

Lerwill,

seismic

W.E.,

profiling

for

coal

Exploration

land _in A . A . Methods,

Fitch

Applied

271-306.

1979. for

coal;

simple

approach

Geophysical

high

Prospecting,

vol. 26, p. 360-393.

DP-5

—251—

IN-SEAM SEISMIC - A TOOL FOR THE COAL INDUSTRY

J. F. DOYLE

B.H.P ILLAWARRA COLLIERIES, 2 6 ATCHISON STREET, WOLLONGONG

NSW

250

-252-

IN-SEAM SEISMIC - A TOOL FOR THE COAL INDUSTRY A Technology package including instrumentation, data processing software, field proceedures, numerical modelling and interpretation know-how has been developed for the routine application of in-seam seismology to the protection of underground coal mining operations. The technology has proven to be highly successful and has been integrated into the operations of B.H.P.'S Illawarra and Macquarie Collieries. The equipment is purpose designed for underground use which results in surveys being conducted quickly, at little inconvenience and at low cost. More than forty surveys have been conducted by BHP in Australia. Substantial real savings in production costs have been realised where ISS has identified mining conditions which would have otherwise impeded longwall operations.

INDRODUCTION Longwall mining is a mechanised method which is now widely accepted as the most efficient mining technique for underground coal extraction. Efficient as it is, the face machinery of a longwall constitutes a very inflexible system. Being capital intensive, its success depends on maintenance of high face productivity. Unexpected geological discontinuities, such as faults of seam thickness throw or less, can retard or interrupt completely a longwall production unit for a long period until the machinery can be relocated to a new area or the faulted area is traversed.

JD-01

—253—

Because of their relatively small magnitude, these geological discontinuities are difficult to detect by traditional surface seismic techniques. In-seam seismic or in-seam long hole drilling are presently the only two techniques available to detect small features ahead of the mine face. Since the late 1970s, it has been demonstrated that discontinuities, undetected by other m e a n s , can be predicted by in-seam seismic (ISS) techniques. The ISS technique involves generating a seismic signal at one point in the seam, and detecting transmitted or reflected signals at some other point in the seam. The signals are then analysed in a computer to determine the nature and location of the discontinuities. There are three methods of conducting in-seam seismic surveys. The first method, called the face-to-face method, was developed in the United Kingdom and West Germany. The second and third methods, face-to-borehole and borehole-to-borehole, have not been applied widely, yet they are particularly relevant in the Australian conditions, where panel advance is the usual means of underground development. The first method is aimed at mine production planning whereas the latter two are more useful during the earlier stages of exploration and mine development. A schematic diagram of the face-to-face survey technique is shown in Figure 1. In this method small explosive shots are set off in some holes drilled in underground

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workings, with detectors installed in other holes some distance away. A transformer detects the instant of shot firing and triggers the timing break circuit. The detectors then capture the seismic signals and transmit them to nearby digitiser modules where they are temporarily stored. At the completion of the shot all data is transferred to the control unit where they are displayed for Field examination and then stored to tape. A face-to-face survey is conducted in two parts: transmission and reflection surveys. The objective of the transmission survey is to determine the seismic velocity for analysis of data from the reflection survey. The reflection survey is aimed at locating reflectors, i.e. geological discontinuities. The field procedure of a borehole-related survey is quite similar to a face-to-face survey, except that the source and/or detectors are installed from the surface in boreholes. The recording station is located on the surface close to the borehole.

FIELD EXAMPLE In late 1984 silling associated with a known dyke caused development delays in 0 panel of Appin Colliery (Fig.2). Concern was felt as to the potential for major delays in the driving of P Panel through a potentially wider sill/dyke zone.

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A reflection

ISS survey was conducted

Panel west of the dyke.

from Blue Panel and 0

Results of this survey were made

available to mine management three weeks after the survey. With the aid of local geology and numerical

modelling

reflectors A and C (Fig. 3) were identified as reflectors

from

a full face of igneous sill.

as a

small scale feature.

Reflector D was identified

Based on the interpretation

presented,

stone driving contracts were awarded prior to the sill being exposed.

Examination of alternative mine

alternatives failed

scheduling

to produce a viable means of developing

the next longwall block north of P Panel. was taken to divert longwall development

The early

mine which has since been achieved with no loss of productivity. subsequently

ISS surveys were conducted

AND F«,

and confirmed

These two

shear zones as reflectors G.E.

The surveys have shown no geological

sufficientto

longwall

in 0 Panel and

in P Panel when it was developed.

surveys delineated

decision

to another area of the

impediment

impede the future development of longwall

blocks

to the north.

SAVINGS ACHIEVED THROUGH THE USE OF IN-SEAM SEISMIC

In-seam seismic technology is only one of a suite of techniques that must be considered underground geological hazards. long-hole drilling

in the prediction of

In-seam seismic surveys and

are the only techniques available that are

capable of results of sufficient accuracy and definition to satisfy the needs of a modern high productivity

longwall

No surface geological or geophysical method can satisfy

mine. this

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criteria.

Site c o n d i t i o n s and g e o m e t r y m u s t g u i d e

the

s e l e c t i o n and a p p l i c a t i o n of e i t h e r or b o t h t e c h n i q u e s to a specific

problem. In the A p p i n case study a gain of a p p r o x i m a t e l y

days was a c h i e v e d .

More intangible benefits were

40-50

gained

t h r o u g h the a d d i t i o n a l time that was made a v a i l a b l e for a c o n s i d e r e d d e c i s i o n a f t e r e x h a u s t i v e e x a m i n a t i o n of all a l t e r n a t i v e mine p l a n s .

A real gain of about one m o n t h was

m a d e by the e a r l y call for s t o n e d r i v i n g t e n d e r s m a d e

possible

by the remote p r o v i n g of the c o n t i n u i t y of the sill by

in-seam

sei s m i c . The e a r l y w a r n i n g a c c e l e r a t e d

the timing of the

d e c i s i o n to d e v e l o p a n e w l o n g w a l l area to the

south.

T h e loss of the p r o d u c t i o n of a p p r o x i m a t e l y

10 000

tonnes of coal from a l o n g w a l l f a c e at A p p i n r e p r e s e n t s a loss of r e v e n u e .

A n y p o s s i b i l i t y of an i m p e d i m e n t to the

continuity

of p r o d u c t i o n from the l o n g w a l l u n i t is s i g n i f i c a n t and m u s t demand

immediate

attention.

In-seam s e i s m i c is the g e o l o g i c a l tool s e l e c t e d as the m o s t s u i t a b l e m e t h o d of e a r l y w a r n i n g of the f e a t u r e s that now assume such i m p o r t a n c e

small-scale

in the m a n a g e m e n t

the l o n g w a l l m i n e s in B H P ' s I l l a w a r r a and M a c q u a r i e Collieries.

As w i t h most t e c h n i q u e s

its i n t e g r a t i o n

into the

g e o l o g i c a l and m i n e p l a n n i n g s y s t e m s of the h o s t c o l l i e r y paramount

to a c h i e v e the m a x i m u m b e n e f i t from its

application.

In-seam s e i s m i c is now a r o u t i n e p a r t of m i n e p l a n n i n g

within

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—257—

BHP and is applied from development roadways in such a manner as to achieve maximum protection to the mine plan.

Oigitisers

Current Transformer Field Control Unit Shot Firer

Figure 1

Face-to-face ISS Survey

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Fig. 2 - Appin Colliery - Mine situation December 1981),

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SCALE

IN METRES

ClMOfX AND HAAO SILL -

\PDH 14 SHCARCOCOAL

CINOE*

CINOE* iCINOE*-

|CINOE«:

Fig. 3 - Appin Colliery - Mine situation June 1985.

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DOWNHOLE AND CROSSHOLE SEISMIC: A NEW TOOL FOR DETAILED DEFINITION OF COAL SEAMS AND GEOLOGICAL CONDITIONS BY R. J. Whiteley, W. M. Holmes & R. D. Dowle

- 261 -

TITLE:

Downhole and Crosshole Seismics: A New Tool for Detailed Definition of Coal Seams and Geological Conditions.

AUTHORS AND AFFILIATIONS: Robert J. Whiteley

Senior Lecturer, School of Mines University of N.S.W.

W. Harvey

Associate Professor, School Electrical Engineering and Computer Science University of N.S.W.

Holmes

Robert D. Dowle

Doctoral Research Student, School of Electrical Engineering and Computer Science University of N.S.W.

CORRESPONDENCE: R.J. Whiteley,

Dept. of Applied Geology, School of Mines University of N.S.W. P.O. Box 1 Kensington, N.S.W. 2033.

EXTENDED ABSTRACT Effective mine planning and immediate response to unexpected geological conditions encountered during mining are essential for efficient exploitation of coal resources. In both situations major unknowns are the detailed configuration of the un-mined seam and geological features such as faults and igneous intrusions. A new high resolution seismic system has recently been developed in Australia which is capable of achieving sub-metre resolution of such features. This system has a bandwidth of upto 3kHz and uses energy sources and receivers within existing or specially sited shallow or deep drillholes. It improves subsurface resolution when compared with surface seismic reflection systems by a factor of at least ten. This improvement is achieved by hardware developments which include a modified marine sparker source and hydrophone receiver array in an adjacent hole. Fig. 1 shows a schematic of the in-field system. Downhole and crosshole seismic data are acquired digitally in the field on micro-computer. Specially designed processing software which includes beam steering, a technique which has been highly successful in radio astronomy and communications, allows coal 1

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seams and geological features to be profiled from above or below or side-scanned. A complete tomographic data set may be acquired and subsets of the data include vertical seismic profiles which compare closely with integrated sonic logs, Yo-Yo reflection profiles for mapping sub-horizontal structures such as seams, side-scan reflection for profiling vertical structures such as dykes and borehole-to-borehole in-seam information. While the system is currently undergoing substantial refinement, to date successful field trials have been completed at three coal mines in the Hunter Valley, N.S.W. between drillholes upto 250 m apart with seams to 150 m depth. Figs. 2 and 3 show examples of upward and downward looking Yo-Yo reflection data from West Wallsend between successive pairs of in-line drillholes each 50 m apart, note RDH 2 is common to both pairs. In Fig. 2 clear continuous reflections have been obtained from both the shallow seams and the upper surface of the deep seam with some evidence of seam splitting in the shallow seams. In Fig. 3 seam continuity has been severely disrupted by an igneous dyke not yet encountered in the workings. The achievement of this downhole and crosshole seismic system represents an important breakthrough in the ability to define seam amd geological conditions prior to mining and, if integrated with other mine planning techniques principally drilling, will provide the Australian coal industry with a significant advantage over other coal producers.

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lAlttWMt Tr\*c* R«e«l««r Hoi«

«Mt«r

Taftla'

ULsmlC '

Swire*

tac«Lv«r

(•9

Hydrophone

Array)

CmI RaClactors

Fig. j Downhole-crosshole v e r y h i g h resolution coal s e a m profiling between b o r e h o l e s . A n i m p u l s i v e s e i s m i c s o u r c e is in o n e b o r e h o l e , a n d a r e c e i v e r is in a n o t h e r , b o t h b e l o w the weathered layer. T h e s o u r c e a n d r e c e i v e r p o s i t i o n s w i l l be varied during the s u r v e y . The transmitted and reflected s i g n a l s a r e r e c o r d e d a n d a n a l y s e d in the i n s t r u m e n t t r u c k .

Pig Jj Processed Yo-Yo section between holes RDH1 end RDH2, which covers the a r e a of the I n t e r p r e t e d d y k e . The c l e a r d i s r u p t i o n s ( a r r o w e d ) of the c o a l s e a a i m a g e s c a n be i n t e r p r e t e d as a h a d i n g d y k e .

P r o c e s s e d r e s u l t s of a Y o - Y o s u r v e y b e t w e e n h o l e s RDH2 and RDH3, not covering the a r e a of the ' i n t e r p r e t e d dyke. Strong coal seam Images are e v i d e n t .

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