Avalanches and Flow Dynamics of a Collapsing Granular Pile

Results of an experimental investigation of the dynamics of flow in a collapsing granular pile are reported. The collapse of the pile is induced by reducing the length of the base support of the triangular pile. The dynamics are studied by using the statistics of the avalanches during the collapse. It is found that the collapse consists of two types of avalanches and two angles of repose are needed to describe the shape of the collapsing granular pile. Corresponding to these two angles of repose, the collapse of the granular pile can be characterized by local and global avalanches. A system-wide global avalanche will be triggered only when a large enough part of the base of the granular pile is removed. Otherwise, small avalanches localized near the tip of the pile will be produced. It is also found that the probability distributions of the avalanches depend on the sizes of the avalanches under consideration. The size of the localized critical tip corresponding to the local avalanche is independent of the size of the whole grain pile and can be estimated theoretically from a simple static friction model. Furthermore, the surface flow dynamics during the global avalanche is also measured. The mean velocity of the grains is found to decay exponentially into the depth of the surface layer. A slipping layer can be identified which has a non-trivial time dependence.