Since the disposal institution of high-level radioactive waste is composed of various underground structures, a geological environment is expected to be changed by the construction of the institution. It is important to grasp the geological environment accurately during and after the construction. Micromechanics-based continuum (MBC) theory proposed by authors can reflect the effects of density, orientation and connectivity of joints as well as the property of the joints. In this article, the MBC analysis is employed to analyze the excavation of disposal tunnel and the change of the environment around the tunnel is discussed with paying attention to joints in a rock mass through the analyses. From numerical results, it is clarified that the behaviors of the rock mass axe strongly affected by the direction of initial stress, the density of joints, etc., and that the excavation of a tunnel may change the geological environment such as permeability.


Japan relies on nuclear power for about one third of its total electricity production. A range of radioactive wastes is an inevitable by-product of the fuel cycle activities associated with nuclear power generation. Radioactive waste also arises through the use of radioactive materials in the fields of medicine, industry and reseaxch. It is necessaxy for all categories of radioactive waste to be managed in a safe and reliable manner and an extensive infrastructure has been developed in Japan for this purpose. Management of high-level waste (HLW) axising from nuclear power production presents a particular challenge and is an extremely important national issue in Japan (JNC (1998)).

Storage techniques should be effective for defined periods of time for HLW. In Japan, it is planned to dispose of high-level waste in deep stable geological formations. The deep geological environment is not affected by climatic change and human activities at the surface. In addition, sites where there are no natural resources are to be selected, thus making the probability of future human intrusion extremely low.

We should, however, consider the transport of radionuclide from the waste to the human environment with flowing ground water. The most important processes are considered to be the chemical reactions controlling the dissolution of radionuclide from the waste and the flow of ground water which is associated with the movement of radionuclide. Thus, we should consider the permeability of geomaterial during not only cavern excavation but also the storage of nuclear waste. In this stage, the disturbed area, which may be extended by the excavation, should be considered. Then, we need to construct the mechanical model, and give us the future behaviors of rock mass.

In general, the rock mass usually includes a great number of joints, which makes the mechanical behaviors of the rock mass complicated. The existences and behaviors of joints in the rock mass govern not only the mechanical behaviors but also permeability of jointed rock mass. For example, in the case of cavern excavation in the jointed rock mass, the sliding and associated opening of the joints which axe initially closed by earth pressure is considered to be the governing mechanism of the mechanical behaviors of the jointed rock mass. With proceeding excavation steps, the region with the opening and sliding of joints is expanded and the loaded region moves outward. The stress redistribution caused by the joint deformation is quite important for the evaluation of cavern safety. Thus, the analytical method that can predict the behavior of the jointed rock mass accurately is indispensable. The number of joints is, however, so large that it is almost impossible to deal wit

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