Physical properties and brittle strength of thermally cracked granite under confinement

Abstract

Effects of thermal crack damage on the rupture processes of a fine-grained granite were investigated under triaxial stress, under water (wet) and argon gas (dry) saturated conditions, and at room temperature. Thermal cracking was introduced by slowly heating and cooling two samples of La Peyratte granite up to 700°C, which were compared to two intact specimens. For each rock sample, a hydrostatic test was first carried up to 90 MPa effective pressure (5 MPa constant pore pressure). The samples were then deformed to failure at a constant strain rate of 2.10-6s-1, at 30 MPa effective pressure. Our results show that (1) permeability of heat-treated specimens was 4-5 orders of magnitude larger than that of intact specimens at low effective mean pressure; (2) nevertheless, at our experimental conditions (2.10-6 s-1), thermal cracking had no significant influence on the brittle strength; (3) similarly, no obvious water weakening effect was observed; (4) however, with increasing stress, elastic anisotropy appeared at lower differential stress in heat-treated specimens than in intact ones, but close to failure, the magnitude of P wave anisotropy was approximately the same for both types of specimens; (5) acoustic emission hypocenter locations and P wave velocity anisotropy in the basal plane demonstrate that strain localization started right at the onset of dilatancy for heat-treated specimens, later in the intact specimens; and (6) inverting wave velocities for crack density, we show that failure was reached for vertical crack densities of 0.35 for dry specimens and possibly 0.5 for water-saturated specimens. ©2013. American Geophysical Union. All Rights Reserved.