Simulating Undrained Tests on Unsaturated Soils

A simple fully coupled constitutive model for unsaturated soils is presented and used to simulate the behavior of unsaturated soils subjected to undrained conditions. The mechanical model is based on the modified critical state model and the effective stress concept. This mechanical model is coupled to a hydraulic model able to simulate the soil-water retention curves during wetting-drying cycles. The hydraulic model uses the grain and pore size distributions to approximately reproduce the structure of soils. Plastic volumetric strains modify the pore size distribution of soil, which in turn modifies the retention curves. The result is a simple general fully coupled constitutive model for unsaturated soils able to simulate different stress paths. The simulation of undrained triaxial tests requires the correct simulation of the hydromechanical coupling phenomenon. Some comparison between numerical and experimental results of undrained triaxial tests show the adequacy of the model.