Effects of Maximum Particle Size on the Shear Behavior of Coarse-Grained Soil: Direct Shear Tests and Discrete Element Simulations

To investigate scale effects on the shear strength and mesoscale load-bearing structure of coarse-grained soil, four specimen groups with maximum particle sizes of 10, 20, 40, and 60 mm were tested in large-scale direct-shear tests under normal stresses of 100–400 kPa. Corresponding three-dimensional PFC3D discrete element models were established and assessed against the shear stress–displacement curves and strength parameters. Particle displacement, contact force chains, coordination number, contact fabric, and the shear stress field were then examined. The simulations agreed reasonably well with the laboratory tests: the mean relative error across 16 peak shear-stress comparisons was 4.09%, and the maximum error was 7.27%. At a given normal stress, peak shear stress increased with maximum particle size. As the maximum particle size increased from 10 to 60 mm, peak shear stress increased by 61.82% and 39.23% under normal stresses of 100 and 400 kPa, respectively; apparent cohesion increased from 6.8 to 25.5 kPa, and the internal friction angle φ increased from 26.8° to 33.9°. The total number of contacts decreased from 60,740 to 8537 and the coordination number from 4.951 to 2.564, whereas the strong-contact load-bearing proportion increased from 71.36% to 82.93%. Thus, the increase in shear strength was associated with load concentration on a smaller number of strong contacts rather than an increase in contact number. Specimens with larger particles developed more prominent strong force chains and clearer inclined load-transfer paths. Their contact fabric became increasingly directional, while the shear stress field changed from a relatively smooth banded pattern to localized patches. The preferred orientations of the strong force chains, contact fabric, and stress field were broadly consistent. Under an equivalent gradation area, increasing the maximum particle size strengthened interlocking among coarse particles and reorganized the load-bearing network from distributed transfer through many contacts to concentrated transfer through fewer strong contacts, thereby increasing the shear strength.