Mizunami Underground Research Laboratory Project (Rock mechanical investigations); MIZ-1 borehole investigations

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

JAEA is advancing the geo-scientific research at the Mizunami Underground Laboratory. In this project, the purpose of rock mechanics investigations is to understand the rock mechanical conditions deep underground and to construct a rock mechanical model of this site. This report describes the rock mechanics results from MIZ-1 borehole investigations. A brief summary is shown as follows. The rock mechanical properties are consistent with the results of similar investigations in the nearby Shobasama site. The AE/DRA and ASR stress measurement give very low confidence results due to very small deformation during drilling. The DSCA and hydraulic fracturing stress measurement show that the horizontal, maximum principal stress is oriented to NW-SE. About 600(m) depth, the in situ stress state is reverse fault type. But below it, the stress state changed to a wrench or normal fault type. In consideration of the investigations and geological model, we proposed a rock mechanical model.