Abstract. Several researchers have reported that Bishop's mean effective stress decreases in unsaturated soils under fully undrained cyclic loading conditions, and unsaturated soils are finally liquefied in a similar manner as saturated soils. This paper presents a series of simulations of such fully undrained cyclic loading on unsaturated soils using an elastoplastic model of the unsaturated soil. This model is formulated using the Bishop's effective stress tensor incorporating the following concepts: the volumetric movement of the state boundary surface containing the normal consolidation line and the critical state line due to the variation in the degree of saturation, a soil water characteristic curve model considering the effect of specific volume and hysteresis, the subloading surface model, and Boyle's law. Comparisons between the simulation results and the experimental ones show that the model agreed well with the unsaturated soil behavior under cyclic loading. Finally, the typical cyclic behavior of unsaturated soils under fully undrained conditions, such as the mechanism of liquefaction of unsaturated soils, the compression behavior, and an increase in the degree of saturation, are described through the proposed simulation results.
The mechanisms of hydraulic collapse (wetting‐induced deformation) are studied through a series of simulations using a model for unsaturated soils. The model is a critical state soil model extended for unsaturated soils using Bishop's effective stress tensor, the state boundary surface (SBS) moving with the change in the degree of saturation, and a rational soil‐water characteristic curve that takes account of density and hydraulic hysteresis. Comparisons with published experimental data validate the proposed model. The simulation results show that the model adequately captures the hydraulic collapse behavior under both isotropic and anisotropic stress conditions. The effects of density, mean net stress, and deviatoric stress on the hydraulic collapse behavior are further investigated through a parametric calculation, and the mechanics of the wetting–induced instability are discussed. The effects of suction history, namely repeated wetting and drying cycles, on the deformation and failure, are also presented. The proposed model provides a validated method for modeling hydromechanical collapse in unsaturated soil and provides a better understanding of the mechanisms involved.
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