Basal force fluctuations and granular rheology: Linking macroscopic descriptions of granular flows to bed forces with implications for monitoring signals

Abstract

Granular flows are ubiquitous in nature with single flows traversing a wide range of dynamic conditions from initiation to deposition. Many of these flows are responsible for significant hazards and have the ability to generate remotely detectable seismic signals. These signals provide a potential for real-time flow measurements from a safe distance. To fully realize the benefit of seismic measurements, basal granular forces must be linked to macroscopic internal flow dynamics across a wide range of flow conditions. We utilize discrete element simulations to observe dry and submerged granular flows under plane-shear and inclined flow configurations, relating bulk kinematics to basal force distributions. We find that force fluctuations scale with non-dimensional shear-rate ($I$), and this scaling tracks three flow regimes that can be described by $\mu (I)$ rheology, as well as a fourth regime that marks a `phase change' from a liquid-like to a gas-like state: (1) an unsteady particle rearrangement regime when $I<10^{-3}$, where basal forces are dominated by low frequencies; (2) an intermediate regime when $10^{-3}<I<10^{-2}$, where basal forces start to become noise-like, (3) a transitional regime at $10^{-2}<I<10^{-1}$, where the increase in basal tractions with increasing shear-rates stalls as the granular bed dilates, partially destroying the contact network and memory stored in the particle contact network, and (4) a fully collisional regime when $I>10^{-1}$, where the signal is nearly flat up to a cutoff frequency. This effort suggests that basal forces can be used to interpret complex granular processes in geophysical flows.

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