Relative Effects of Angularity, Grain Size, and Sorting on Auto-Acoustic Compaction in Granular Flow

Abstract:

There are thought to be three main rheological regimes exhibited within granular media in the presence of shear stress, each dependent on a dimensionless number, I, representing the relative values of collisional stress between grains and confining stress. At slow shear rates, when I<<1, the granular flow is in the quasi-static regime, and shear stress is supported elastically through multi-grain networks (force-chains). At high shear rates, when I>>1, the flow is in the grain-inertial regime, and shear stress is supported through the transfer of momentum occurring in grain-to-grain collisions.

Experiments conducted using a torsional rheometer found that at intermediate shear velocities, where I approaches 1, force-chain collapse in angular sand samples produces sound waves capable of vibrating the shear zone enough to cause compaction (van der Elst et al., 2012; Lu et al., 2007). Whether or not a granular mixture exhibits this auto-acoustic compaction effect during flow has been observed to be dependent on angularity, grain size, sorting, and mineral strength: more angular grains produce more noise and compact more at intermediate shear velocities than spherical grains do, and smaller angular grains produce more noise and compact more than larger angular grains. We use the same experimental set up to explore the relative importance of the effects of angularity and grain size, comparing various grain sizes and sorting in both spherical and angular granular mixtures.

Accurate assessment of the effect of angularity and grain size on rheology of granular flow will serve as a helpful predictive tool for modeling granular processes including landslides, rockslides, fault ruptures, and desert dune migrations. Grain size and shape can vary greatly system-to-system but are also often easy to observe in the field. We hope to be able to use these types of small scale grain observations to form predictions about the behavior of the larger scale process of which they are a part.