ON THE STRESS DEPRESSION UNDER A SANDPILE

The observed minimum in the normal stress beneath the highest portion of a 'sandpile' (a pile of granular material) is a counter-intuitive r esult that has long remained unexplained. In this paper, we suggest th at spatial size differentiation, where the larger particles are separa ted from smaller particles within the pile, creates an increase in the internal horizontal stress distribution, which in turn gives rise to a minimum in the normal stress, similar to what is observed experiment ally. We develop a simple analytic model for the normal stress that en capsulates the existence of horizontal contact forces between adjacent colevel particles. We compare this analytic model to the results of a n n-body simulation code, where we obtain close quantitative agreement for small sandpiles. Finally, experimental results from a real sandpi le are replicated by an n-body code, but only for a distribution of ma terial where the largest particles are at the top of the pile.