Model for the Prediction of the Strain-Softening Stress–Strain Relationship for Unsaturated Soils Associated with Large Shear Deformation

Local shear zones typically develop in a progressive manner in geostructures constructed with or within unsaturated soils due to the influence of wetting and drying cycles. The unsaturated shear strength of soils will reduce associated with large shear deformation in these shear zones over a long period of time (i.e., strain-softening behavior) and can significantly influence the long-term performance of the geostructures. In this study, a disturbed state concept (DSC) model is proposed for predicting the strain-softening stress–strain relationship of unsaturated soils associated with large shear deformation. In the DSC-based model, the apparent stress–strain relationship was expressed as a weighted average of a hyperbolic stress–strain relationship at the relative intact state and a horizontal stress–strain relationship at the fully adjusted state (i.e., the shear strength is constant at the residual value at the fully adjusted state) with a disturbance function as the weight. In addition, a formulation approach of the disturbance function was introduced. The proposed DSC-based model includes several mechanical parameters that are predicted by extending existing models in literature based on two stress state variables. The step-by-step procedures for determining the DSC model parameters are succinctly described. In addition, two sets of suction-controlled ring shear tests were conducted based on axis-translation technique on two different soils including a coarse- and a fine-grained unsaturated soil, respectively. The comparisons between the measured stress–strain curves and the predicted results using the proposed model suggested that the proposed DSC-based model provides a reasonable prediction for the strain-softening stress–strain relationship for different soils associated with large shear deformation.