Exploring the micromechanical origin of shear response in granular materials induced by size non-uniformity

This study investigates the role of particle size distribution (PSD) in the shear response of granular materials using discrete element modeling (DEM). Three series of DEM samples, two gap-graded and one continuously graded, are prepared under different initial packing densities and sheared quasi-statically to the critical state. The DEM results indicate that the PSD crucially influences the macroscopic stress at the peak state but does not have an impact on it at the critical state. Microscopically, the PSD affects the granular structure and causes significant inhomogeneity in the contact network. The origin of the phenomenological observations can be traced through the stress-force-fabric analysis. At the peak state, it is found that the anisotropy in normal contact force, which is stronger with wider polydispersity, plays the predominant role in determining the overall stress response. When the particles have rearranged sufficiently upon shearing at the critical state, the geometric part of anisotropy starts showing dependence on the PSD and compensates for the mechanical part of anisotropy, thereby leading to an independence of overall stress on size non-uniformity.