Model for the Prediction of Residual Shear Strength of Unsaturated Soils

The residual shear strength (RSS) of unsaturated soils is a crucial parameter for the reliable analysis and design of geo-structures constructed with or within unsaturated soils undergoing large shear deformation. For investigating the RSS of unsaturated soils, two sets of data are specifically generated on the poorly graded sand with silt and Indian Head till using suction-controlled ring shear tests and three more sets (i.e., silty sand (SM), silty clayey sand, and fat clay) are gathered from the literature. A model is proposed extending two approaches for predicting the RSS for both coarse- and fine-grained unsaturated soils. In this model, the suction contribution was calculated considering the loss of degree of saturation due to shearing, which was described as a nonlinear function of degree of saturation. The capability of the proposed model is validated with the five sets of data using two different approaches. The best-fitting approach that is based on three fitting parameters provides good predictions. The approximate approach performs well for four studied soils, except for SM soil; this approach is simple for use in engineering practice because no fitting parameters are required. The proposed model is valid for the suction range where degree of saturation is higher than the residual degree of saturation.