A Fortune 100 company requested that Geosyntec provide a technical assessment and evaluation of a 2,200-foot long gabion-faced combination gravity and mechanically stabilized earth wall at one of its major distribution centers in the eastern United States. Other retaining structures on the project site experienced significant movements or collapse. Previous consultants expressed concern regarding the long-term stability and distress for this wall such that the owner would be required to initiate major rehabilitation activities to stabilize it. The owner was actively considering complete wall rehabilitation at an estimated cost of $2.5 million.
Geosyntec's Scope of Services
Geosyntec's work included a preliminary assessment of the condition of the wall. Geosyntec then reviewed available wall movement data and performed reliability-based wall stability assessments to identify portions of the wall that represented the greatest potential risk for continuing adverse movements or distress. Based on this assessment, Geosyntec recommended and implemented a focused subsurface investigation and laboratory testing program to assess the condition of the wall backfill soils. Soils were collected for laboratory triaxial strength testing. Geosyntec used the test results and prior experience to develop statistical relationships for "most-likely" and "worst-case" shear strength parameters for the wall backfill soils. This information was used to conduct additional reliability-based wall stability computations. These computations resulted in a conclusion that no part of the wall was in imminent risk of failure; however, site observations indicated the need to implement specific maintenance and rehabilitation measures for parking lot pavements, inlets, curbs, and gutters to control surface water and subsurface flows towards the wall facing.
Geosyntec recommended and implemented a long-term monitoring program consisting of wall movement measurements at 15 cross section locations. These measurements will allow for the evaluation of incremental future movements and assessment global movement patterns in the very flexible gabion wall. This monitoring program will be executed for approximately two years. Depending on the monitoring results, it may be possible to suspend the surveying program. These results are anticipated to indicate that no or a relatively small portion of the wall may need some rehabilitation or rebuilding.
Inconclusive wall movement data only were available to render an opinion on the condition of the wall. The use of wall stability analyses augmented with reliability-based computations and site-specific data on the wall backfill soils were used to develop an opinion regarding the current condition of the wall. The approach implemented by Geosyntec significantly reduced the uncertainties associated with the current condition of the wall and allowed the owner to, for the first time, quantify the risk associated with the current wall. Geosyntec's analyses provided the owner with sufficient information to justify a performance monitoring program at relatively low cost in lieu of a major rehabilitation effort or complete wall replacement.