Formation mechanism of extension fractures induced by excavation of a gallery in soft sedimentary rock, Horonobe area, Northern Japan

Tokiwa, Tetsuya; Tsusaka, Kimikazu; Matsubara, Makoto*; Ishikawa, Taiki*; Ogawa, Daisuke*

This paper focuses on the formation mechanism of fractures induced by excavation of a gallery in soft sedimentary rocks in the Horonobe area of Japan. Detailed fracture mapping of the gallery indicates that the fractures consist of both pre-existing fractures (shear fractures) and EDZ fractures (extension fractures). EDZ fractures correspond to bedding planes or potential joints inferred by paleo-stress field, and the EDZ fractures terminate against pre-existing fractures. Therefore, even for excavations in soft sedimentary rocks, formation of the EDZ fractures are controlled by pre-existing fractures and weakness planes associated with bedding planes and potential joints.