The Japan Atomic Energy Agency (JAEA) is constructing the Mizunami Underground Research Laboratory at Mizunami, Japan to establish general techniques for the assessment of the deep geological environment. The facility including two 1,000 m shafts and sub-stages at 100 m depths between two shafts is currently under construction in the sedimentary rocks. This study aims to evaluate the environmental changes around a large underground facility. To this end, hydrochemical changes in response to shaft excavation are assessed based on the observation of hydraulic head and groundwater chemistry around the facility.
The observations indicated that rock formations with low hydraulic conductivity act as barriers to hydraulic disturbances, while higher conductivity zones provide a preferential flow path. Groundwater flow to the drifts creates chemical changes by mixing among chemically different groundwaters in higher conductivity zones. It is therefore meaningful to monitor the water pressure and chemistry at highly conductive rock formations during construction and operation of underground facilities. These investigations will provide the basic information on hydrochemical buffer capacity of the natural environment. Furthermore, observations suggest that grouting of conductive rock formations is important for maintaining the groundwater at near preconstruction levels so as to retain the buffer capacity of the rock formations used for safety assessment.
