ROOM-TEMPERATURE CREEP IN SATURATED GRANITE

Cylindrical samples of granite were deformed at 26-degrees-C, constant confining pressure (60 MPa), and constant pore pressure (20 MPa). Axial and volumetric strain were determined from changes in the output of resistance foil strain gauges bonded to the rock surface. In addition, dc electrical resistivity was measured parallel to the sample axis, During each experiment (typically lasting from 1-2 weeks), the deviatoric stress sigma(d) applied to the sample was cycled between 70% and 90% of the short-term failure strength. The bulk of the experiments were conducted in the secondary or ''steady state'' creep regime. Inelastic volumetric strain rate was found to obey the law log10(epsilon(v)) = A - B sigma(d) + C log10(epsilon(v)), where B = 0.074 +/- 0.003 MPa-1 and C almost-equal-to -4 (compressive stresses are negative). The C coefficient represents a strain-hardening-like term. The stress dependence is of the same form as the stress dependence measured for mode I crack growth in double cantilever beam experiments. The observed creep behavior is analyzed in terms of stress corrosion and crack growth models including a formulation based on energy release rate for characteristic microcracks.