Geometrical analysis of sand piles on small platforms

The underwater geometry of small sand piles is relevant to the removal of noncolloidal sediment by filtration. The critical size range for clogging occurs when noncolloidal particles are about one-tenth the diameter of the media grains. No previous experimental work has examined the formation of sand piles at this scale. Sand pile formation was investigated on small platforms (roughened spheres, flat surfaces, and irregular stones) in the critical size range for clogging. The size of sand piles formed on platforms of irregular crushed stone increases with sand grain diameter. This did not occur for spherical media. Further, this observation contrasts with previous measurements of the angle of repose, which showed a weak decrease with sand grain size. A geometric model for small piles is presented to account for the increase in size of the sand pile as a function of grain size and platform shape.