Effect of volume fraction on granular avalanche dynamics

We study the evolution and failure of a granular slope as a function of prepared volume fraction, phi(0). We rotated an initially horizontal layer of granular material (0.3-mm-diam glass spheres) to a 45 degrees 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 phi(0) is an element of [0.58-0.63] and differed above or below the granular critical state, phi(c), defined as the onset of dilation as a function of increasing volume fraction. For phi(0) - phi(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 theta(0) = 7.7 +/- 1.4 degrees. and occurred intermittently prior to the onset of an avalanche. Avalanches occurred at the maximal slope angle theta(m) = 28.5 +/- 1.0 degrees Granular material at phi(0) - phi(c) > 0 did not experience precursor compaction prior to avalanche flow, and instead experienced a single dilational motion at theta(0) = 32.1 +/- 1.5 degrees. prior to the onset of an avalanche at theta(m) = 35.9 +/- 0.7 degrees. Both theta(0) and theta(m) increased with phi(0) and approached the same value in the limit of random close packing. The angle at which avalanching grains came to rest theta(R) = 22 +/- 2 degrees, was independent of phi(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 phi(0), with phi(0) - phi(c) < 0 precursor flow extending deeper into the granular bed and occurring more rapidly than precursor flow at phi(0) -phi(c) > 0. Our study elucidates how initial conditions-including volume fraction-are important determinants of granular slope stability and the onset of avalanches.