Undrained Tests

 When undrained triaxial tests are performed, two main phenomena occur. First, the compression of the sample produces an increase in the degree of saturation and therefore, a reduction in the value of suction. Second, with the reduction in the sizes of pores, the retention curves shift on the axis of suction. Thereafter, the simulation of undrained triaxial tests requires the correct simulation of the hydromechanical coupling phenomenon. A fully coupled constitutive model for unsaturated soils is used herein 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. The hydraulic model uses the grain and pore size distributions to approximately reproduce the structure of soils. This model is able to simulate the soil-water retention curves during wetting-drying cycles. Plastic volumetric strains modify the pore size distribution of the soil, which in turn affects the retention curves and, therefore, the current effective stress. Some comparisons between numerical and experimental results of undrained triaxial tests show the adequacy of the model.

Keywords: Effective stresses, Undrained tests, Constant water tests, Constitutive model, Unsaturated soils, Elastoplasticity, Suction stress, Net stress, Yield surface, Anisotropic hardening, Preconsolidation stress, Critical state, Suction hardening, Triaxial tests, Isotropic compression