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

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. Plain Language Summary Depending on the speed of movement, flowing granular materials can behave like a solid, a liquid, or a gas. In the natural world, flowing granular materials-like sands, gravels, powders, and boulders-move through complicated channels and pathways, often changing their speed of movement either across the channel or vertically with depth. These types of natural granular flows make up many natural hazards, like landslides, debris flows, avalanches, and earthquakes. We examine how a granular material will transition from slow, solid-like movement to very fast, gas-like movement in a single granular flow with different parts moving at different speeds. We find that the onset of slow movement and the nature of the transition between the slower and faster sections of flow are both dependent on how fast the fastest part of the flow is going. The higher the speed of the faster gas-like part of the flow, the more acoustic noise is produced by grains banging into each other, and this noise in turn affects the strength and resistance to flow in the slower parts of the flow.