Numerical study on the shear strength of granular materials under the low confining pressure

The relationship between stress and strength in granular soil differs under low confining pressure as compared to high confining pressure, which is of great importance for the geotechnical engineering. Numerical simulations using discrete element method (DEM) are conducted to model consolidated-drained triaxial compression tests, with the confining pressure σ_c ranging from 1 to1000 kPa. From the analysis of the shear strength of granular soils under various confining pressures and void ratios, it is found that the peak stress ratio η_p under a low σ_c is significantly lower than that under the high σ_c, particularly for samples with the same initial void ratio. As the confining pressure increases, η_p for samples with the same initial void ratio increases up to 29 % under the low σ_c while stabilizing under the high σ_c. Conversely, η_p for samples with the same void ratio after the consolidation can increase up to 13.5 % under the low σ_c while declining under the high σ_c. In addition, the critical confining pressure distinguishing the low and high confining pressures is about 200 kPa for dense samples with initial void ratio of 0.605 used in the study, and it increases as the sample void ratio increases. From the microscopic analysis, the variation of η_p under the low ${\sigma }_c$ can be attributed to the enhanced anisotropy of normal contact force and contact normal under the low confining pressure. Finally, a modified Mohr-Coulomb failure criterion is proposed, which can determine the strength of granular soils under the low confining pressure with different void ratios.