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, φ0. 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 φ0[0.58-0.63] and differed above or below the granular critical state, φc, defined as the onset of dilation as a function of increasing volume fraction. For φ0-φc<0, slopes experienced short, rapid, precursor compaction events prior to the onset of a sustained avalanche. Precursor compaction events began at an initial angle θ0=7.7±1.4' and occurred intermittently prior to the onset of an avalanche. Avalanches occurred at the maximal slope angle θm=28.5±1.0'. Granular material at φ0-φc>0 did not experience precursor compaction prior to avalanche flow, and instead experienced a single dilational motion at θ0=32.1±1.5' prior to the onset of an avalanche at θm=35.9±0.7'. Both θ0 and θm increased with φ0 and approached the same value in the limit of random close packing. The angle at which avalanching grains came to rest, θR=22±2', was independent of φ0. 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 φ0, with φ0-φc<0 precursor flow extending deeper into the granular bed and occurring more rapidly than precursor flow at φ0-φc>0. Our study elucidates how initial conditions - including volume fraction - are important determinants of granular slope stability and the onset of avalanches.