The progressive fracture of Lac du Bonnet granite

The strength of intact rock is made up of two components: the intrinsic strength, or cohesion; and the frictional strength. It is generally assumed that cohesion and friction are mobilized at the same displacements such that both components can be relied on simultaneously. Damage testing of samples of Lac du Bonnet granite has shown that as friction is mobilized in the sample, cohesion is reduced. This progressive loss of intrinsic strength and mobilization of friction is modelled using the Griffith locus based on a sliding-crack model. There appears to be a maximum cohesion that can be relied on for engineering purposes, and this strength is less than half of the unconfined compressive strength measured in the laboratory.