Whether the shear displacement can exert a significant control on the fault hydraulic conductivity of rocks or not is common major concerns in both engineering fields and earth science fields. To clarify the relation between hydraulic conductivity and inner structures in rocks, we have measured three principal strains and hydraulic conductivity in the intermediate principal stress direction during deformation as a function of three principal stresses. The specimens were deformed under the true triaxial compression stress state where the intermediate principal stress is not equal to either the minimum or the maximum principal stress. In brittle faulting regime, the hydraulic conductivity decrease with increasing the axial strain until the onset of dilatancy, then increase significantly with increment of the axial strain. In the specimens under the effective confining pressure lower than 10 MPa, the final hydraulic conductivity value is larger than the initial value. On the other hand, in both brittle-ductile transition and fully ductile regimes the hydraulic conductivity after the onset of dilatancy increases slightly. Deformation experienced or failure induced specimens show a final value lower than the Initial value before the deformation. We used X-Ray CT with high resolution of only 10 micron to visualize the fracture pattern in the stressed specimen. Using 2D images obtained by X-Ray CT, we can get easily more detail information about inner structure change caused by the difference with three principal stresses.
The So-called triaxial compression test has been proved to be the most adequate and convenient testing method for the study of the mechanical characteristics of rocks tor wide variety of confining pressures. In the conventional triaxial compression test, the longitudinal axial load is applied parallel to the axis of a cylindrical rock specimen through rigid piston and the other stresses are applied by the fluid pressure. The relatively homogeneous stress distribution can be produced in the specimen. However, the intermediate principal stress is fixed equal to the minimum principal stress, in few cases, to the maximum principal stress, that is extension test under confining pressure, because the stress State in the confining pressure test is essentially axially symmetric. According to the confining pressure experiments, failure strength increased remarkably with increasing of the confining pressure, and relative transition from brittle to ductile behaviors is observed by the increase of the confining pressure. Thus, conventional triaxial experiment has been executed to elucidate the confining pressure effect on strength and deformation of rocks. However, these confining pressure experiments are not adequate for decision of mechanical and hydraulic conductivity properties of rocks around the underground open spaces. The following reasons are considered.
In most of the underground facilities, the existence of the water is very serious problem in excavating and sustaining the underground open spaces. Previous studies are not sufficient for estimating the real rock mass behaviors with considering the mechanical and hydro-geological aspect.
In general, the fractures in real underground spaces are not distributed randomly.