Long-Term Monitoring of a Drilled Shaft Retaining Wall in Expansive Clay: Behavior before and during Excavation

This project seeks to provide insight into the distribution of lateral earth pressures below the ground surface in expansive clay soils, and into how the pressures are affected by moisture cycles causing shrinking and swelling of the expansive clay. A full-scale test wall is being constructed in Manor, Texas on a site underlain by approximately 50 feet of the Taylor Formation, a stiff, highly plastic clay. In order to estimate the lateral earth pressures and moisture content in the soil behind the test wall, the wall and retained soil are instrumented with optical strain gauges, inclinometers, and moisture sensors. This wall will be monitored for a period of at least two years. This paper addresses the construction of the wall and its short-term response during excavation in order to set the stage for the start of long-term monitoring. The major conclusions to date are that the instrumentation survived construction and is working, residual stresses developed in the drilled shafts prior to excavation due to concrete curing and moisture changes in the soil, and the wall responded immediately in response to stress relief during excavation, rotating outward with a top-of-wall deflection of approximately 1/200 the height of the wall. Interpretation of the long-term monitoring data will need to account for the residual stresses that are present before and immediately after excavation.