The shear strength of rock and rock joints

Rock joints exhibit a wide spectrum of shear strength under the low effective normal stress levels operating in most rock engineering problems. This is due to the Strong influence of surface roughness and variable rock strength. Conversely, under the high effective normal stress levels of interest to tectonophysicists the shear strength spectrum of joints and artificial faults is narrow, despite the wide variation in the triaxial compression strength of rocks at fracture. In Part I of this review, empirical non-linear laws of friction and fracture are derived which explain this paradoxical behaviour and which can be used to predict or extrapolate shear strength data over the whole brittle range of behaviour. Under higher confining pressures the behaviour of rock ceases to be brittle as the brittle-ductile transition is reached. Expressions are derived which quantify this condition and explain the variable transition behaviour of rocks as dissimilar as limestone and shale. At still higher confining pressures the Mohr envelopes describing failure of intact rock eventually reach a point of zero gradient on crossing a certain line, defined here as the critical state line. This critical state is associated with a critical effective confining pressure for each rock. It appears that the dilation normally associated with the shearing of non-planar joints and faults may be completely suppressed if the applied stress reaches the level of the critical effective confining pressure. The empirical laws of friction and fracture were developed during a review of laboratory-scale testing on rock and rock joints. In Part II of this review these laws are applied to the interpretation of full-scale features. The following topics are investigated; the conjugate shear angle of shear joints and faults, the scale effect on frictional strength, the lack of correlation between stress drops measured in laboratory-scale faulting experiments and those back-calculated from major earthquakes, the strength corrosion caused by moisture, and finally the possible effect of fault dilation and water pressure changes at shallow depth in the crust.