Studies on grouting performance in Mizunami Underground Research Laboratory (Contract research)

Grouting has practical importance for the reduction of groundwater inflow into excavations during construction of underground facilities. Two grouting approaches are commonly used to control excessive groundwater inflows into underground structures: pre- and post-excavation grouting. The method utilized is basically contingent on geological conditions, required performance and other practical considerations. However grout material used in either approach is usually, though not exclusively, cement. Considering the performance assessment of a radioactive waste repository, the initial conditions of the geological environment and the performance of the engineered barrier system could be adversely affected by a high pH plume generated from grout such as ordinary Portland cement. Therefore, a quantitative estimation of the effectiveness of grouting and grout material is essential. Studies of the grouting performance were carried out in the Mizunami URL being excavated in crystalline rock. The URL is being constructed in Gifu Prefecture, Central Japan, for the establishment of the scientific basis for geological disposal of radioactive wastes. The aims of the present study were to evaluate the applicability of existing grouting technology and to develop the methodology to determine the distribution of grout and changes in hydraulic properties of the grouted rock volume. The target rock for the investigation is the volume of rock around a planned refuge niche (4-m wide x 3-m high x 6-m long), where pre-excavation grouting was performed at 200-m depth from ground surface. The targeted rock mass volume for the pre-excavation grouting extends three meters into the walls, roof and floor of the excavated niche. Ordinary Portland cement was used for grout, color coded. After excavation of the refuge niche, horizontal and downward plunging boreholes were drilled. Water inflow and pressure were measured during drilling. The borehole layout was based on information from pre-excavation grouting and excavation of the sub-stage. Borehole studies included core logging, borehole TV, resistivity logging and hydraulic packer testing in all boreholes. Resistivity tomography between boreholes was also used to estimate the extension of grout in two and three dimensions. All measurements extended beyond the pre-excavation grouted volume because of very limited pre-investigation information on the rock mass. The results were integrated and groundwater flow analysis of pre and post grouting conditions was done to estimate quantitatively the effectiveness of pre-excavation grouting. The initial hydraulic conductivities were assumed to be two orders of magnitude higher than after pre-excavation grouting. The results suggest that current pre-excavation grouting technology is effective for reduction of groundwater inflow into excavations and that hydraulic conductivity in the surrounding rockmass can be reduced by more than one order of magnitude with pre-excavation grouting. Successful grout penetration and distribution throughout the rock mass selected for pre-excavation grouting was confirmed by core logging and resistivity tomography. Groundwater flow simulations infer that inflows without any pre-excavation grouting would be about ten times higher. Moreover, a block-scale hydrogeological model has been generated using discrete fracture networks and the grouting of individual fractures in terms of reducing the water inflow rate was evaluated quantitatively. This study has been performed as a part of the Project for Grouting Technology Development for the Radioactive Waste Repository funded by Ministry of Economy, Trade and Industry (METI), Japan. A CD-ROM is attached as an appendix. (J.P.N.)