Mizunami Underground Research Laboratory Project (Rock mechanical investigations); In situ stress measurements and the laboratory tests using core on GL.-100m level of research gallery

Hirano, Toru; Nakama, Shigeo ; Yamada, Atsuo*; Seno, Yasuhiro*; Sato, Toshinori*  

Japan Atomic Energy Agency is advancing the geo-scientific research at the Mizunami Underground Research Laboratory. In Phase II of the research, improvement on the previous conceptual model of the geological environment is one of the objectives. So the additional rock mechanical investigations on the GL.-100m level research gallery was conducted. As a result, the average rock physical and mechanical properties are consistent with the results from MIZ-1 testing. This means that the surface based investigations in MIZ-1 are valid at this level. Almost all in situ stress measurements indicate that the maximum principal stress is oriented in the NW-SE direction and match the observations in MIZ-1 at GL.-200m depth. But the observed stress level is not the expected value extrapolated from the MIZ-1 results. This suggests that the in situ stress has likely been decoupled at the rock boundary located about GL.-150m level between the Toki Lignite-bearing formation and the Toki granite.