Three-Dimensional Particle Shape and Stress–Dilatancy Behavior of Carbonate and Silica Sands at Low Normal Stresses: A Direct Simple Shear Investigation

Abstract

This multiscale study investigates the influence of three-dimensional (3D) particle shape on the stress–dilatancy behavior of granular materials under low-stress simple shear conditions. High-resolution X-ray micro-computed tomography (micro-CT) was first used to quantify the particle shapes of 11 different sands, spanning silica, carbonate, and mixed compositions. The images with a reconstructed voxel size of 5.5    μm were segmented using a watershed-based algorithm, which isolated more than 10,000 grains per material. This enabled particle-level quantification of 3D shape descriptors for each sand. The analyzed particle-size distributions were validated against those measured from mechanical sieve analysis. In the second phase, direct simple shear tests were conducted on three selected materials, UWA superfine silica, ledge point, and glass beads, considering relative densities of 0.4 to 0.9 and normal pressures of 25 to 150 kPa. The results show that particle shape affects peak friction, excess friction, and dilation response, but the magnitude of this effect depends on density and normal stress. Particle-shape effects are strongest at lower density and lower normal stress, where particle rearrangement and dilation are more readily mobilized. As relative density or normal stress increases, the responses of the three materials converge, indicating that the mechanical response becomes less sensitive to particle shape. These results suggest that particle shape should be treated as a state-dependent modifier of stress–dilatancy behavior at low stress, rather than as a single fixed material correction.