Preliminary analysis of the excavation disturbance due to construction of the Mizunami underground research laboratory

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Japan Nuclear Cycle Development Institute (JNC) is presently carrying out investigation and researches of the Phase I of the Mizunami Underground Research Laboratory (MIU) project in Mizunami, Gifu Prefecture, JAPAN. The project consists of the following three phases; (1)Surface-based Investigations Phase I, (2)Construction Phase II, (3)Operations Phase III. Geomechanical investigations in the first phase include laboratory tests to determine the mechanical and physical properties of the Toki granite and stress measurements using hydraulic fracturing and AE/DRA in three deep boreholes, AN-1, MIU-1 and MIU-2. The purpose of these investigations and preliminary analysis is to obtain baseline information and to predict the excavation disturbance due to constructiou of the underground research laboratory (URL). This is necessary for detailed planning of geomechanical studies in the second and third phases. Therefore, a 3-D elastic analysis using the boundary element method (code name: Examin3d) was carried out. The code used can readily model the complicated layout of underground structures such as the URL. The extent of damage due to stress concentration and deformation around the drifts was estimated, based on the results of uniaxial compression tests (strength, strain) for the Toki granite. The extent of stress redistribution is defined for any area with over 1mm displacement in the analysis. In this analysis, two different structural styles of the bedrock were assumed [Case 1: Bedrock is isotropic and homogeneous] [Case 2: Bedrock is comprised of three zones having different rock properties and in situ stress conditions depth dependent.] Five sets of input parameters, based on variations in elastic modulus and poisson's ratio, were used to consider the effect of fractures in bedrock. Baseline data for determination of input parameters and in situ stress conditions adopted the results from the AN-1 borehole, which are expected to represent the minimum ...