The Relation Between Hydraulic and Mechanical Apertures of Smooth Joint Under Low to High Confining Pressure
Xuan-Xinh NGUYEN#+, Jia-Jyun DONG
Graduate Institute of Applied Geology, National Central University, Taiwan

Fluid flow through rock mass is a critical issue for rock engineering practice and geoscience from shallow to great depth. Among others, the hydraulic aperture (e) of a single joint is a key variable for predicting hydraulic conductivity. Usually, the mechanical aperture (E) of rock joints under different normal stress can be predicted via famous Barton model. If the relation between mechanical and hydraulic apertures is available, the hydraulic aperture at different depth can thus be calculated. In this study, laboratory measurements of mechanical/hydraulic apertures of sawed, polished aluminum samples using gas flow via YOKO 2 system under a confining stress from 3 MPa to 120 MPa were conducted. Different from the widely adopted normal displacement method for measuring the joint closure, we measured the mechanical aperture from the joint volume divided by joint area, directly. Based on the measurements, we found the relationship between hydraulic and mechanical apertures is non-linear. The traditional assumption of linear relation between mechanical and hydraulic apertures could not be valid under a wide spectrum of confining stress, even for a smooth joint. Our measurement shows a contradict trend to the widely used non-linear relation between E/e and e. We argue that the estimation of initial aperture for converting joint closure curves into mechanical aperture for normal displacement method accounts for the contradict trend. Joint apertures (e and E) cannot totally close even when the effective normal stress goes up to 120 MPa (~5 km). The residual hydraulic and mechanical apertures of smooth aluminum joint are about 14–16 µm and 33-37 µm, respectively. Finally, we demonstrated clearly that the influence of matrix (intact rock) pore volume on joint volume measurement is significant even for low porosity granite sample (porosity < 1%) and the matrix permeability contributes few to the measured hydraulic aperture.