Anticrack and Avalanche Triggering

Abstract

anticrack is a form of slope failure characterized by the nucleation and propagation of shear-loaded, mixed-mode fractures that delaminate the weak layer over a wide area. The resulting non-linear collapse wave can drive avalanche hazard and triggering. In this article we calculate the state of stress perturbation generated by a simple, model anticrack. We show that it generates a shear wave with an amplitude that depends on the strength of the weak layer and can be interpreted in terms of a Griffith stress concentrator. This stress concentration produces en-echelon fractures, similar to those observed in the field. We show that these en-echelon fractures contribute to the large scale deformation of the collapse wave and the consequent increase in avalanche kinetic energy.

In models in which brittle fracture is assumed, a crack is characterised by a single size parameter, the radius or half-length of the crack, and a unique characteristic mechanical energy, called the crack energy V(r). In addition to the stress applied to the crack, this energy is also dependent on cohesion, geometry, applied strain and any material defects (e.g. pores).

In a continuum description of the stress field in the weak layer the mechanical energy of an anticrack can be divided into a contribution proportional to t2 and a portion that is proportional to s2. We use a two-dimensional BEM code to calculate the state of stress induced by a model anticrack CB, and compare it with the normal stresses obtained with the Eshelby approach. Near the anticrack tip the anticrack solution is very close to the Eshelby solution, but for any distance beyond this point the mismatch drops to less than 1%.