Influence of Stress Path and Particle Quantity on Granular Soil Critical State and Fabric: Insights from DEM Simulations

Abstract Critical-state soil mechanics provides a fundamental framework for describing the shearing behavior of granular materials. This study employs the discrete-element method (DEM) to investigate the critical-state behavior of granular materials under direct shear, direct simple shear, and true triaxial compression loading paths. Simulations are conducted using sphere-cluster particles whose parameters were adopted from prior validated studies of a poorly graded sand. The evolution of stress ratio ( q / p ′) and void ratio is analyzed across a range of initial relative densities, confining pressures, and two particle assemblages with different particle quantities. The evolution of fabric anisotropy is quantified using fabric tensors for contact normal forces and particle orientations to examine the micromechanical origins of critical-state behavior. This paper also demonstrates that bulk properties remain consistent across different particle sizes. The peak strengths and dilation angles can be matched across the selected particle quantities and test types, whereas the dilation and displacement required to reach critical state depend on the imposed boundary conditions. Direct shear simulations reveal greater sensitivity to particle quantity, with larger particles producing pronounced dilation due to rigid confinement. Direct simple shear reduces boundary effects and yields consistent stress–strain behavior across assemblages, although volumetric expansion remains affected by particle size. In contrast, true triaxial simulations show that both stress–strain and volumetric responses are independent of particle quantity, as volume change occurs in all three directions. Results show convergence to a unique critical state characterized by constant stress state and volume, and a unique critical-state line is observed in both stress and volumetric spaces. Fabric anisotropy measures approach stable critical values at large strains, independent of initial density, loading path, and particle quantity, indicating the existence of a unique critical-state fabric.