Diminished pore pressure in low‐porosity crystalline rock under tensional failure: Apparent strengthening by dilatancy

Abstract

Rupture tests on internally pressurized, thin‐walled hollow cylinders of Westerly granite with impermeable inner membranes suggest that the conventional, or Terzaghi, effective stress law does not describe tensile failure at high internal pressurization rates near 6 MPa/s. Unjacketed and saturated samples, with an initial pore pressure and for which the inner cavity pressure was increased rapidly with respect to the diffusivity, display substantially increased apparent tensile strengths and deformational moduli much higher than similarly configured but more slowly pressurized tests. Alternatively, the properties of completely dry test pieces with no pore pressure show little, if any, dependence on pressurization rate. Further, the behavior of the rapid unjacketed tests was similar to that for completely dry samples. These observations cannot be explained by the predicted undrained response, but they provide indirect evidence for diminished pore pressure effects reminiscent of dilatant hardening observed in compressive failure experiments. Calculated pore pressure diffusion rates support this suggestion as pore pressure perturbations cannot be damped out on the time scale of the rapidly pressurized tests. It is not clear if these effects are produced by elastic microcrack dilatancy, of which the nonlinear stress‐strain curve of granites is symptomatic, or the irreversible production of new porosity as in compressive shear failure tests.

The Elastic Strain Energy of Damaged Solids with Applications to Non-Linear Deformation of Crystalline Rocks

“…In contrast, the behavior of stiff crystalline rocks is approximately linear up to the stress level corresponding to the onset of acoustic emission (AE) signifying brittle yielding. However, measurements of the volumetric deformation reveal significant dilation under shear loading before yielding in both soft (e.g., soil) and stiff (e.g., granite) materials (PATERSON, 1978;SCHOCK and LOUIS, 1982;LOCKNER et al, 1992;SCHMITT and ZOBACK, 1992). Dilatancy was first scientifically described by Reynolds in 1885, who coined the term, and is assumed to be associated with opening and closure of micro-cracks.…”

“…Nonlinear elastic deformation of damaged rocks and other brittle materials is well-established by observations on different scales (NISHIHARA, 1957;BRACE, 1965;ZOBACK and BYERLEE, 1975;BRADY, 1969;SCHOCK, 1977;AMBARTSUMYAN, 1982;ALM et al, 1985;SCHMITT and ZOBACK, 1992;LOCKNER and STANCHITS, 2002;BAZARAN and NIE, 2004). Distributed rock damage in the form of cracks, joints and other internal flaws develops as part of the rock formation and can increase significantly during tectonic loading.…”

Evolution of poroelastic properties and permeability in damaged sandstone

“…exhibit deviatoric stress induced damage due to the nucleation and propagation of microcracks [13][14][15][16][17]. The main consequences of the induced damage include nonlinear stress-strain relations, degradation of elastic properties and induced anisotropy, volumetric dilatancy, material softening and permeability variations [18][19][20][21][22][23][24]. A number of laboratory investigations have contributed to the evaluation of permeability during rock damage and cracking, for instance [25][26][27][28][29][30][31][32][33].…”

A dynamic ball compression test for understanding rock crushing

“…The strength of rocks is affected by the rate of dynamic loading. [19][20][21] The loading rateσ is obtained from the time evolution of the local tensile stress using Eq. (3).…”