The effect of a gouge layer on rupture propagation along brittle shear fractures in deep and high-stress mines

The presence of fault gouge and the generation of wear material between two sliding rock surfaces plays a critical role in slip weakening and propagation of ruptures along underground brittle shear fractures forming ahead of tabular excavations in deep and high stress gold mining. We performed two types of friction experiments: one with a fault gouge layer between two sliding surfaces, and the other without a fault gouge layer ‘rock-on-rock’, both under room dry conditions at slip velocities ranging from ~1.0 mm/s to 1200 mm/s. These friction experiments revealed a remarkable difference in the frictional weakening behaviour, e.g., rock-on-rock friction experiments show weakening behaviour at lower slip velocity (~5 mm/s) and generally has lower frictional strength than those with the intervening fault gouge between sliding surfaces. This study shows that the existence of the fault gouge layer between sliding rock surfaces delays the onset of fault weakening (i.e., slip weakening displacement of gouge layer experiments is larger compared to rock-on-rock experiments). It is proposed that flash heating may be the main active weakening mechanism within both our gouge and rock-on-rock experiments, and provides a feasible account for the observed weakening. The observed slip weakening displacement (D_c) differences may be attributed to the presence of a gouge layer between sliding surfaces, which has many more contacts during sliding compared to rock-on-rock experiments, thus reducing the average slip velocity per contact, consequently, the potential for activation of flash heating which delays the onset of weakening. Here we suggest that we may be able to describe brittle shear fracture rupture propagation process along underground brittle shear zones by conducting low, intermediate and high slip velocity friction experiments with and without an intervening fault gouge between sliding rock surfaces. These findings should have important implications for the modelling of rupture propagation processes in underground shear zones, a phenomenon that influences the severity of rockbursts, and hence the safety of mine workers and mining operations.