This paper shows the application of two post-grouting works to a gallery at 500 m depth of Mizunami Underground Research Laboratory (MIU) in Japan. The ground water pressure was around 3.5 MPa during the campaign. A maximum grouting pressure was set at 5.0 MPa for the first post-grouting and 5.5 MPa for the second. Three new grouting concepts were applied to the post-grouting works; a new grout material, a new injection system, and a new post-grouting zone. As for a grout material, "durable liquid-type colloidal silica grout (CSG)" was applied to seal the narrow fractures. As for an injection system, "complex dynamic grouting method" was applied to improve the penetrability of the grout material. As for a post-grouting zone, "outside of the pre-grouted zone" was targeted to reduce the risk of erosion and leakage of fresh grout.
The first post-grouting work in 2014 was designed by the new concepts combined with the ordinary ones for the comparative study. The reduction of the sectional inflow indicated that the hydraulic conductivity of the post-grouted rock mass to be lower than 10−9 m/s by the back calculation. The second campaign in 2016 was designed by totally latest concepts. As a result, it was measured that all the dripping spots be lower than 1 l/min, which satisfied the severe criterion to perform the post-grouting for a deep repository in Sweden.
The grouting works were successful in reducing the abundant water inflow from the rock mass with many fractures. It can be concluded that the developed grouting methodology in MIU is applicable for constructing the watertight tunnels in hard rock with lots of fractures and for the future disposal sites with a severe inflow requirement.
In Japan, as geological disposal of high-level radioactive waste is legislated to be at a depth of at least 300 m, it is one of the mandatory tasks to develop the technology for reducing groundwater inflow in the deep underground with high water pressure. The Japan Atomic Energy Agency is operating the Mizunami Underground Research Laboratory (MIU) project in Japan in order to establish a firm scientific basis for the safe geological disposal. The regional geology consists of Cretaceous granitic basement covered by Tertiary and Quaternary sedimentary rocks from the 170 m depth to the surface. The water inflow into MIU is required to be the more strictly reduced in the deeper excavation with the higher water pressure. Therefore, performing and developing the rock grouting technology in the conductive zones has been an essential countermeasure during the construction and to prepare for the future disposal project. Several pre-grouting works before the excavation and then two post-grouting works at 500 m depth have been conducted, see Fig. 1. Details of the pre-grouting works in MIU are provided in .