DEM Investigation of the Critical State Behavior of Granular Soils: Effect of Intermediate Principal Stress and Particle Size Distribution

This study investigates the roles of the intermediate principal stress ratio (b) and particle size distribution (PSD) in governing the critical state behavior of granular soils using the discrete element method (DEM). A series of π-plane analyses is conducted to isolate the b effect under constant mean effective stress. The simulations are performed on both spherical and multisphere-clumped particle assemblies, encompassing a wide range of PSD and initial density. The results reveal that both b and PSD significantly influence the macroscopic response of soils, particularly regarding the dilation and peak strength. Specifically, the intermediate principal stress ratio (b) governs the critical stress ratio, while its impact on the critical state void ratio remains minimal. In contrast, PSD has a pronounced effect on the critical state in the void ratio–mean effective stress plane, inducing a distinct downward shift and counterclockwise rotation with increasing uniformity coefficient Cu. Microscopic analyses, including the mechanical coordination number and fabric anisotropy, are employed to bridge the particle-scale mechanisms with the macroscopic responses across diverse loading conditions. Notably, a distinct correspondence between stress ratio, void ratio, and fabric anisotropy emerges at the critical state, signifying the uniqueness of critical state conditions. These findings advance the understanding of granular soil behavior and provide insights for improving critical state-based constitutive models.