Column collapse of granular rods

We investigate the collapse of granular rodpiles as a function of particle (length/diameter) and pile (height/radius) aspect ratio. We find that, for all particle aspect ratios below 24, there exists a critical height Hₗ below which the pile never collapses, maintaining its initial shape as a solid, and a second height Hᵤ above which the pile always collapses. Intermediate heights between Hₗ and Hᵤ collapse with a probability that increases linearly with increasing height. The linear increase in probability is independent of particle length, width, or aspect ratio. When piles collapse, the runoff scales as a piecewise power law with pile height, with rf~ ̃ \~H^1.2±0.1 for pile heights below ̃ \~Hc≈0.74 and rf≈H^0.6±0.1 for taller piles.