The aim of this paper is to study spalling in soft rock excavation. The large test cavern in soft rock was excavated to investigate the Excavation Disturbed Zone (EDZ) of the radioactive waste disposal project in Japan. During the excavation, the rock-fall and the induced crack, which was the low dip angle near the cavern surface caused by spalling, occurred. These spalling are similar to the failure for the stress concentration due to the excavation of the hard rock at the deep underground. The competence factor of the rock, which is the ratio of rock strength to in situ stress, is smaller than two, which means the strength of the rock is low relative to the in situ stress, and this site corresponds to the rock mass with relatively high rock stress. Therefore, we have carried out the stress paths simulation test, which are the triaxial compression tests in consideration of stress path, which assumed the change in the stress around the cavern during the underground openings. As a result, the splitting failure has been confirmed under low confining stress, which range is less than 0.4 MPa. On the other hand, in the confining stress range of more than 0.4 MPa, the shear failure has confirmed. The induced stress around the cavern decreases due to the underground openings. Therefore, it is thought that the crack confirmed in the test cavern is the splitting failure under stress decrease. Furthermore, the failure zone surrounding the cavern was proved by the numerical analysis applying the failure criterion based on the results of the above-mentioned triaxial test.
Japan Nuclear Fuel Limited (JNFL) has a plan of a new facility for Low-Level Radioactive Waste (LLW) of underground disposal with the engineered barrier. This facility can accept the relatively higher LLW from nuclear power plant operations and in core materials from the reactor decommissioning. JNFL site is located in the legislated development area in Rokkasho-mura, Aomori Prefecture in the northern part of mainland Japan, which is about 700 km away from Tokyo. The detailed investigation of geology and ground water has been carried out since 2002 in order to collect various data for the safety review. As a part of the investigation, the tunnel and the test cavern have been excavated to acquire the knowledge on mechanical stability of openings and the excavation disturbed zone (EDZ), which is shown in Figure 1.
Fig. 1. available in full paper.
The excavation of the 1 km long approach tunnel (the portal level EL. 8.0 m, incline of 1/10) to the altitude of EL. -86.0 m underground has been finished in 2004. Moreover, the large-scale test cavern has been completed in June 2005. Prior to the test cavern excavation, the three measurement tunnels had been excavated within a 20 m vicinity of the test cavern and the measuring instruments had been installed in order to examine the rock behavior.
