Wall effects on granular heap stability

– We investigate the effects of lateral walls on the angle of movement and on the angle of repose of a granular pile. Our experimental results for beads immersed in water are similar to previous results obtained in air and to recent numerical simulations. All of these results, showing an increase of pile angles with a decreasing gap width, are explained by a model based on the redirection of stresses through the granular media. Two regimes are observed depending on the bead diameter. For large beads, the range of wall effects corresponds to a constant number of beads whereas it corresponds to a constant characteristic length for small beads as they aggregate via van der Waals forces. A characteristic of a sand pile is that it forms a non-zero angle to the horizontal. Two angles can be defined for a heap of granular matter: the angle of repose θ r , under which no flow can occur, and, a few degrees larger, the maximum angle of stability θ m first noticed by Bagnold [1]. This angle, also called the angle of movement, is the one at which an avalanche spontaneously occurs at the surface of the pile, making the slope angle relax to the angle of repose. Between these two angles is a region of bistability as the heap can be static (" solid state ") or flowing (" liquid state "). The values of these two angles have been known for long to depend on many parameters, namely the shape, the roughness, the size distribution and the packing fraction of grains, as well as the packing history [2]. Humidity, by introducing cohesion through capillary bridges between grains, is known to strongly increase the stability of a heap [3]. The presence of close lateral walls, by changing the boundary conditions, also increases the stability of a heap, as both angles θ m and θ r increase when the gap width between the confining walls decreases [4]. This is often explained qualitatively by the presence of particle arches between the walls [5,6,7]. Particle arches or divergence of force networks lead to many remarkable effects. The saturation of the pressure at the bottom of containers, known as the Janssen effect [8], makes hourglasses flow at constant speed. In silos, arch formation may lead to a complete jamming of the flow, with potential damages for industry. Most of laboratory model experiments on granular …