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 H-l below which the pile never collapses, maintaining its initial shape as a solid, and a second height H-u above which the pile always collapses. Intermediate heights between H-l and H-u 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 r(f) similar to(H) over tilde (1.2 +/- 0.1) for pile heights below (H) over tilde (c) approximate to 0.74 and r(f) approximate to H-0.6 +/- 0.1 for taller piles.