Micro-macro and rheology in sheared granular matter

Soil, made up of countless interacting grains is a perfect example of granular materials. When soil is sheared, the flow is confined to narrow regions called flowing zones. In this thesis, DEM simulations are used to study the flowing zones induced by the geometry called the splitbottom geometry, with an aim to link the microscopic properties to the macroscopic bulk behavior. To begin with, we study the pairwise collisions between two cohesive particles. A contact model, which takes all essential effects into account is introduced. With increasing impact velocity, a stickreboundstickrebound behavior is observed. First sticking originates from the shortrange noncontact attractive forces, while the second appears due to the plasticity induced cohesion and dissipation. To study the effect of particle properties on the bulk behavior, first focus is the contact friction. Both the shear resistance and deviatoric fabric first increase and then saturate with increasing friction, while contact number density decreases. Increasing friction increases heterogeneity in spatial distribution of both the normal and tangential forces. Next, cohesion is introduced. To determine the intensity of cohesive forces, a nondimensional parameter, Bond number Bo which compares attractive forces to external compression forces is defined. Bo captures the crossover from essentially noncohesive freeflowing granular assemblies Bo<1 to cohesive ones Bo>1. As next step, the effect of particle softness and gravity are studied. In literature, bulk behavior has been assumed to be independent of both. However our analysis, shows that the shear resistance of the material decreases systematically with increase in either. While, the shear resistance can be described as a unique power law, when analyzed against a nondimensional number, which is ratio of time scales related to both. Structural anisotropy shows similar behavior, leading to an interesting interpretation that shear resistance accompanies anisotropy in the contact network. Finally, we look at the rheology of granular flows. Both local shear resistance and structural anisotropy increase with increasing strain rate. This shows that the shear resistance increases with strain rate mainly due to an increase in structural anisotropy, which indicates that the mesoscopic contact network dominates the behavior even for fast flows.