Instrumented Geogrid Reinforced Mechanically Stabilized Earth Wall Undergoing Large Settlement

The goal of this project was to explore the effects of large settlement on the geogrid reinforcement of a mechanically stabilized earth (MSE)wall. Geogrid-enforced MSE walls have been constructed to support new bridge approach embankments at the Indian River Inlet in Sussex County, Delaware. These embankments, however, have been constructed over poor foundation soil, a layer of soft clay about 60 feet thick. A large magnitude of the settlement is expected, and therefore the embankments have been instrumented with settlement plates, piezometers, and inclinometers. Along with this instrumentation, at the south abutment the geogrid reinforcement in a 34.5-foot-high section of MSE wall 1 has been instrumented with strain gages. The use of strain gages is necessary to verify that the geogrid reinforcement is not overstressed, as MSE wall design does not account for large foundation settlement. Attachment of strain gages to the high-density polyethylene geogrid used in this project is challenging and requires a unique technique. Proper calibration of the strain gages allows for resistances read to be converted to strains and forces in the geogrid. Four months after the completion of construction, the ultimate predicted settlement has already been exceeded, with settlement continuing. High excess pore water pressures are dissipating at a slow rate. Large horizontal movements have been measured. Strain measured in the geogrid has exceeded three percent, a value significantly higher than seen in similar applications. This report presents the effects of the large magnitude of settlement on the MSE wall using measured field data provided from the strain gages, settlement plates, piezometers, and inclinometers. Based on field calibration, the ultimate consolidation settlement is predicted.