Countermeasures planned for reducing water inflow into deep shafts at the Mizunami Underground Research Laboratory

The Mizunami Underground Research Laboratory (MIU) is currently being constructed. The MIU design consists of two 1,000 m-deep shafts with several research galleries. The goals of the MIU project are to establish techniques for investigation, analysis and assessment of deep geological environments, and to develop a range of engineering expertise for application in deep underground excavations in crystalline rocks such as granite. The diameter of the Main and the Ventilation Shafts are 6.5 m and 4.5 m, respectively. Horizontal tunnels to connect the shafts will be excavated at 100 m depth intervals. The Middle Stage, at about 500 m in depth, and the Main Stage, at about 1,000 m in depth, will be the main locations for scientific investigations. The Main and the Ventilation Shafts were 180 m and 191 m deep, respectively, in November 2006. During construction, water inflow into the shafts has been increasing and affecting the project progress. In order to reduce the water inflow into the shafts, pre- and post-excavation grouting has been planned. A post-excavation grouting test has been undertaken in the Ventilation Shaft and the applicability of several techniques has been evaluated. This paper describes an outline of the MIU project, its work plan and the results of the post-excavation grouting test. (author)