This study investigates the influence of correlations between particle morphology and gradation on the critical shear strength of three-dimensional granular assemblies via numerical simulations. While grain shape is acknowledged to playa central role in the mechanical behavior of granular media, only a few works have explored the combined effects of grain shapes varying with grain size. Employing three-dimensional discrete- element simulations, we explore the shear behavior of samples with diverse particle size distributions, where grain shapes (ranging from spheres to very angular polyhedra) are assigned based on the relative size of each particle. Using drained triaxial shear tests, we analyze the macroscopic behavior up to large deformation levels. Micro-mechanical analyses are also conducted to understand the underlying mechanisms governing the observed macroscopic behavior, highlighting the role of grain connectivity and force transmission. Surprisingly, well graded samples composed of coarse angular grains and fine rounded ones are not capable of developing higher shear strengths than uniform samples with the same diversity on grain shapes. Conversely, samples where coarser grains are rounded and fines are angular, show constant shear strength as the particle size distribution becomes broader. These findings underscore the importance of simultaneously considering grain size and shape distributions for the assessment of realistic granular soil behavior.

The applicability of particle-scale modeling using the discrete-element method (DEM) is typically evaluated by comparing simulation results with stress-strain responses observed in elementary tests. This validation at the global level may not guarantee that the simulation can capture realistic particle-level motion. Thus, this study investigated the applicability and limitation of two types of DEM models, through the comparison with experimental results of biaxial shearing tests on bidisperse granular assemblies comprising circular (round) and hexagonal (angular) particles under various confining pressures. Experimental data wherein particle rotations were identified by novel image analysis technique were used to evaluate whether the DEM models could accurately reproduce macroscopic stress-strain relationships and microscopic particle responses. Experimental findings suggested that particle rotations play a crucial role in granular deformation and are influenced by the particle shape. A detailed DEM model with precise particle shapes effectively replicated both macroscopic stress-strain relationships and microscopic responses, including particle rotation and interlocking at global and local levels. Conversely, a simpler ad hoc DEM model, which incorporates rolling resistance for circular particles, could imitate the stress-strain relationships of hexagonal particles but fell short in replicating microscopic responses accurately.