The effect of volume fraction on granular slope stability

We study the stability of granular slopes as a function of the prepared volume fraction 0.58$<\phi<$0.62. A bed of 250$\mu m$ diameter glass beads with an initial slope angle $\theta$=0$^\circ$ and initial $\phi$ is slowly rotated at constant angular velocity to a final angle of $50^\circ$. We monitor the motion of grains at the top surface and observe that the angle at which continuous surface flow occurs is sensitive to $\phi$ and increases from $\theta_0 \approx 26^\circ$ at low $\phi$ (loosely packed) to $\theta_0 \approx 32^\circ$ at high $\phi$ (closely packed). Prior to the uniform failure at $\theta_0(\phi)$ the grain motion during tilting differs between the loosely packed to the closely packed regimes. Tilting loosely packed beds results in rapid intermittent grain rearrangement at the surface; the angle at which these begin is $\theta \approx 15^\circ$s. In the closely packed beds grain motion at the surface is not observed until $\theta \approx 31^\circ$, prior to continuous failure.