PROGRESSIVE ROCK FAILURE: OBSERVATIONS, MECHANISMS AND INTERPRETATIONS

All processes responsible for the deformation of rocks must necessarily be applied for some finite time before macroscopic failure ensues. Hence, all rock failure can inherently be described as progressive. By far the most common type of experiment performed to measure rock deformation and failure is the short-term strength test, where rock samples are forced to failure by increasing the applied stress at a constant strain rate. Such tests are generally conducted at relatively high strain rates, even though it is well-known that rock strength is strain-rate dependent; with apparent strength decreasing with decreasing strain rate. The alternative approach is to study longer-term strength by appling a constant stress that is a high percentage of the short-term strength, and simply allowing the rock to deform (strain) progressively over time until failure eventually occurs. Results from these two end-member approaches can be reconciled through the concept of subcritical crack growth where the rate of crack propagation is controlled by the stress-concentration at crack tips and the local environmental conditions.

Under the conditions prevailing in the near-surface and the shallow crust, healing processes will be occurring in rocks contemporaneously with fracturing processes. Just like deformation, the healing processes can also be purely mechanical or environmentally driven (hydro-thermo-chemo-mechanical).

Here, we consider how the juxtaposition of all these competing fracture propagation and fracture healing mechanisms controls the overall progressive deformation and failure of rocks under the range of stress and environmental conditions expected to prevail in the sub-surface and shallow crust of the Earth.