Effective Stress-Based Limit-Equilibrium Analysis for Homogeneous Unsaturated Slopes

Abstract

With a suction stress–based effective stress representation, stability analysis of unsaturated engineered and natural slopes can be performed effectively in the same manner as the classical limit-equilibrium (LE) methodologies. This paper presents an analytical framework for effective stress LE analysis of unsaturated homogeneous slopes under steady one-directional (vertical) flow. The proposed log spiral failure surface–based LE method involves only two additional hydromechanical parameters for unsaturated soil, approximating the inverse of the air-entry pressure and pore-size distribution. Both parameters are used to describe seepage and effective stress variations in unsaturated soils. Unlike most other LE formulations, the method is statically determinate. A parametric study was performed, and stability charts for general use are presented. The impact of infiltration and evaporation on the stability of slopes for four hypothetical soil types was studied. It is shown that the apparent cohesion due to suction stress may contribute substantially to the stability of slopes. Specifically, different seepage rates can significantly impact the stability of clayey slopes. This impact progressively decreases in silty slopes and further diminishes in loess and sand. Because the proposed method is statically free of assumptions, it can serve as a benchmark for rigorous slope stability methods that can deal with more complex problems.