ABSTRACT
A very important property of granular materials is their ability to flow (or yield), i.e. to undergo large deformations without stress change, as soon as the stresses and the void ratio obtain their critical values. This sort of flow should be attributed as `plastic' flow and distinguished from the flow of fluids. The latter has a pronounced viscous (rate-dependent) character. Plastic flow occurs as soon as the stress state T and the strain rate D fulfil the condition h(T, D) = 0. In the theory of elastoplasticity the condition in terms of T is called the yield (limit) surface, and the condition in terms of D is called the flow rule.
