THE PROGRESSIVE FRACTURE OF LAC DU BONNET GRANITE

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