FOUNDATIONS & WALLS
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BY BRIAN FRALEY
Contractor Takes On Challenging Soil Nail Wall UMA Geotechnical Construction constructed eight permanent soil nail walls along a roughly two-mile stretch of North Carolina’s Interstate 73— all dwarfed by Wall 4.
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orth Carolina Department of Transportation (NCDOT) led a $176 million project to reduce congestion and improve access to the Piedmont Triad International Airport in Greensboro. Part of the project consisted of a taxiway bridge over Interstate 73 leading to the airport. UMA Geotechnical Construction constructed all of the eight required permanent soil nail walls along a roughly two-mile stretch of the Greensboro Western Loop along I-73 to make way for the project. The total project required 76,000 sq. ft. of wall, 42,000 of which was Wall 4. For comparison, the average size wall was 3,700 sq. ft. UMA was a subcontractor to Colo.based Flatiron Construction Corp., which had been retained by NCDOT to build a 1.5-mi. divided highway around Greensboro between Battleground Avenue and Lawndale Drive. UMA submitted a proposal in Nov. 2016, work began mid-2017, and engineers submitted drawings for Wall 4 in 2018. Construction finished in April 2019. The job was essentially to install soil nails and a temporary shotcrete facing to allow Flatiron to finish it off with a castin-place concrete wall. However, due to adverse geotechnical soil conditions, a simple install became increasingly more challenging.
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Flatiron handled excavation and the cast-in-place concrete wall, which required UMA to work between its crews for most of the project. Flatiron’s first step was to cut the hill down 30 ft. to reach the road base with an impressive fleet of four Komatsu excavators and 18 articulated loaders. This mass excavation ultimately exposed Wall 4. UMA’s drilling crew worked the wall from one side to the other. Each lift the excavation crew cut out provided a successively lower working platform for the drilling crew. THE DRILLING PROCESS It took more than 2,000 soil nails to complete Wall 4. The length ranged from 15-50-ft, although the vast majority were 30 ft. The Casagrande C7’s 30-ft stroke allowed UMA to drill most holes with a single stroke. A chevron bit was used to drill and air was used to extract the cuttings, leaving an open 6-in.-diameter bore hole. The rig has no carousel to hold additional drill rods so deeper holes required
UMA installed a 4-in.-thick, 4,000 psi temporary shotcrete facing to hold the wall in place in preparation for Flatiron Construcion Corp. to build the final cast-in-place wall. Photo Credit: UMA Geotechnical Constrcution
the support of an EZ Spot UR grapple mounted on a mini excavator. The C7 would drill the 30-ft. hole and the grapple was used to set another drill rod to accommodate the additional depth. UMA used nearly 61,000 li. ft. of Grade 75 epoxy-coated threaded bar produced by Skyline Steel. Temporary casing was required in areas with collapsing soil and rock, but that was only about 1,400 li. ft. Of the roughly 61,000 li. ft. of bar installed, about 5,500 li. ft. was in rock and the remainder in soil. Almost 800 tons of Type I/II cement was supplied by Roanoke Cement and mixed on site with a Colcrete Grout plant. Aiming for a 1.85 specific gravity, a mud balance was used to measure the specific gravity of the grout and ensure the proper mix, which included
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6/10/21 9:03 AM
