Reversible Compaction in Sheared Granular Flows and Its Significance for Nonlocal Rheology

Abstract

Naturally occurring granular flows like landslides often have a gas-like rapidly moving layer immediately adjacent to a slower quasistatic layer. Determining the nature of coupling between these regimes is critical to capturing flow behavior. Using image analysis and a rheometer, we measure dilation, acoustic energy, and velocity profiles for quartz sand sheared in a geometry that imposes multiple flow regimes at once. We show that acoustic energy resulting from grain collisions in the fastest part of flow causes (a) weakening and compaction of the transitional regime layer between quasistatic and fast flow and (b) increasing slow flow via reduced friction. Thus, the volume of the entire shear zone is governed by competing effects of dilation near the boundary and compaction in the more distant transitional layer. This understanding of the partition of volume change across the shear layer can guide models of natural shear zones and interpretation of deposits.