Naturalistic Granular Flows: Using Experiments to Apply Granular Physics to Geophysical Shear Systems

Granular flows are ubiquitous in nature and deviate in potentially important ways from idealized systems. Shearing in fault zones, landslides and debris flows involve water-saturated, rough particles of varying composition and particle size accelerating through a range of velocities while abutting stationary grains. In addition, the types of observations that can be made in nature also differ from laboratory and experimental situations. Here we present a range of experiments that attempt to isolate these naturalistic features and elucidate their implications. Specifically we find that: (1) Natural, rough particles emit acoustic energy during shear, which can affect the flow behavior at intermediate velocities, (2) The acoustic energy observed in the lab depends on the particle diameter cubed, (3) Mineralogy affects the intermediate to high velocity flow behavior primarily through the combination of elastic, fracture and plastic material properties captured by the critical slip distance required for fracture nucleation, (4) high and low shear rate regions of the flow are coupled through acoustic waves, creating a non-trivial boundary condition at the base of a landslide or other natural shear system, (5) Fluid-saturation can result in dilation at high velocities regardless of the initial compaction state and therefore dilatory drainage is expected even in repeated activation of the same shear flows, and (6) preserved, fine-grained apparent shear bands in the geological record can be indicative of fine grains migrating at high velocities. These experimental observations suggest important directions for theoretical development in order to bring granular physics to bear on natural systems.