Unsaturated behavior of excavations in residual soil at the Auburn University National Geotechnical Experimentation Site

Two excavations were instrumented at the Auburn National Geotechnical Experimentation Site (NGES) in Opelika, AL. The excavations were constructed approximately 6 m deep × 30 m long with a vertical face. The primary goal of this experiment was to determine the boundary conditions that resulted in failure of the excavation. In doing so, conclusions were drawn regarding the accuracy of common laboratory test methods for estimating the strength properties of residual soil.
The instrumentation plan was designed to monitor real-time pore water pressures (positive and negative) surrounding the excavation throughout the course of each 1-year test period. Time-lapse cameras were used to identify when failures occurred, and the approximate geometry of the failure planes. Undisturbed soil samples were taken during the subsurface exploration and used in conjunction with previous soil test results to accurately define the material properties and layering based on common laboratory test methods. In addition to common laboratory tests, unsaturated triaxial tests were also conducted, and soil-water characteristic curves were measured to further define the unsaturated properties of the soil.
In both excavations, failure was observed along a similar plane, which began at the bottom of the excavation, and propagated to the surface (approximately 2 m behind the face of the excavation) along existing tension cracks that developed during the construction of the excavation. Based on these results, recommendations are provided as related to the most appropriate test methods for determining the strength properties of residual soil for use in geotechnical design.