Search

To ensure the long-term integrity of a rock mass, understanding the time-dependent fracturing is essential. For this reason, the study of subcritical crack growth in rock has been conducted by various researchers. It has been clarified that the influence of water is significant for the increase of the crack velocity in rock. Even though the surrounding environment is alkali, the crack velocity in rock decreased if the concentration of calcium ion in water is high. This phenomenon is basically microscopic. Even though subcritical crack growth happens in a rock mass, it is impossible to detect this phenomenon, because the aperture of the crack is on the order of micrometer. Since the study of subcritical crack growth is fundamental, it is often considered that its application to rock engineering project is difficult. However, industry-academia collaboration related to radioactive waste disposal has been started after obtaining the result with high reproducibility and clarifying the influence of surrounding environment on the change of the crack velocity in various rock materials. In addition, some international collaboration has been started to research the influence of subcritical crack growth on the weathering in rock and time-dependent deformation in a rock mass under excavation. It is suggested that the future development is possible even though we conduct some fundamental studies which may be useless apparently for rock engineering projects and natural phenomena.

It is important to understand the long-term migration of radionuclides when considering rock engineering projects such as the geological disposal of radioactive waste. The network of fractures and pores in a rock mass plays a major role in fluid migration as it provides a pathway for fluid flow. The geometry of a network can change due to fracture sealing by some fine-grained materials over long-term periods. Groundwater usually contains finegrained minerals such as clay minerals, and it is probable that the accumulation of such minerals occurs within a rock fracture upon groundwater flow, thereby decreasing the aperture of a fracture and the permeability. It is therefore essential to conduct permeability measurements using water that includes fine-grained minerals to understand the permeability characteristics of a rock; however, this has not been studied to date. In the present study, we use a macro-fractured granite sample to investigate the change of permeability that occurs under the flow of water that includes two different amounts of clay. Findings showed that clay accumulated in a fracture and that the permeability (hydraulic conductivity) of the granite sample decreased over time, which was greater in for the higher clay content. We concluded that the accumulation of clay minerals in the fracture decreased the permeability of the rock. Furthermore, we consider that the filling and closure of fractures in rock is possible under the flow of groundwater that includes clay minerals.