For many reasons including the efficient land use, environmental and energy saving issues, the demand for the underground construction is highly increasing. The underground structures include nuclear waste repositories, road tunnels, food storage, and so on. Each one of them is usually designed to have a long life span. Therefore, it is necessary to estimate and investigate the hydraulic and mechanical coupling behavior of rock to assess the long-term stability and the environmental effects. In this study, a torsional shear testing apparatus to characterize the hydro-mechanical behavior of a rock joint was devised. Shear stress was driven by twisting the end of a sample in the torsional shear testing apparatus. The results showed that the torsional shear test underestimated the peak shear strength of a rock joint. It is because the torque is a function of the radial distance from the axis of rotation resulting in the radial variation of the shear stress. Fluid flow through rock joints is mainly dependent on the joint roughness, contact area, initial aperture, etc. To examine the dependency, the relationship between the hydraulic and the mechanical aperture for shear-flow was established by measuring the initial aperture. It showed that the mechanical aperture and the hydraulic aperture increased linearly with the increase of the dilatancy. The difference between the hydraulic and mechanical apertures will describe the deviation from the behavior predicted by the parallel plate model.
Recently, various deep underground utilization, such as storage of oil and radioactive waste disposal sites, has drawn much attention. For the appropriate development of them, it is important to examine and to evaluate the mechanical and hydraulic properties of rock joints. It is well known that the hydraulic properties of a rock joint are strongly controlled by aperture and surface roughness of rock joints. Early attempts to model the hydraulic properties such as cubic law assumed a laminar flow through parallel plates (Bandis, 1985). However, in the real surfaces of rock joints have some roughness, not planar. Thus there are several inconsistencies, such as the applicability of the cubic law and the effect of mechanical aperture on flow in joints. The hydromechanical behavior of deformable rock joints subjected by normal stress was studied by Witherspoon (1981), Cook (1988) etc. and is well understood. Recently, research for the examination of the changes in fluid flow rate with the shear displacement of rock joints was being carried out (Makurat, 1992 and Esaki,1998 and Olsson, 1993). In this study, torsional shear testing apparatus was designed to characterize hydromechanical behavior of a rock joint subjected by normal and shear stress. Prior to the experiments, initial aperture was measured using roughness measuring system. The changes in shear stress and fluid flow rates with shear displacement was observed during shear-flow test, and the relationship between mechanical aperture derived from joint closure model and initial aperture.
For laminar flow within ideal smooth parallel plates, flow rates are expressed with the following expression.