Estimation of crack tensor for evaluating excavation disturbance of research gallery at the Mizunami Underground Research Laboratory

Japan Atomic Energy Agency has been implementing the Mizunami Underground Research Laboratory (MIU) Project in order to develop the comprehensive investigation techniques for the geological environment and the engineering techniques to construct a deep underground laboratory in crystalline rock. In the rock mechanical study in the MIU Project, the development of the evaluation method for the excavation disturbed zone due to excavation of shafts and research galleries is one of the important issues. Therefore, the equivalent continuum model base on crack tensor theory applied to predict the rock mass behavior including excavation disturbance in FY2004. The parameters for the analysis were determined based on the results of deep borehole investigations from surface. In this study, crack tensor based on the geological observation in the MIU was calculated and the numerical simulation was done to compare the measured displacement in the MIU. Then, calculated crack tensor based on the geological observation compared with it in FY2004 work's and estimated from pilot borehole investigation. Moreover, the crack tensor at 500m in depth was re-estimated from these result and numerical simulation was carried out to understand realistic rock mass behavior due to excavation. The results are as follows: 1) For the ventilation shaft at GL-350m, the crack tensor deformation analysis based on FY2004 work's results showed that the calculated displacement was smaller than the measured displacement. Geometrical parameters of fractures in FY2004 work's result were different from one based on geological observation in the shaft. Therefore, the crack tensor of FY2004 work's results seems to be underestimated. 2) Large discontinuities with NE strike and high dipping observed in the ventilation shaft were major reason for the difference of crack tensors determined by borehole investigation from surface and geological observations in the ventilation shaft. Therefore, the crack tensor the obtained from pilot borehole investigation in the ventilation shaft was calculated as well and compared with each results. It was found that the fabric tensor is similar with it of geological observation and the value was medium. 3) The crack tensor around GL-500m was estimated by relation of statistical quantities of fractures between GL-300m and GL-500m. Consequently, the deformation analysis based on the estimated crack tensor showed an increase in convergence and stress in the support system compared to FY2004 work's results. (author)