DEM Analysis of Mechanical Behavior of Unsaturated Silt under Strain-Controlled Constant Stress Ratio Compression Tests

Abstract

The three-dimensional discrete element method (DEM) is employed to investigate the macro- and microscopic mechanical behaviors of unsaturated silt subjected to one-dimensional (K0) and constant stress ratio loadings. A DEM sample was prepared by installing a contact model considering rolling and twisting resistances and the van der Waals force. Capillary forces were installed at contacts to represent the unfilled water effect. Strain-controlled one-dimensional and constant stress ratio compression tests were then simulated on the specimens with different constant matric suctions and stress ratios (η = q/p). The discrete element simulation results reproduce the primary macroscopic mechanical behavior observed in laboratory tests. Shear failure occurs in specimens compressed under stress ratios exceeding the strength slope M, while volumetric yielding occurs in specimens compressed under very small stress ratios η ≪ M. The compression test under a constant stress ratio larger than M reaches the critical state characterized by not only a constant stress state and void ratio, but also a constant effective particle number, (mechanical) an average coordination number, and fabric anisotropies of contact normals and contact forces. The contribution of normal contact forces to the deviator stress predominates over tangential contact forces throughout the simulation. The capillary stress (capillary force-induced stress tensor) anisotropy decreases under isotropic compression but increases under anisotropic compression (η = 1.25). The mechanical average coordination number versus mean stress is independent of the stress ratio and the matric suction. Fabric evolutions indicate that the rearrangement of contact forces is much quicker than that of contact normals at the beginning of compression.