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Massive Excavation Project Required No Vibrations From Construction
Graham said sheet pile shoring was ruled out because of the vibration that would be created during the installation and removal. It was estimated that an I-beam system with timber lagging with soil stabilization would require one month to install – too much time to be accommodated by the schedule.
The solution was the installation of a 90-foot long by 33-foot wide by 16-feet-deep beam and plate pit with five sets of 80-ton Mega Brace struts.
Graham said the beam and plate system with the hydraulic Mega Brace struts provided by URTS required the drilling of only half as many caissons for the I-beams, as compared to a timber lagging system.
“This,” he explained, “allowed the shoring system to be installed, the concrete pad formed and poured, 10 precast concrete vault/tank sections installed, shoring components removed and the excavation backfilled, all within one month. And the entire system was recovered afterwards, whereas with a timber lagging solution, everything below five feet would have been left in the ground.”
Appears to be simple enough, but with an excavation this size so near an historic building in a highly-developed urban area, there were numerous challenges that required adjustments to the original design of the shoring (see sidebar).
Work on the excavation and shoring began with surveyors staking out 30 points for drilling holes 31 to 36-feet deep with eight-foot, 6-inch horizontal spacing. A skid-steer loader removed spoil and added a mixture of concrete and fly ash into each caisson. Water was added as drilling continued, turning the mixture into a slurry to stabilize soils in the caissons.
Iron Woman used a crane to set HP14 x 117 pounds, 45-foot long I-beams into the drilled caissons, and a skid-steer backfilled to the engineer-prescribed depth while crew members kept the beams aligned and square.
United Rentals’ Graham describes the procedure Iron Woman personnel used to dig and shore the excavation.
“A 40-metric-ton-class excavator then dug the entire 90-foot-long, 35-foot wide excavation to a depth of five feet. An in-out ramp was constructed at one end. Twenty-by-eight-foot steel plates were placed between the vertical I-beams and pushed down by the excavator as it dug. Plates were one-inch and 1 1/4 inches thick.”