Shear strength of unsaturated granular soils: three-dimensional discrete element analyses

Abstract

To investigate the shear strength behavior of unsaturated granular soils, the three-dimensional discrete element method has been used to model soils in triaxial compression tests. A simple but effective contact model with an attractive capillary force was implemented. The effects of matric suction and packing density on shear strength of unsaturated granular assemblies were examined. The Mohr–Coulomb strength parameters (apparent cohesion and friction angle) were fitted for each matric suction examined. The results show that matric suction can increase the strength and modulus of granular soils and lead to increased dilation. The peak friction angle depends on the packing density but seems independent of matric suction. The apparent cohesion increases with matric suction non-linearly at a decreasing rate. Similar values of cohesion were observed for both dense and loose assemblies, which can be explained by the anisotropic distribution of capillary force network. Based on the microscopic observations, the stress-induced anisotropy of contact distributions leads to an anisotropic distribution of capillary water and, as a consequence, the capillary stress is anisotropic, imposing a shear effect on an assembly in parallel with that imposed by the externally applied loading. Consequently, the strength of unsaturated granular soil is controlled by the combined effects of packing density, and the magnitude and degree of anisotropy of the capillary stress. A new shear strength function for unsaturated soils, considering the anisotropic effects of matric suction, is proposed and validated using experimental data in the literature.