CFD-DEM model to assess stress-induced anisotropy in undrained granular material

This paper presents CFD-DEM simulations of undrained true triaxial tests on a granular soil. A CFDEM solver that considers compressible fluid and moving meshes has been developed and calibrated against the results of laboratory testing on Nevada sand. The effect of the intermediate stress ratio (b) on the macro- and micro-mechanical features of undrained granular assemblies was studied. The tests were repeated at different confining pressures, and the trends were similar to each other. The lowest effective frictional resistance was observed for specimens for which b = 0 and the highest value was obtained for specimens with b = 0.5. An increase in b caused samples to exhibit the greatest dilative behavior and increased the absolute maximum negative excess pore pressure. The stress-force-fabric equation that represents the stress state of the assembly in terms of the fabric anisotropy and contact force anisotropy tensors was used to study the micromechanics of the undrained granular samples. The tests were varied from compression to extension in order to qualitatively and quantitatively analyze the evolution of anisotropy under anisotropic loading conditions. Observations on the micro structural evolution of undrained samples were in agreement with those already reported for drained samples.