At just over 30 feet, the height of Wall 4 towers over you when stand- ing at its base. Once completed, it is 42,000 square feet and just over a quarter mile long. To provide context, 12,000 square feet is an average sized wall. Of the seven other walls UMA built on the Greensboro Western Loop, the average size was 3,700 square feet. At 42,000 square feet, Wall 4 accounted for 55 percent of the design-build project’s total square footage of 76,000. “Most project prequalifications that enable your company to bid on a soil nailing project require that you have completed 2,000 soil nails in the last five years,” says DeSpain. “This one wall is in excess of How did UMA install soil nails on a wall that was higher than 30 feet in some locations? In lifts, of course. 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 feet to reach the road base with an impressive fleet of four Komatsu excavators and 18 ar- ticulated 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 Equipment Spread UMA completed most of Wall 4 with a single drilling rig – a Casagrande C7 – one of the largest soil nail rigs on the market. The supporting equip- ment included concrete pumps, forklifts, grout plants, and a water truck. Despite having a single crawler drilling rig doing the work, UMA’s crew kept pace with Flatiron’s top-notch excavation team. A balance was required to work in between Flatiron’s crews. The excavation crew, hindered by wet weather and cold temperatures, per- severed to provide UMA with as much access as conditions permitted. The cast-in-place wall crew, on the other hand, required more time to that requirement.” Working in Lifts UMA’s drilling crew worked the wall from one side of Wall 4 to the other. Flatiron’s excavation crew worked in lifts, making cuts to provide a successively lower working platform for the drilling crew.
handle formwork, pour concrete, wait for it to cure, and remove the formwork due to the sheer magnitude of this enormous wall. Using a single rig on this massive wall was intentional, according to DeSpain. “We designed the wall around the 30-foot stroke of the ma- chine,” he says, noting that this simplified the operation by allowing the operator to work alone without support from others. “There were days where he was drilling 100 soil nails in a day by himself.” UMA bought the Casagrande C7 Drilling Rig specifically for the Greensboro Western Loop project. It was the first of two C7s that UMA has purchased from North Carolina-based International Drilling Equip- ment. “We felt that it was appropriate to buy a new machine because of the size and the length of the project,” DeSpain says. “We basically had one crew on the entire project for two years moving from wall to wall.” The Drilling Process It took more than 2,000 soil nails to complete Wall 4. The length ranged from 15- to 50-foot-long, although the vast majority were 30 foot. The C7’s 30-foot 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 six-inch-diameter bore hole. The rig has no carousel to hold additional drill rods so deeper holes required the support of an EZ Spot UR grapple mounted on a mini excavator. The C7 would drill the 30-foot-hole and the grapple was used to set another drill rod to accommodate the additional depth. UMA used approximately 61,000 linear feet of Grade 75 epoxy-coated threaded bar produced by Skyline Steel. Temporary casing was re- quired in areas with collapsing soil and rock, but that was only about 1,400 linear feet. Of the roughly 61,000 linear feet of bar installed, about 5,500 linear feet 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 spe- cific gravity, a mud balance was used to measure the specific gravity of UMA installed a four-inch-thick, 4000-psi temporary shotcrete facing to hold the wall in place in preparation for Flatiron’s crew to build the final cast-in-place wall.
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csengineermag.com
november 2020
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