Physical properties and brittle strength of thermally cracked granite under confinement

Abstract

[1] 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 ı 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 -6 s -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.

Inelastic Compaction in High‐Porosity Limestone Monitored Using Acoustic Emissions

“…As shown in Figure c, we recorded about 7 times less AE during the hydrostatic experiment performed in wet conditions than in dry conditions. Similar observations were made in low‐porosity crystalline rock (see, e.g., Wang et al, ). Two mechanisms at least could explain such differences: the increased attenuation in the presence of fluids (since the triggering threshold remained the same in all our experiments) and subcritical crack growth that would radiate less acoustic waves (triaxial creep experiments on sandstone have shown that an increased efficiency of stress corrosion cracking reduces the mechanical work required; i.e., there is an “energy deficit,” to bring a sample to failure; Brantut et al, ).…”

“…Acoustic waveforms were amplified at 40 dB and recorded at 10 MHz sampling rate using a Richter system (Itasca Ltd.). A complete description of the acoustic system is given in Ougier‐Simonin et al () and Wang et al (). A classical ultrasonic pulse transmission technique was used for P wave velocity measurements.…”

Geomechanical rock properties of a basaltic volcano

“…The variable effect of thermal stressing at similar temperatures has also been noted by changes in crack content in other rocks such as andesite (Heap et al, 2014a) and granite (Wang et al, 2013). Although heating and cooling can both induce cracking, there seems to be no systematic response as a function of starting porosity.…”

Can Precursory Moment Release Scale With Earthquake Magnitude? A View From the Laboratory

“…It is unlikely that the absence of foreshocks in all fluid‐pressurized conditions results solely from an increased attenuation of seismic waves since the sensors were located <1 mm away from the fault. Previous experiments have indeed shown that AEs are systematically observed prior to the failure of intact or thermally cracked, dry or water saturated granite specimens (Wang et al, ). The absence of recorded foreshocks in all fluid‐pressurized experiments might nevertheless be due to the high recording threshold of our AE triggered setup (0.001 V).…”