Uncommon MSE Wall Failure Calls for Unique Geotechnical Solutions By Brian M. Fraley, Fraley Construction Marketing
Stabilizing a Mechanically Stabilized Earth (MSE) wall in proximity to an occupied five-story glass office building was a task unto itself for UMA Geotechnical Construction and its team of engineers. The need to revise plans three times during construction and work near the wall when it unexpectedly started rapidly failing, however, proved to be one of the greatest challenges on this project. “A 300-foot section of the wall began to rapidly move horizontally and vertically simultaneously” says UMA’s President Brian DeSpain. “We’ve worked on hundreds of earth retention projects over the years, but never one with this magnitude of movement.” The Office Building and MSE Wall The site included a modern, five-story glass office building in Fort Mill, South Carolina with frontage on Interstate 77 and atop a winding MSE wall overlooking a lake. The wall in question had been constructed and was functioning without any major signs of movement for five years. Below the wall was a thick buffer of trees on a downhill slope leading to the banks of Lake Patricia. The building sits on a site of several acres with extensive parking and detention basins. The existing MSE wall was 1,033 feet long and 32 feet tall at its highest point. Behind the building were five large emergency power generators on a concrete slab to keep the building systems up and running in case of a power outage. Monitoring Movement of the MSE Wall UMA was contacted by Terracon Consultants and Engineering Consulting Services (ECS) in summer 2021 to look at the existing MSE wall, which had been showing small signs of movement over an extended period of time. The movement was limited to the middle 300 feet. ECS got involved in 2018 when it surveyed the top of the MSE wall and began monitoring settlement, according to ECS’ Marc Plotkin, P.E., D.GE who specializes in earth retention and deep foundations. Plotkin served as the company’s principal engineer at the time. “This is a steel-frame building that was built in 2016,” he says. “We were seeing distortions and movement of the columns. We got involved in the project about 2017 or 2018 because the building and generator enclosure were exhibiting signs of settling and distress.” ECS conducted a forensic study to identify the cause of movement in the building and the best solution. The generator enclosure was skewed at a 30- to 45-degree angle at the time. There were many factors to con -
sider including the proximity of the patio to the MSE wall, the grade of the slope between the slab and the wall, and the backfill material. Plotkin recalled that the MSE wall looked solid leading up to the failure. ECS was only measuring vertical deformations at the time and seeing minimal lateral movement. The team was surprised and concerned that the movement was concentrated on the two exterior building corners closest to the MSE wall, as opposed to being evenly distributed across the entire face. Increased movement and steepening of the slope near the generator enclosure exacerbated their worries; however, the wall showed no visual signs of distress. “Based on where we were seeing the settlement of the building founda - tion, the distortions in the building were primarily tied to the quality and consistency in composition of the fill beneath the building, and not tied to the retaining wall,” says Plotkin. “Ultimately what we saw is that part of the retaining wall failed without impacting the structure.” Although ECS did not see an increasing rate of settlement in 2018, it recommended underpinning two of the corner building column founda- tions with 8 to 10 cased micropiles in 2019. The generator enclosure was underpinned with 10 to 12 helical piers. The intent was to prevent further deformation of the building columns and save the generator enclosure. Safety First The safety of the building occupants and field workers was paramount. Employees at the office building were able to continue working dur - ing construction thanks to underpinning that was done in 2019, ECS set up inclinometers on the wall and pinned locations on the building that would serve as a warning system if it needed to be evacuated at any point. The existing micropile underpinning would allow more than enough time for that to occur. ECS had installed inclinometers at the end of the underpinning and started seeing an accelerated movement in the inclinometers in 2021, which meant the slope and wall were starting to move. Cracks opened up in the generator enclosure and along the rear of the building. 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.
November 2022 csengineermag.com
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