Internal avalanches in a granular medium

Avalanches of grain displacements can be generated by creating local voids within the interior of a granular material at rest in a bin. Modeling such a two-dimensional granular system by a collection of monodisperse discs, th e system, on repeated perturbations, shows all signatures of self-organized criticality. During the propagation of avalanches the competition among gr ains creates arches and in the critical state a distribution of arches of d ifferent sizes is obtained. Using a cellular automata model we demonstrate that the existence of arches determines the universal behavior of the model system. [S1063-651X(98)51212-3].