Articles | Volume 7, issue 2
Research article
01 Apr 2016
Research article |  | 01 Apr 2016

The Mohr–Coulomb criterion for intact rock strength and friction – a re-evaluation and consideration of failure under polyaxial stresses

Abigail Hackston and Ernest Rutter

Abstract. Darley Dale and Pennant sandstones were tested under conditions of both axisymmetric shortening and extension normal to bedding. These are the two extremes of loading under polyaxial stress conditions. Failure under generalized stress conditions can be predicted from the Mohr–Coulomb failure criterion under axisymmetric shortening conditions, provided the best form of polyaxial failure criterion is known. The sandstone data are best reconciled using the Mogi (1967) empirical criterion. Fault plane orientations produced vary greatly with respect to the maximum compressive stress direction in the two loading configurations. The normals to the Mohr–Coulomb failure envelopes do not predict the orientations of the fault planes eventually produced. Frictional sliding on variously inclined saw cuts and failure surfaces produced in intact rock samples was also investigated. Friction coefficient is not affected by fault plane orientation in a given loading configuration, but friction coefficients in extension were systematically lower than in compression for both rock types. Friction data for these and other porous sandstones accord well with the Byerlee (1978) generalization about rock friction being largely independent of rock type. For engineering and geodynamic modelling purposes, the stress-state-dependent friction coefficient should be used for sandstones, but it is not known to what extent this might apply to other rock types.

Short summary
This was an experimental study of the strength of two rock types, over a range of pressure conditions that correspond to depths in the Earth’s crust ranging up to 12 km. Tests were carried out under different stress regimes to simulate extremes of the range of loading geometries encountered in the Earth and hence how these affect failure strength and resistance to frictional slip on faults. These experiments will promote understanding of rock behaviour in oil, gas and water reservoirs.