The ISRM Suggested Methods for Rock Characterization, Testing and Monitoring: 2007-2014

The figure on the front cover of this ORANGE BOOK emphasizes the general non-linearity of shear strength envelopes for intact rock, for stress-induced fractures, and for rock joints-the latter showing great similarity to rock fill due to common, highly stressed asperities and contact points. A combination of triaxial and direct shear tests is illustrated as if at the same scale. The reality may be widely different sample sizes. Although nonlinear behavior has been a focus of rock mechanics developments for the last half century, it remains convenient to consider an approximately 'linear' behavior when the range of stress is limited. Thus the Coulomb and Mohr-Coulomb criteria continue to be utilized in shear test interpretation, despite the availability of non-linear alternatives. When interpreted in a strictly correct manner, only intact rock displays true cohesive and tensile strength, and could be extrapolated into the tensile sector of this diagram. The critical viewer will note the superposition of several different stress levels, as the five illustrated 'samples' are seldom tested over the same range of stress. Missing from the test samples is their aggregate: the rock mass, which may have at least four of the symbolic samples contributing to its shear strength, each at different magnitudes of strain. (Figure modified from Barton, 2006.