Elastic and transport properties of an in situ jointed granite

Abstract

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.