Fully Softened Shear Strength for Soil Slope Stability Analyses

Abstract

The fully softened shear strengths of soils, mudstones, and shales of different plasticities and gradations were measured at low (10 and 25 kPa), intermediate (50 and 100 kPa), and high (200, 300, and 400 kPa) effective normal stresses. A torsional ring shear apparatus was used for the strength measurement. The effects of the change in nonlinearity of the fully softened shear strength envelope over the utilized normal stress ranges on slope stability analyses were investigated. An empirical correlation was developed to predict the fully softened shear strength as a function of plasticity index and a wide range of effective normal stresses. The correlation is presented as a revised version of those previously developed for a limited number of normal stresses that do not cover the range mobilized in the vast majority of reported long-term first-time landslides. A numerical expression was also introduced to allow for direct incorporation of the fully softened shear strength correlation in slope stability software. Comparisons with measured fully softened shear strength data available in the literature are given to verify the reliability of the presented correlation and numerical expression for use in slope stability analyses.