Fully Softened Shear Strength Measurement and Correlations

The fully softened strength (FSS) is usually the controlling shear strength for cut slopes in stiff-fissured fine-grained soils, shales, and claystones, engineered fill slopes, and compacted embankments with no prior slope movement. The FSS is used to model the shear strength of shallow slope soils because it represents the shear strength remaining after the effects of overconsolidation, compaction, desiccation, or other strengthening processes have been removed because of wet-dry cycles, applied shear stresses, stress relief, progressive deformation or failure, swelling, freeze-thaw cycles, weathering, or all of these. An empirical correlation is frequently desired for such slopes because the slopes can be long, such as a levee system, and the borrow material is not completely defined, which makes comprehensive testing difficult. This article presents the mode of shear for the FSS condition, a comparison of existing FSS correlations, power function coefficients to estimate the FSS envelope, and recommendations for modeling the stress-dependent FSS envelope in stability analyses.