Time-dependent Transmissivity of Joints. Progress Report, July 1, 1981-June 30, 1982 PDF Download

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When rock is subjected to a stress cycle during which cracks open and close, hysteresis is observed in the stress-strain curves, which has usually been attributed to friction. Scanning electron microscope studies, however, have shown that shear cracks are scarce in rock that has been deformed in the dilatant region. Stevens and Holcomb (1980) and Holcomb and Stevens (1980) have argued further that crack friction models are incompatible with the rheology of dilatant rock and are virtually untenable. They asserted instead that cracks are reversible in the sense of the Griffith energy balance. Here we demonstrate that two nominally flat rock surfaces in contact subject to a cycle of applied normal stress exhibit all the properties of crack opening, closing, and hysteresis exhibited by a dilatant rock and that in this case the mechanism must be friction. It is concluded that an axial crack in dilatant rock will exhibit frictional interactions on opening or closing if the crack walls are not in perfect registration, and that there is no justification for accepting the reversible Griffith crack concept.