A novel pulse-decay-balance method for smooth rock joint aperture measurement

Abstract

A novel pulse-decay-balance (PDB) method was proposed to simultaneously measure hydraulic and mechanical apertures (e and E) under stress. The YOKO2 system was modified into a closed system to measure the e and E of saw-cut and foliated slate samples under a confining (normal) stress of up to 60 MPa. The hydraulic aperture was determined based on analysis of the pressure decay curve, and the mechanical aperture was determined based on the joint volume measurement (JV method) by determining the balance pressure. The hydraulic aperture estimated using the proposed method was comparable to that measured in steady-state tests. The hydraulic apertures of the saw-cut and foliated slate samples estimated using the simplified and general analytical solutions were in good agreement, especially for smooth joints under high normal stress. However, the error in the hydraulic aperture estimation, induced by neglecting the specific storage of the rock joint, is non-negligible for rough rock joints under low normal stress. A regression function based on the stress-dependent E and e fitted well with the experimental data to describe the E-e relationship. Our testing results, which show a decreasing trend in the E/e ratio with decreasing e, contradict the existing (E/e)-e relation, at least for smooth joints. The measurable e via the closed YOKO2 system was several tens of microns. Although the measurable E is below 1 μm, the best measurement range of E is roughly 1000 to 100,000 μm. A standard sample size for the conventional triaxial test was suggested to harmonize the optimum measurement ranges of the hydraulic and mechanical apertures. In this case, an analytical solution for the pulsed decay that considers the specific storage of the rock joint should be used to reduce the error in estimating hydraulic apertures.