Intact Granite Research Articles

Finite strain rock plasticity: stress triaxiality, pressure, and temperature effects

An elasto-plasticity theory is used to model the deformation of geological materials under various confining pressures and moderate temperatures. The effects of material hardening (or softening due to volumetric strains) are included, and the corresponding elasto-plastic rate constitutive relations are developed. To study the influence of pressure and temperature on the constitutive parameters, we use some published data of laboratory experiments on certain rocks. It is shown that over a wide range of pressures and low to moderate temperatures, when the rate effect can be ignored, the model can be used to describe the behaviour of geological materials. Based on this theory, dilatancy (i.e., inelastic volumetric expansion) of an intact granite is studied under conventional triaxial stress states. The effect of pressure and temperature on the magnitude of dilation and on the stress (measured relative to the peak stress) at the onset of dilatancy is investigated. It is found that, consistent with experimental data, the theory predicts this stress to be about 50% of the peak stress, but its specific value depends on pressure and temperature. As an illustration, stress-strain curves for intact granite at relatively shallow crustal depths are then predicted for possible application to the study of crustal deformation and for the prediction of fault behaviour.

Elastic and transport properties of an in situ jointed granite

In situ elastic and transport properties were measured as a function of compressive stress to 200 bars for a variety of load psths on a 3-m cube of jointed granite near Laramie, Wyoming. The specimen contained 3 vertical joints which are parallel to a set of well-developed microfractures. Measurements were made parallel and normal to the joints across the entire block and within intact areas containing only microfractures. The loads were applied by eight 1.2 × 2.4 m flatjacks, 2 on each of the 4 sides. The measured properties included deformation, compressional velocity, electrical resistivity and fluid permeability. Load paths included uniaxial, biaxial and proportional stress and uniaxial ‘strain’. ‘Direct shear’ tests were also conducted at 2 normal stresses. Field data were compared with measurements made on oriented specimens in the laboratory. Results based on differences in rate of change of fluid flow, elastic moduli and on seismic wave propagation indicate that the joints close at 15–30 bars normal stress, but that microcracks remain open at the highest stresses attained. Changes in fluid permeability along the joint and elastic modulus with stress are greater than those of either seismic velocity or electrical resistivity. Modulus and velocity increased with stress while permeability decreased by a factor of 4. Even when ‘closed’ permeability along the joint is 3 orders of magnitude greater than for intact granite. Intact granite exhibits a marked anisotropy with respect to both modulus and seismic velocity.

Fracture data and stress-strain behavior of rocks in triaxial compression

Test results are reported for a recently completed experimental research program on rocks subjected to triaxial compression. Sandstone, marble, granite and shale specimens were tested at confining pressures as high as 90,000 psi corresponding to mean stresses of up to 143,000 psi. Recognizing that the largest potential experimental error in such tests results from making strain and load measurements external to the vessel, special load and strain-measuring devices were designed and fabricated for use inside the pressure vessel. The specimens were carefully machined cylinders with length-to-diameter ratios of two and with diameters ranging from 4/16 in. to 1 in. The confining pressure was held constant during each run, but varied from 0 to 90,000 psi over the tests. Results are reported in the form of: A number of tests were run on granite specimens which had been previously fractured. Results from these tests showed good agreement with tests on intact granite, providing the confining pressure was above 30,000 psi.