Microstructural anisotropy as the origin of dilatancy

Dilation in sheared granular materials has been widely observed, yet its micromechanical origins remain debated. Using discrete element method (DEM) simulations of slow granular flows, we find that grain rearrangements occur intermittently, alternating between jammed phases, where the contact network remains stable, and yielding phases, characterized by rapid plastic rearrangements. Our analysis reveals that, under constant confining stress, granular materials dilate during jammed phases and compact during yielding phases. We show that as the material is sheared, the contact network develops anisotropic features. By incorporating these microstructural details into a macroscopic linear elastic framework, we derive a constitutive equation that explains the volumetric expansion in sheared granular materials. These findings highlight the essential role of elastic grain deformation and microstructural anisotropy in stress transmission and dilation, providing new insights into the mechanics of static and quasi-static granular materials.