Initial stress measurement by CCBO at GL.-500m closure test drift of the Mizunami Underground Research Laboratory

Kuwabara, Kazumichi; Sato, Toshinori  ; Takayama, Yusuke  ; Tanno, Takeo*; Kato, Harumi*; Itamoto, Masaharu*

It is very important to understand an initial stress state of the rock mass in doing a design, the construction of large-scale facilities under the ground. Groundwater recovery experiment will be conducted at the MIU GL.-500m closure test drift. This report described what stress measurement were performed by the CCBO (Compact Conical-ended Borehole Overcoring technique) to understand an initial stress state of intact rock and a stress state of the -500m gallery wall surface. As a result, the magnitude of principal stresses were 16.8 MPa, 10.2 MPa and 7.5 MPa. The maximum principal stress level was approximately same at the GL.-500m level. The direction of maximum principal stress is approximately 60 from a horizontal plane. The width of stress redistributed zone was about 2m from the wall of research galleries. Numerical analysis was carried out to examine an outbreak condition of core disking.