Rock masses, which commonly contain a large number of discontinuities like joints, are treated as homogeneous, anisotropic porous media. The corresponding permeability tensor k, is formulated in terms of a symmetric, second-ran k tensor pij, which is dependent not only on the geometry of related joints (density, size, orientation and aperture of joints) but also on the earth pressure at any depth. Two case studies are carried out; that is, the ventilation drift at Stripa Mine, Sweden, and the underground cavern for an oil storage at Kikuma Test Plant, Japan, where the large scale hydraulic conductivity tests have been done together with the extensive survey of joints. It becomes clear that a statistical interpretation on the joint data collected from an in-situ rock mass provides a powerful tool to determine the permeability tensor. The mean permeabilities of the both sites accord well with the theoretically predicted ones. On the basis of this study, the permeability at great depth is considered.
In the recent topics of rock hydraulics, much attention is focused on ground water flow through various geological discontinuities to solve some problems concerning the geothermal energy, earthquake and deep underground burial of high level nuclear waste. For example, the repository of high level nuclear waste will be buried about 300 m to 1000 m below the surface in rock masses (e.g. Runchal and Maini, 1980). One of the most serious problems to be solved is the isolation of the high level nuclear waste form the biosphere. Ground water flow through geological discontinuities (called cracks) is believed to be the most significant way of radionuclide migration. Field tests can be a practical solution to overcome the present difficulty. Large scale hydraulic conductivity tests were carried out to investigate the hydraulic properties of a low-permeable, jointed granue, for example, at Stripa mine in Sweden (Wilson, Witherspoon, Long, Galbraith, Dubois and Mcpherson,]983) and at Kikuma in Japan (Hoshino, 1983 and 1984). In addition to such field tests, many theoretical (or numerical) studies were also done to give a sound basis for predicting the overall permeability of rock masses; e.g., Parsons (1966), Snow (1969), Wilson and Witherspoon (1974), Long, Remer, Wilson and Witherspoon (1982), and Robinson (]984). Among others, Oda, Hatsuyama and Ohnishi (1987) have also proposed a theory in which discontinuous rock masses are treated as homogeneous, anisotropic porous media. By extending the previous study, we will propose a set of equations which make it possible to predict the permeability of jointed rock masses at great depth.
In order to formulate a permeability tensor for discontinuous rock masses, the following idealization was accepted (Oda, et al., 1987):
Any crack has a shape similar to a penny, with a diameter r and a uniform aperture t, whose orientation is identified by a unit vector n normal to the principal plane. Her, r, t and n are random variables.
