A Study on rock mass behaviour induced by shaft sinking in the Horonobe Underground Research Laboratory

Tsusaka, Kimikazu; Tokiwa, Tetsuya; Inagaki, Daisuke; Hatsuyama, Yoshihiro*; Koike, Masashi*; Ijiri, Yuji*

Japan Atomic Energy Agency has excavating three deep shafts through soft sedimentary rock at Horonobe Underground Research Laboratory. In this paper, the author discussed rock mass behaviour induced by the 6.5 m diameter shaft sinking. They conducted geological mapping in an excavation face and boreholes digged around the shaft wall, field measurements such as convergence measurements and monitoring of rock displacements using multi-interval borehole extensometers around a shaft at around 160 m and 220 m in depths, and three-dimensional numerical analysis which models the shaft excavation procedure such as timing of installation of support elements and setting and removal of a concrete form. As a result, it was clarified that remarkablely large compressive strains occurred within about 1 m into the shaft wall since the rock mass behaviour was controlled by the concrete lining and that the behaviour would predominantly be induced by the fractures closing which opened significantly and propagated during excavation steps before the installation of a concrete lining and the directions where the strains occurred heavily depended on the fracture orientation around the shaft.