Micromechanical Mechanisms Governing the Shear Behavior of Sand–Clay Mixtures with Different Clay Contents: Laboratory Tests and DEM Simulations

Understanding how clay content alters force transmission and particle rearrangement is essential for explaining the shear behavior of sand–clay mixtures. Laboratory simple shear tests and three-dimensional discrete element simulations were conducted on clean sand and mixtures containing 10%, 20%, 30%, and 40% clay. After calibration against the laboratory shear responses, the simulations were used to track contacts, force transmission, and particle rearrangement. Increasing clay content raised the coordination number but reduced the proportion of sand–sand (S–S) contacts. Despite their declining number, S–S contacts retained a disproportionately large share of the summed normal contact force and a higher force-to-contact ratio than sand–clay (S–C) and clay–clay (C–C) contacts. As clay content increased, force transmission shifted from a predominantly sand-based skeleton toward greater participation of S–C and C–C contacts, while shear resistance decreased. Non-affine displacement was concentrated mainly in the lower part of the specimens, with larger departures from locally affine motion during the earlier shear interval. Together, these observations relate the differences in shear response among the mixtures to changes in contact structure, normal-force distribution, and localized particle rearrangement.