DEM Investigation of the Effect of Gradation on the Strength, Dilatancy, and Fabric Evolution of Coarse-Grained Soils

Design of geosystems built on coarse-grained soils with broader gradations are typically based on methodologies developed for clean sands without explicit consideration of the effects of gradation, potentially leading to uncertainty in performance predictions. This study investigates the effect of changes in gradation on the shear strength, stress-dilatancy behavior, critical state parameters, and fabric evolution of coarse-grained soils using three-dimensional (3D) discrete element method (DEM) simulations. The simulations of monotonic isotropically-consolidated drained and undrained triaxial tests were conducted on specimens with coefficients of uniformity (CU) between 1.9 and 6.4 composed of nonspherical particles following the calibration of parameters against experimental triaxial data. Results are used to evaluate the peak and critical state shear strengths, dilatancy responses, critical state lines, shear-induced pore pressures, and fabric evolution. Notably, an increase in CU leads to increases in peak shear strength, total dilation, rate of dilation, negative pore pressure magnitude, and rate of pore pressure generation. The results show that the state parameter better captures the effect of gradation than the relative density because the former accounts for the difference between the initial and critical states. The trends in triaxial parameters are compared with established frameworks to highlight the differences in response resulting from variations in CU. The particle-level measurements indicate that gradation affects the packing characteristics and contact force transmission, where broader gradations result in greater interlocking between coarser particles, and the presence of coarser particles increases the anisotropy of the strong force networks. The finer particles provide resistance to buckling within these strong force networks. Additionally, particles smaller than D10 are inactive in stress transmission, and the percentage of particles inactive in stress transmission decreases with an increasing CU. The combination of macro- and microresults contributes to understanding the mobilization of stress and its dependency on dilatancy in soils of varying gradation.