Post-Peak Fully-Softened Strength and Curved Strength Envelope in Shallow Slope Failure Analysis

Abstract

It has long been recognized that highly plastic clays and stiff fissured clays may become "fully softened" in shallower zones of slopes and undergo significant strength loss over time. Consideration of fully softened strength (FSS) in clay slopes has begun to come into more frequent use in slope stability analyses in recent years. However, using peak FSS values for first time slides in many cases requires unusually high pore pressure assumptions to produce factor of safety (F) values near 1. This indicates that the average FSS along the slip surface is actually between residual strength and peak FSS. A research study was conducted to evaluate a range of post-peak FSS values at 25, 50, and 75 percent incremental difference between residual and peak FSS from existing correlations and available FSS test results. Power curves were fit to the data points to develop coefficients for the full range of peak FSS, post-peak FSS, and residual values. These coefficients were used to perform limit equilibrium analysis of a range of slope ratios and heights representative of approximately 60 case history shallow slope failures observed by the authors. Pore pressure assumptions were varied in the analyses to produce F values near 1. This study illustrates that the analyses are more accurate and predictive when more realistic pore pressure assumptions are made while using post-peak values of FSS for stability analysis of slopes in non-water retention facilities such as highway slopes.