Hydro-mechanical double-yield-surface model for unsaturated sand and clay

Hydro-mechanical coupling behavior analysis with a sophisticated model for both unsaturated sand and clay is still a challenge. In this study, a hydro-mechanical coupling model with two plastic deformation mechanisms, i.e., loading collapse and shear sliding, for unsaturated soils is formulated using the Bishop’s stress as the stress variable and using the void ratio and the effective degree of saturation as the state variables. An expression for the critical state line related to the effective degree of saturation in the void ratio-soil skeleton stress semilog plane is explicitly implemented, which is combined with a non-associated flow rule for the shear sliding yield surface to guarantee the satisfactory simulation of the dilation or contraction during shear for unsaturated soils including sand and clay. The predictive capability of the model to reproduce the main features of unsaturated soil behavior is analyzed by simulating the triaxial tests on silty sand and kaolin.