Multiscale Force Networks in Highly Polydisperse Granular Media

We investigate highly polydisperse packings subjected to simple shear by contact dynamics simulations. A major unsolved issue is how granular texture and force chains depend on the size polydispersity and how far they influence the shear strength. The numerical treatment was made possible by ensuring the statistical representativity of particle size classes. An unexpected finding is that the internal friction angle is independent of polydispersity. We show that this behavior is related to two mechanisms underlying the stability of force chains: (i) The class of largest particles captures strong force chains, and (ii) these chains are equilibrated by weak forces carried by increasingly smaller particles as the size span broadens. In the presence of adhesion between particles, the Coulomb cohesion increases with size polydispersity as a result of enhanced force anisotropy.