The generators were removed due to a concern that the helical piles could have a buckling failure. After the MSE wall began to shift later - ally and vertically, a tension crack opened adjacent to the generator foundations. As the MSE wall moved, the soil mass behind moved with it. This rapidly exposed the helical piles underpinning the generators. Approximately 10 to 15 feet of the helical pile shafts were exposed within a very short period of time. UMA redesigned the repair plan to include deconstructing a portion of 300 linear feet of the failed 30-foot-high MSE wall, while simultane - ously constructing a soil nail wall to retain the soil below the building and underpinning the building with micropiles. UMA proposed to the owner and the design team that a 4,600-square- foot permanent soil nail wall with reinforced shotcrete facing was required since a 3:1 slope to connect the building with the grading below the MSE wall was impossible. The 250 soil nails needed to be long enough to stabilize the excavation to permit construction of the slope. These nails ranged from 20 to 50 feet long. Within 10 feet of the building, a gradual transition with grading was impossible. The path of the wall meandered to avoid the tension cracks that formed during the failure. The grading was between the building and MSE wall and also required additional underpinning along the rear side of the building. UMA ac - complished that with four additional micropiles to stabilize the build- ing close to the failed MSE wall. UMA had just gotten equipment on-site to start phase one, when the wall started moving very rapidly and shifted 10 feet laterally and 10 feet vertically, but the soil mass stayed intact. In fact, very few blocks dropped from the MSE wall. Tension cracks were discovered parallel to and behind the wall and a depression in the ground formed behind the geogrid. Cracks widened daily, leaving openings large enough to swallow a compact car. Additional cracks opened at the foot of the wall, which slopes downward to the lake. As the wall moved forward it heaved the ground, toppling trees, and cracking the ground. UMA’s crews worked between the wall and the failed MSE well. Crews exercised caution and engineers continued to closely monitor slope stability to ensure the safety of the workers and building occupants.
The property contains Piedmont Residual Soil, which is relatively stable, according to Plotkin. “At some point the slope above the wall started to move sufficiently enough to develop tension cracks. They continued to open up, to the point where we're able to leave a really good water intrusion location. It allowed that tension crack to propa - gate to the MSE wall, until eventually as we would get rain, that ten- sion crack would fill, load the wall, and force it to shift laterally. “Continuing to work, and the fact that they (UMA) engineered a plan as we were out there in the field is really what saved the structure,” says Plotkin. Project Team Collaboration Wins the Day UMA’s work on the MSE wall concluded safely and successfully in Spring 2022. Despite the previous challenges, the project was unevent - ful. The project received the GeoVirginia 2022 Outstanding Geotech - nical Engineering Project award on April 13. “This was a very unique and exciting project in our industry.” says UMA’s Brian DeSpain. “I hope it will help to provide some lessons learned to future wall designers.” Aside from the geotechnical feats, perhaps the greatest lesson on this project was the importance of a well-functioning project team, from an engaged owner to a strong partnership between a contractor and mul- tiple engineering firms to agree on a Design-Build approach on the fly. UMA proposed to the owner and the design team that a 4,600-square-foot permanent soil nail wall with reinforced shotcrete facing was required since a 3:1 slope to connect the building with the grading below the MSE wall was impossible.
November 2022 csengineermag.com
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