Hydrostatic compression and triaxial compression tests including post-failure region were carried out on Inada granite to measure permeabilities at various stages during deformation using the transient-pulse method. Granites, generally, have an orthorhombic anisotropy due to their inherent microcrack fabric, and the Inada granite has the distinct anisotropy of this kind. In this study, specimens in nearly three anisotropic directions, perpendicular to each other (the velocities of longitudinal wave are 4.59 km/s, 5.00 km/s, and 4.82 km/s), were measured. The results of permeability tests under hydrostatic compression indicate that Inada granite also exhibits the anisotropy in permeability and that, for example, the permeabilities at about 60 MPa effective confining pressure with 5 MPa pore pressure were 7.0 X 10-14 m/s, 1.4 X 10-13 m/s, and 9.8 X 10-14 m/s Under triaxial compression condition, the anisotropy in permeability was preserved. The changes in permeability are well explained by the closure of pre-existing microcracks and the progressive development of microcracks (dilatancy). The permeabilities have dropped down to minimum values, 5.3 X 10-13 m/s, 1.5 X 10-12 m/s, and 8.6 X 10-13 m/s, then they have increased proportionally to the dilatation to the increase in circumferential strain between the onset of dilatancy and the peak strength. However, in the post-failure region, the changes in permeability have accelerated greatly due to the localization of microcracks and fault formations.
The transport properties in underground rock structures, such as cavities for compressed air energy storage (CAES), underground oil and LNG storages, will become increasingly important for us to study. In underground rock cavities, the exact long-term estimation for the groundwater flow regime around these openings is indispensable for safety and environmental assessments, which closely relate to structural design and site selection. In general, rocks masses are inhomogeneous. Even within the same rock mass, there exists wide varieties of rocks exhibiting different degrees of damage (deformation), ranging from intact rocks, which contain only pre-existing microcracks, to apparently sound rocks that contain stress-induced microcracks due to stress concentrations, to even severely jointed, or faulted rocks. This variety of in-situ rocks corresponds to every each stage of the stress-strain curves; pre-existing crack closure, elastic deformation, dilatancy with microcracking, localization of microcracking, fault formation and shear sliding along fault. It is of interest to establish the relationships existing between the extent of deformation and the transport properties of rocks. Generally, when considering rock masses, their permeability is greater than that of intact rocks; and weak planes such as joints and faults influence macroscopic flows. Furthermore, even in impermeable rocks such as granite, permeabilities must drastically increase, in the post-failure region. However, for class II rocks, continuous changes in permeability from the elastic region to the post-failure region have not been observed. Researching permeability properties of rocks during their faulting process in the laboratory is essential to estimate the in-situ permeabilities. Granites, in general, have an orthorhombic anisotropy due to their inherent microcrack fabric; and the Inada granite used in this study also has this kind of anisotropy.
