Long-Term Strength of Compacted High-PI Clays

Moisture infiltration into earth slopes and structures constructed of high-plasticity clays will lead to a reduction in soil suction and strength that can ultimately result in stability failure. This research presents arational analysis of this process that: establishes a relationship between soil suction and strength, and provides a framework for predicting the time rate of strength loss due to moisture infiltration into an earth mass. This research utilizes a generalized form of the Mohr-Coulomb failure criterion to characterize the effects of suction on soil strength. A diffusion equation governs changes in soil moisture and suction. This equation is in general non-linear for unsaturated soils; however, this research uses a linearized formulation through appropriate transformation of variables. This research also presents methods for estimating the necessary material parameters needed for input into the moisture diffusion model. The suction-strength relationships and moisture diffusion analyses are applied to back-analyzing a number of shallow slope failures in Texas high-PI (plasticity index) clays. The moisture diffusion analysis is also extended to typical Texas Department of Transportation earth- retaining structures to predict decreases in suction during the life of these structures. Based on these suction predictions, estimates of soil strength within an earth structure as a function of location and time are possible.