MECHANISMS OF SHEAR FAILURE IN ARTIFICIAL FRACTURES OF SANDSTONE AND THEIR IMPLICATION FOR MODELS OF HYDROMECHANICAL COUPLING

Direct shear tests, in which the behaviour of the rock surfaces during shear could be continuously observed, were Used to study the shear fa ilure of a profiled and clean discontinuity artificially prepared from natural sandstone. Displacement transducers were used to measure the normal and shear displacements. A series of strain gauges glued on the sides of the upper block provided information on the change of the st ress field occurring close to the discontinuities whilst shear displac ement increased, and these changes were then compared with the behavio ur of the profiled surface. The results of the laboratory tests, the s equence of photographs taken for most of them, and the results conduct ed with a sample of similar shape made from the same rock material and tested in a rotary shear machine, allowed several stages and mechanis ms of failure to be defined: static friction and mobilization of initi al shear stiffness; mobilization of sliding; mobilization of brittle f racture; post-peak failure of the teeth; descent of the teeth; gliding and ploughing; commencement of second cycle of shearing. In many resp ects these stages are similar to those occurring between sliding surfa ces of metal and suggest that the analyses developed in tribology may be relevant to the development of constitutive models for predicting t he hydromechanical coupled behaviour of a discontinuity with shear dis placement. Such models will have to consider these different stages of shear, because the original discontinuity changes its geometry with d isplacement and is filled with gouge which changes its grain size with displacement.