The secondary soil nail wall design would utilize 20- to 30-foot-long #8 steel tendons and a shotcrete facing to retain the wall from further movement. The placement of this wall would be located above the fail - ing MSE wall and below the building and generators to hold both in place to facilitate the construction of the MSE wall repair. Helical piles were installed and attached to the generator slabs in an effort to try and take some of the weight off of the slope above the MSE wall in hopes that this would reduce the amount of movement that was being experienced. “Our intent was to remove the surcharge load from the generators,” says TGC’s Harshman. “We did not want to stabilize that load with the soil nails.” Wall Failure During Construction UMA had begun implementing the first soil nail wall design when the MSE wall failed in lateral sliding. “Over the course of weeks, the wall moved an additional 10 feet horizontally and 10 feet vertically,” says UMA’s Brian DeSpain. “There was no option but to abandon the secondary repair plan.” “One of the unique things here was that this project required a full redesign, and basically punting from when UMA mobilized to what ultimately had to happen,” says Plotkin. “If the wall hadn't failed, it would have been a matter of trying to stabilize the MSE wall in place.” “We came up with a soil nail stabilization system for the wall,” says Harshman. “There was no chance of saving the generator enclosure at this point.” Nearby was a loading dock area, access drive, collector cabinet, and electrical service lines that were connected to the generator. The place - ment of the soil nail wall would allow these areas to stay in place and save the building. The MSE wall was directly below an occupied modern, five-story glass office building in Fort Mill, South Carolina. UMA and its engineers had to revise plans three times during construction and work near the wall when it unexpectedly started rapidly failing.
Very few blocks dropped from the MSE wall when it failed.
There were many contingencies in place to monitor the safety of the site, but Plotkin ultimately bore the weight of determining if the site was safe to work on. He gave UMA credit for seeing the project through. “I don't know if every contractor would have stuck it out,” says Plotkin. “They were essentially working on top of a failed soil mass.” The Initial Design Approach UMA was hired directly by the owner of the building. The wall had been in place for five years at the time. UMA brought in Triad Geotechnical Consultants (TGC) of Greens- boro, N.C. as a consulting engineer to evaluate the preliminary designs for pricing, and ultimately to develop the final design. TGC and UMA have partnered on numerous Design-Build projects in the past. During its first five years in existence, the wall moved one foot hori - zontally and one foot vertically and was showing small signs of de- formation on the face. The owner initially wanted UMA to design and build an anchoring system to stabilize the face of the MSE wall. The intent was that soil nails would be drilled through the face from a safe area atop the MSE wall, followed by spraying a six-inch shotcrete face over the block. That design was nixed due to accelerated wall move - ment after construction began. The primary design was created that focused on protecting the building and generators because of the accelerated wall movement.
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csengineermag.com
November 2022
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