Jp. Pereira et Mh. Defreitas, MECHANISMS OF SHEAR FAILURE IN ARTIFICIAL FRACTURES OF SANDSTONE AND THEIR IMPLICATION FOR MODELS OF HYDROMECHANICAL COUPLING, Rock mechanics and rock engineering, 26(3), 1993, pp. 195-214
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.