The Mizunami Underground Research Laboratory (MIU) Project, conducted by the Japan Atomic Energy Agency (JAEA) is a comprehensive research project investigating the deep underground environment of fractured crystalline rock. As part of the project, development of a descriptive model on a scale of several tens of meters of the geological discrete fracture network (GeoDFN) and hydrogeological discrete fracture network (HydroDFN) of the different hydrogeological units encountered in the MIU has been ongoing since the Surface-based Investigation Phase (Phase I) of the MIU Project. In Phase I outcrop data, geophysical investigations from the surface as well as BTV and hydraulic test data from deep boreholes were gathered. During the Construction Phase (Phase II) of the URL in the MIU project, new data has become available, such as geological description of the tunnel walls, BTV and hydraulic test data from boreholes drilled underground.

This paper presents the methodology for constructing the GeoDFN and HydroDFN models using the data from Phase I and II. In particular, a methodology to determine the parameters for GeoDFN and HydroDFN models, such as the fracture transmissivity distribution, fracture size distribution and fracture density, are investigated. The up-scaled equivalent continuous porous media (ECPM) models, taking into account hydraulic heterogeneity on scales of up to several kilometers, have been developed based on the HydroDFN model. The results of groundwater flow analyses using the heterogeneous ECPM and homogeneous CPM hydrogeological models have been compared to confirm the importance of modeling hydraulic heterogeneity for understanding of groundwater flow condition on a scale of several kilometers. By investigating the results of this study, recommendations for both the next Phase and site investigation programs in general are extracted.

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