INITIATION AND PROPAGATION OF DILATANT RUPTURE ZONES IN GEOLOGICAL MATERIALS.

This paper examines recent work on the initiation of localized rupture in geological materials. Attention is focused on the hypothesis that localization may be considered as an instability in the constitutive description of homogeneous deformation and the approach is placed in the context of the general problem of uniqueness. In terms of plasticity theory, the appropriate constitutive descriptions for soil and rock, due to their frictional nature, involve strain-softening, non-normality of the strain increment to the yield surface, and yield surface vertices. It is emphasized that these violate the assumptions of classical plasticity theory, and, as a consequence, geological materials are more likely to exhibit non-unique modes. In the particular case on non-uniqueness corresponding to the formation of a narrow rupture zone, these issues are given concrete and precise interpretation in terms of physically meaningful parameters. The theoretical framework is used to interpret some experimental observations.