Contact Force Distribution Beneath a Three-Dimensional Granular Pile

A simple elasto-optical method has been used to measure the normal force distribution under three-dimensional conical piles, for a range of granular materials. The method is based on the measurement of contact diameters of a monolayer of steel balls between the pile and an elastomeric substrate. Experiments on sandpiles confirm the existence of counter-intuitive dips in the pressure at the centre of the pile, as reported previously in the literature; the dips were also found in piles of small (ca. 180 $\mu$m diameter) glass beads. They were suppressed, however, on increasing the glass bead diameter by a factor of ca. 3. Variations in surface roughness were found to have little effect on the pressure profiles. The probability density function for the normal contact force was approximately negative exponential in form.