Effect of volume fraction on granular avalanche dynamics

Abstract

We study the evolution and failure of a granular slope as a function of prepared volume fraction, ϕ(0). We rotated an initially horizontal layer of granular material (0.3-mm-diam glass spheres) to a 45° angle while we monitor the motion of grains from the side and top with high-speed video cameras. The dynamics of grain motion during the tilt process depended sensitively on ϕ(0)∈[0.58-0.63] and differed above or below the granular critical state, ϕ(c), defined as the onset of dilation as a function of increasing volume fraction. For ϕ(0)-ϕ(c)<0, slopes experienced short, rapid, precursor compaction events prior to the onset of a sustained avalanche. Precursor compaction events began at an initial angle θ(0)=7.7±1.4° and occurred intermittently prior to the onset of an avalanche. Avalanches occurred at the maximal slope angle θ(m)=28.5±1.0°. Granular material at ϕ(0)-ϕ(c)>0 did not experience precursor compaction prior to avalanche flow, and instead experienced a single dilational motion at θ(0)=32.1±1.5° prior to the onset of an avalanche at θ(m)=35.9±0.7°. Both θ(0) and θ(m) increased with ϕ(0) and approached the same value in the limit of random close packing. The angle at which avalanching grains came to rest, θ(R)=22±2°, was independent of ϕ(0). From side-view high-speed video, we measured the velocity field of intermittent and avalanching flow. We found that flow direction, depth, and duration were affected by ϕ(0), with ϕ(0)-ϕ(c)<0 precursor flow extending deeper into the granular bed and occurring more rapidly than precursor flow at ϕ(0)-ϕ(c)>0. Our study elucidates how initial conditions-including volume fraction-are important determinants of granular slope stability and the onset of avalanches.

The dynamics of legged locomotion in heterogeneous terrain: universality in scattering and sensitivity to initial conditions

“…• , exceeding the maximum angle of stability [14] for most granular media used in locomotion studies [15] [3]. To generate states of arbitrary heterogeneity, a 3-axis motor system (Copley, STA25, STB25, XTB38) was installed above the trackway, enabling the motor end-effector to move in three dimensions.…”

Rapid In Situ Characterization of Soil Erodibility With a Field Deployable Robot

“…The dune stoss face is composed of gypsum sand that has been compacted and mildly cemented under deposition and is now being exposed again by erosion due to dune migration (McKee, 1966). It has been documented that compaction can influence the yield stress and rheology of granular materials (Gravish & Goldman, 2014;Nedderman, 1992;Richard et al, 2005). It is also likely that cementation-which is more common in White Sands than other dune fields due to the hygroscopic nature of gypsum (Schenk & Fryberger, 1988)-plays a role in strengthening the soil.…”

“…It is also likely that cementation-which is more common in White Sands than other dune fields due to the hygroscopic nature of gypsum (Schenk & Fryberger, 1988)-plays a role in strengthening the soil. Future laboratory experiments could and should tune granular compaction (e.g., Gravish & Goldman, 2014) to explicitly examine its effect on shear strength as measured with our robotic plowing test.…”

Precursors and triggering mechanisms of granular avalanches

“…Beside the numerical simulations of 2D and 3D [19] inclined granular packings, various experimental methods have been used to expose the secrets of these strange happenings: direct observation of the surface or the side of the box with a camera [20,21,10], X ray imaging technique [22], interferometric technique based on diffusive wave spectroscopy [17], and acoustic methods.…”

“…Most of these parameters affect the contacts between grains, their capacity to influence the precursors is not surprising. In the same way, the preparation of the packing plays an important role [22,24,20], sometimes neglected by authors who get results seemingly paradoxical. It never should be forgotten that it is the force network which is submitted to strong modifications during the loading.…”