On the pressure dependence of strength of rocks and the coulomb fracture criterion

The observed microfracturing before faulting in rocks provides a basis for a physical model for the internal friction and the cohesive stress of the Coulomb criterion. Based on recent experimental results in the literature, the applicability of the Coulomb criterion is reexamined. The pressure dependence of strength of brittle rocks can be predicted approximately by the Coulomb criterion using measured values of sliding friction, except for a low-pressure region for certain rocks and near the brittle-ductile transition pressure. Deviation at low pressure seems to be explained either by the effect of cracked state or by different fracture mechanisms at low confining pressures, and deviation near the transition pressure is attributed to the increase of local yielding or fracturing before faulting.