Assessment of Seismic Liquefaction through detailed Seismic Study of Soil- A case study by IRJET Journal

Assessment of Seismic Liquefaction through detailed Seismic Study of Soil- A case study

Saurabh

Kumar1, R. Roshan2, Azhar Ali3

1Saurabh Kumar – Assistant Manager, KEC International Ltd., Mumbai, India

2R. Roshan - Assistant Manager, KEC International Ltd , Mumbai, India

3Azhar Ali - Assistant Manager, KEC International Ltd., Mumbai, India ***

Abstract - Soil liquefaction has caused several damages in the past to lifeline and structures. Liquefaction is a phenomenon where soils suffer loss in shear strength due to cyclic loading such as an earthquake. The mapping of liquefaction of soil can be done using seismic, geotechnical, and topographical parameters. Obtaining a suitable seismic parameter is a key challenge that can aid to qualitative mapping of Liquefaction potential of a soil stratum. This paper presents a comparative study conducted at a site in Gujarat to evaluate the liquefaction potential of soil using two different techniques, involving the usage of the obtained Peak Ground Acceleration (PGA). Bhuj (2001) earthquake was one of the most devastating earthquakes that occurred in Gujarat in recent years with a magnitude of 7.7. The site is 105 km from the epicenter of Bhuj (2001) earthquake for which attenuation relationship is incorporated, and the PGA is assessed using Linear Ground Response Analysis using the SPT information Plasticity of soil plays a key role in determining the Liquefaction potential of the soil stratum. This paper devises a significant impact in the field of liquefaction assessment for soils with moderate to high plasticity and intends to add to the body of knowledge on liquefaction studies.

Keywords: Liquefaction,Plasticity,Clays,Sands

1. INTRODUCTION

The strength and serviceabilityof a structure determineitsstability. Liquefaction canlead to ground subsidence,lateral spreads,disruptionofvitalservices,andsignificantdamageto infrastructure.Cyclicliquefactionisaphenomenawhereby cyclicloading,likeanearthquake,resultinginlossofitsshearstrengthpartiallyorcompletely

2. LITERATURE REVIEW

The first conventional method to assess liquefaction using the Standard Penetration Test (SPT) was the streamlined processputoutbySeedandIdriss(1982)[1].Althoughliquefactionwaslongthoughttobelimitedtosandysoildeposits, afewobservationsmadeaftermanyearthquakesshowedthatitmayalso occurinfine-contentsoilswithmediumtolow plasticity. The Haicheng and Tangshan earthquakes in 1975 and 1976 caused silty sand to liquefy into slightly sandy silt soils.Wang(1979)[2]wasthefirstresearchertopointoutthisphenomenonandproposedacriterionthatclayeysoilscould onlybeliquefiableifallthreeofthefollowingconditionsweremet- percentofparticleslessthan0.005mm<15%,liquid limit(LL)<35%andratioofwatercontentandliquidlimit(wc/LL)>0.9(Wang,1979[2];SeedandIdriss,1982[1]).This standard was termed as Chinese criteria due to its origin. Threshold-level plastic soils are generally immune to flow liquefaction failures and the ensuing deformation and strength loss. The study's findings led to the conclusion that the liquefactionresponseofsoilcanbedefinedbyaspecificrangeoftheplasticityindexandthewatercontenttoliquidlimit ratio. The study categorized liquefaction response into susceptible, moderately susceptible, and non-susceptible classes. Bray and Sancio (2006) [3]reviewed previous works and defined a new set of PI and wc/LL range criteria. Boulanger & Idriss(2006)[4]categorizedsoilsas“sand-like”and“clay-like”consideringtheirplasticityindexvalues.Theimportanceof PIasaparameterforanalyzingtheliquefactionpotentialofsaturatedclayeysoilswasverifiedbyGratchevetal.(2006)[5]

ThecorrelationbetweenCyclicResistanceRatio(CRR)andSPT-NvalueispresentedIndianStandardCode1893(Part1): 2016[6].Itisoneofthemostsimplifiedapproachestodeterminetheliquefactionpotentialofsiltandsand.SPT–Nvaluesis oneofthemostpopular,oldest,andfrequentlyusedin-situtestsforthesubsoilinvestigationbecauseofitssimplicity

FactorofSafety(FOS)iscalculatedtodetermine thelikelihoodofliquefaction.FOSiscalculatedusingSPT-N.SPT-N isthe most common parameter for the prediction of liquefaction. FOS is the ratio of CRR to the Cyclic Stress Ratio (CSR) using equation (1). CRR is considered the fundamental parameter for the estimation of liquefaction, which is mainly observed

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during medium and strong seismic excitation in sand-slit mixture and based on the soil properties. If FOS <1, the soil is susceptibletoliquefactionandifFOS ≥1,soilisnotpronetoliquefaction.Whenthedesigngroundmotionisconservative, earthquakerelatedpermanentgrounddeformationisgenerallysmall,ifFOS≥1.2(IS1893Part-I,1893)[6] �� (1)

CSRiscalculatedusingtheEqn.(2)proposedbySeedandIdriss(1971)[7]. �� 065× ��

(2)

Where, amax isPGA,g=accelerationduetogravity(9.81m/s2), σv istotalverticalstress, σ ’ v iseffectiveverticalstressand rd is stressreductionfactorwithdepth.

Where,

rd =1–0.00765z;0≤z≤9.15m (3)

rd =1.174–0.0267z;9.15≤z≤23m (4)

Beyond23mfromgroundlevel,ISCodedoesnotprovideforthereductionfactor.

CRRisascertainedbycorrectingCRR7.5forearthquakemagnitude,highoverburdenstresslevelandhighinitialshearstress usingtheequation(5):

7.5() CRRCRRMSFKK   (5)

Where,CRR7.5 isStandardcyclicresistanceratiofora7.5magnitudeearthquakecorrelatedwiththecorrectedSPTdata,

��75 1 34 (��1)60�� + (��1)60�� 135 + 50 (10×(��1)60��+45)2 1 200 (6)

Where,term (N1)60cs dependson (N1)60 andgivenas:

(��1)60 ��+��(��1)60 (7)

Where,

α = 0 β = 1 forFC≤5percent

�� *176 (190 ��2)+ �� 0.99+ �� 15 1000 for5percent<FC<35percent

α =5 β = 1.2 forFC≥35percent

(N1)60 is the SPT blow count normalized to an overburden pressure of about 100 KPa and a hammer energy ratio or hammerefficiencyof60%.

(N1)60=CN N60 (8)

CN isCorrectionfactorforoverburden

�� (�� ′ )05 ≤1.7 (9)

N60 = N *C60 (10)

N is the Uncorrected SPT blow count which is the quantification of resistance to penetration of a sampling spoon under dynamicorstaticloading Thisiscorrectedtoanoverall60%efficiencyfactorgivenbelow.

��60 �� (11)

CHT -CorrectionfactorforNon-standardhammerweightorheightoffall,

CHW -CorrectionfactorforNon-standardhammerweightorheightoffall,

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CSS -CorrectionfactorforNon-standardsamplersetup,

CRL -Correctionfactorforshortrodlength,

CBD -CorrectionfactorforNonstandardboreholediameter,

MSF-magnitudescalingfactorgivenbythefollowingequation:

MW –Magnitudeofearthquake.

σ=Correctionforhighoverburdenstresseswhenoverburdenpressureishigh(depth>15m)

(12)

�� (��′ ��/��)(�� 1) (13)

Where,

σv'-effectiveoverburdenpressure,

Pa -atmosphericpressure,

f isanexponent,dependentontherelativedensity(Dr),forDr is40-60%,f=0.8~0.7andforDr is60-80%,f=0.7~0.6

Kα = Correction for static shear stresses required for sloping groundand is not necessary in normal engineering practice andvalueassumedtobeunity.

IdrissandBoulanger (2004)[8]providesoneoftheapproachestoascertaintheliquefactionpotential ofsiltsandclaysof plasticity index ≥ 7%. For liquefaction prediction of the soil deposits, the CSR7.5 that indicates the loading imposed by earthquakesonthesoilisprovidedinthefollowingequationbelow: (�� )75 065 (

(14)

In which, σv and σ ’ v are respectively the total and effective overburden pressure at a depth z; amax is the peak horizontal groundaccelerationin g’s; rd isastressreductionfactor TochangetheinducedCSRtothereferenceearthquakemagnitude of7.5,themagnitudescalingfactor,orMSF,isemployed. Kσ representsthecorrectionfactorforeffectiveoverburden Stress reduction coefficient (rd) is calculated analytically and is a function of depth (z) and earthquake magnitude (M) Given below is the equation for rd which is valid to a depth up to z ≤ 34 m, where z is depth in meters and M is earthquake magnitude.

��(��) ��(��)+��(��)�� (15) ��(��) 1.012 1.126��( �� 1173 +5.133) (16)

��(��) 0.106+0.118��( �� 1128 +5.142) (17)

Whereasthefollowingequationisvalidfor z >34m;

�� 012��(022��) (18)

Equationsusedtoascertain CRR, andtheresultant FOS fromthecorrectedblowcount (N1)60 havebeenprovidedbelow:

(19)

Where,thetermSu isundrainedShearStrength.Inthis paper,Liquefactionanalysisofsoil isdoneusingIS1893:2016[6] forSandLikesoil(PlasticityIndex<7)andIdrissandBoulanger2004[8]forClaylikesoil(PlasticityIndex≥7).

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3. CASE STUDY

According to IS 1893:2016 [6], as given in Figure 1, the Kutch region of Gujarat is classified as a Zone V (high seismicity zone),makingitvulnerable toearthquakes.Thisareahasseentwosignificantearthquakesinrecordedhistory: the1819 Kutchearthquakeandthe2001Bhujearthquake.Withanepicentreat23.4Nand70.43Ewithafocusof18km,theBhuj earthquake had a magnitude of Mw = 7.7. The Bhuj earthquake had an impact on the Great Rann of Kutch, but there was only modest liquefaction at the location. A project located at the Great Rann of Kutch was selected for this investigation. The project involved the construction of Substation. Geotechnical Investigations conducted at the project site show subsurfaceprofileasshowninTable1forTestlocation1andTable2forTestlocation2

Table-1:SubsurfaceprofileforTestlocation1

Table-2:SubsurfaceprofileforTestlocation2

55-11.5

AccordingtoapriorSPTnearthetestsite,thebedrockwasroughly30metersdeep Thisassumptionwasmaintainedfor the current study For Test Location 1, the average SPT N values range from 0 to 10 between 1.5 m and 8 m and then increasetoarangeof10-20from8mto16m,providedinFigure-1.ThesoilatTestLocation1consistsofSandytoClayey Silt with a Plasticity Index greater than 7% up to 8 m, exhibiting clay-like behaviour during liquefaction. Below 8 m, the soil becomes less plastic (Plasticity Index < 7%) up to 18 m, exhibiting sand-like behaviour, which is more prone to liquefaction,asshowninFigure-2.Similarly,forTestLocation2,theaverageSPTNvaluesareintheorderof0to10from 1.5mto11.5mandincreaseto10-20from11.5mto18m,giveninFigure-1.ThesoilconsistsofSandytoClayeySiltwith a Plasticity Index greater than 7% up to 5.5 m, exhibiting clay-like behaviour during liquefaction. Below 5.5 m, the plasticitydecreasesuptoadepthof11.5m(PlasticityIndex<7%),andthesoilexhibitssand-likebehaviour,whichisalso prone toliquefaction. Following 11.5m,clayeysilt ofintermediate plasticityhaving Plasticityindex> 7%is encountered. Shearwavevelocity(Vs)isalsooneoftheimportantinputparametersforGroundResponseAnalysis(GRA)studieswhich indicatesthevariationsofsoilstiffnessalongwithdepth.

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Sand Like (Pl >7)

Clay Like (Pl >7)

Test location-1

Test Location-2

For simplicity's sake, the epicentre of the Bhuj earthquake is considered a point source. This is approximately 105 kmaway from the test location. Tests were carried out at locations 1 and 2 down to a depth of around 30 meters. The energy emitted during an earthquake transmits in the form of shear waves, which, depending on the kind of soil strata theypassthrough,mightamplifyorde-amplify.Whentheshearwavesreachthegroundsurface,thebedrockacceleration varies according to the different soil strata. A fundamental input for any liquefaction analysis is the PGA during an earthquake, which is derived from strong motion records that are accessible close to the epicentre. Attenuation relationships are employed to determine the reduced acceleration at the bedrock because the study location is 105 km fromtheepicentre.Forthisstudy,IyengarandRaghukanth2010[9]attenuationrelationship(seeEq.20),isapplied. ��Y C1 +C2(M 6)+C3

where M, Y, R refer to the magnitude, acceleration in g, and hypo-central distance in km, respectively. The values of the various constants C1, C2, C3 and C4 and error ε are obtained from Iyenger and Raghukanth 2010 [9] for western central regionsofIndia.

In the present study, multiple correlations of shear wave velocity (Vs) based on SPT-N values were used to calculate the shear wave velocity (Vs) profiles, presented in Table 3. In this investigation, the average value of these correlations was utilizedduetotheuncertaintyintheselectionofshearwavevelocity(Vs)profiles.

Figure-1:SPTNvsDepth

Figure-2:PlasticityIndexvsDepth

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Table-3:EmpiricalcorrelationsfortheanalysisofshearwavevelocityobtainedfromtheobservedSPT-NValue

S. No.

References

1. OhbaandToriumi(1970)[10]

2. OhsakiandIwasaki(1973)[11]

3. ImaiandYoshimura(1972)[12]

4. OhtaandGoto(1978)[13]

5. SeedandIdriss(1981)[14]

6. Imai(1982)[15]

7. Athanasopoulos(1970)[16]

8. Lee(1990)[17]

9. Iyisan(1996)[18]

10. Yokotaetal.(1981)[19]

11. MhaskeandChoudhury(2010)[20]

12. HanumantharaoandRamana(2008)[21]

13. Kalteziotisetal.(1992)[22]

14. Jafarietal.(2002)[23]

15. Dikmen(2009)[24]

16. HasancebiandUlusay(2007)[25]

17. Maheswarietal.(2010)[26]

4. RESULTS AND DISCUSSION

Correlations

Vs=84*N0.31

Vs=82*N0.39

Vs=91*N0.337

Vs=85.35*N0.348

Vs=61*N0.5

Vs=97*N0.31

Vs=107.6*N0.36

Vs=57.4*N0.49

Vs=51.5*N0.516

Vs=121*N0.27

Vs=72*N0.4

Vs=82.6*N0.43

Vs=76.2*N0.24

Vs=22*N0.85

Vs=58*N0.39

Vs=90*N0.309

Vs=95.64*N0.301

Thegroundresponseanalysis(GRA)recordofaccelerationvs.timeisscaledusingthevalueofbedrockaccelerationfound usingEq.(20),anda1-DlinearGRAofsoilisperformedusingthesoftwareprogramDEEPSOILv7.1todetermineground acceleration.Theaccelerationatbedrockwasobtainedas0.18g.

Figure-3:AccelerationtimehistoryatbedrockandatgroundsurfaceanalyzedusingDEEPSOIL

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Thelowestsoillayerineachtestwasassumedtoextendtoadepthof29.5m sincetheSPTswerenotconducteduptothe bedrock.Thewatertableisatgroundsurface.TheresultsfromtheGRAaredepictedinFigure.3,anditisinferredthatPGA is0.36gwhichconformstotheIS1893:2016[6]recommendation,i.e.,0.36gforSeismicZoneV.Thepeakvalue derivedat groundsurfacewasincorporatedtocalculatetheCSRandLiquefactionanalysiswasdonebasedontheobtainedPGA.

FOS<1 Liquefiable

FOS≥1 Non-Liquefiable

IS 1893 IDRISS & BOULANGER (2004)

LiquefactiondoesnotoccurpredominantlyforsoilshavingPlasticitygreaterthan7.Inthisstudyliquefactionanalysiswas carried out from a depth of 1.5 m, which is the ground water level. As the liquefaction analysis method provided in IS 1893:2016 [6] is not applicable to fine grained plastic soils, such soil layers are evaluated using Idriss and Boulanger (2004) [8] for Soils having Plasticity Index greater than 7. This study aims for a comparative analysis of Factor of Safety againstliquefactioncalculatedusingboththemethodsforPlasticsoilshavingPlasticityIndexgreaterthan7. PGAof0.36g evaluated from GRA was applied to ascertain the CSR Figures 4 and 5 show the variation in FOS with depth for SPT N value and Undrained Shear Strength at the two test locations considered in this study. It can be observed that there is a differenceintheFOSbetween1.5mto9mfortestlocation1,1.5mto4.5mand9mto18mfortestlocation2bythetwo methods This differenceoccurs asthestrata inthesedepthsare classifiedasclay like(PI>7).AsthePlasticity increases, thetendencyofsoiltoliquefydecreases

5. CONCLUDING REMARKS

The objective of this work was to compare liquefaction potential of plastic soils by different simplified approaches for moderately liquefied locations in Gujarat. Using the test results, factors of safety against liquefaction were established fromthemethodsgiveninIS1893:2016[6]andIdrissandBoulanger(2004)[8]atallthetestlocations.Thecomparison ofboththeapproaches was established graphically, showing thevariation oftheFOSagainstdifferentlevelsof Plasticity Index.Themajorconclusionsdrawnfromthisstudyaresummarizedbelow:

 The liquefaction assessment at the location, based on damage indices calculated using both methods, revealed a significantdiscrepancyinthepredictedliquefactionpotentialofplasticsoils.

Figure-4:FOSvsDepthforTestLocation1

Figure-5:FOSvsDepthforTestLocation2

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 The FOS against liquefaction for plastic soils was found to be higher using the Idriss and Boulanger (2004) [8] equationscomparedtothevaluesobtainedbasedonIS1893:2016[6]inbothtests.

 The Idriss and Boulanger (2004) [8] method provides a more accurate assessment of liquefaction potential for plasticsoilsthanIS1893:2016[6],whichisprimarilydesignedtoevaluateliquefactionriskinsandysoils.

 SinceSPTmeasuresvarioussoilstresscomponents,conductingConsistencytestsandShearStrengthtestsatthe location can offer a more comprehensive understanding of soil properties, enabling the selection of appropriate groundimprovementtechniques.

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