Crushing-induced angularity governs critical state strength in granular soils

This study investigates the impact of particle size and shape covariation due to crushing on the micro- and macroscale mechanical behaviors of granular soils. A validated two-dimensional discrete element model was employed to perform numerical simulations capturing the effects of particle breakage. To replicate the mechanical transformations induced by particle breakage, sample gradations were modified by gradually increasing the proportion of finer particles. Two scenarios were considered: (a) modifying gradation alone and (b) simultaneously altering particle size and shape to more accurately represent realistic crushing behavior. The findings indicate that changing gradation alone led to a downward shift of the critical state line (CSL) on the mean stress (p)–void ratio (e) plane, without a significant change in the critical state friction angle. However, with increased angularity resulting from shape alteration due to crushing, compressibility decreased more gradually, and the critical state friction angle increased, resulting in a more pronounced downward shift of the CSL in the p-e plane. This shift became prominent with greater particle angularity variation. On the microscopic level, round particles exhibited a “ball-bearing” effect due to their higher rotational mobility, which inhibited stable force chain development. In contrast, angular particles resisted rotation and acted as stable force-transmitting bridges, thereby enhancing shear resistance. These findings highlight the need to account for particle shape distribution and gradation covariation to accurately predict mechanical responses in granular systems subjected to particle crushing.