Numerical study on the shear strength under low confining pressure of sandy soils with different particle shapes

The shear strength of sands with varying particle shapes demonstrates significantly different variations with decreasing confining pressure. Aiming to investigate the effect of particle shape on the shear strength variation with confining pressure from the macro and micro scales, this paper reconstructed a series of irregular particles based on the shape of sandy soils and then conducted DEM simulations of triaxial compression tests under the confining pressures ${\sigma }_c$ widely ranging from 2 to 400 kPa. The result indicates that as ${\sigma }_c$ increases, the peak stress ratio of spherical particles rapidly increases and then stabilizes while that of irregular particles slightly increases and then decreases above the critical ${\sigma }_c$. Moreover, the value of critical low ${\sigma }_c$ decreases with the increasing particle irregularity. An improved strength envelope is proposed to reflect the nonlinear variation of shear strength with low confining pressure and particle shape. The microscopic analysis reveals that the nonlinear increase in peak stress ratio of spherical particles results from the increase of normal contact force anisotropy under low ${\sigma }_c$ and the decrease of normal contact force anisotropy under high ${\sigma }_c$. In contrast, the irregular particles interlock with each other and reach sliding limit under low confining pressures. As ${\sigma }_c$ increases beyond the critical value, the normal contact force anisotropy decreases, leading to a decline in peak stress ratio.