As part of the research and development program on geological disposal of a high-level radioactive waste, the Horonobe Underground Research Center, a division of the Japan Atomic Energy Agency (JAEA), has been implementing the Horonobe Underground Research Laboratory(URL) Project. This paper describes mechanical properties of siliceous rocks distributing around the Horonobe URL on the basis of triaxial compression and isotropic consolidation tests. The triaxial tests were carried out under drained and undrained conditions since pore structure change in the sedimentary soft rocks with large effective porosity was expected. The stress-strain behavior changed from strain-softening to ductle behavior with high effective confining pressure and the pore pressure increased gradually even after the peak strength for the diatomaceous mudstone. On the other hand, strain-softening behavior was founded in total duration of the tests regardless of the effective confining pressure value and the pore pressure decreased rapidly after the peak stress for the siliceous mudstone. The diatomaceous mudstone yielded under hydrostatic pressure of 10 MPa in isotropic consolidation. This yielding was regarded as pore collapse due to the change of hydraulic conductivity which was estimated from volume change under isotropic consolidation test, effective porosity and microscopic observation before and after the yielding.
As part of the research and development program on the geological disposal of a high-level radioactive waste which is generated in the process of reprocessing of spent fuel, the Horonobe Underground Research Center, a division of the Japan Atomic Energy Agency (JAEA), has been implementing the Horonobe Underground Research Laboratory (URL) Project with the aim at investigating sedimentary rock formations. The Horonobe URL Project is planned to carry out over a period of 20 years and composed of three phases, namely "Phase 1: Surface-based investigations", "Phase 2: Construction phase" (investigations during construction of the underground facilities) and "Phase 3: Operation phase" (research in the underground facilities). Surface- based investigation phase has been conducted from 2000 to 2006 and Construction phase has started from 2006.
This paper describes mechanical properties of siliceous rocks around the Horonobe URL from triaxial and isotropic consolidation tests carried out to understand mechanical behavior of rocks distributing around the URL.
The Horonobe URL is located in the northern part of Japan (Figure 1). Figure 2 shows the geological map around the URL. Neogene siliceous sedimentary rocks composed of diatom with the thickness over 500 meters distribute around the URL. Regarding the sedimentary sequences, diatomaceous mudstone layer called the Koetoi formation is located on siliceous mudstone layer called the Wakkanai formation. Effective porosity of diatomaceous mudstone is about 60% and siliceous mudstone is about 30%. Effective porosity of the rocks around the URL is very high compared with sedimentary rocks distributing in Japan since the origin of the rocks is diatom whose porosity is very high due to nano-order pores. Uniaxial strength of diatomaceous mudstone is less than 5 MPa and siliceous mudstone is around 20 MPa.