Numerical simulations for describing generation of excavation damaged zone: Important case study at Horonobe underground research laboratory

The aim of the present research was to establish a case study for the prediction of the unknown EDZ (Excavation Damaged Zone) distribution using a numerical analysis calibrated by replicating the trends in the EDZ observed from one of the representative underground research fields in Japan (Horonobe URL). In this study, a 2D numerical analysis using a damage model, which can determine rock deformation and fracturing simultaneously, is presented. It was calibrated to reproduce the excavation of the gallery at the Horonobe URL at a depth of 350 ​m. Simulated results show an excellent agreement with the extent of the measured EDZ and capture the failure modes of EDZ fractures suggested by the in-situ observations. Finally, the calibrated numerical analysis was used to realistically estimate the EDZ formation for the geological disposal of high-level radioactive waste (HLW) under the same environment as that of the above-mentioned galley at the Horonobe URL. Consequently, it was shown that the tensile/shear hybrid fractures dominantly constituted the EDZ and propagated to a maximum extent of about 0.3 ​m from the cavity wall during the cavity excavation for the HLW disposal. Overall, the calibrated numerical analysis and resulting estimations, targeted for the environment at the depth of 350 ​m at the Horonobe URL, where mudstone is located, should be useful for predicting the trends in the EDZ distribution expected in the implementation of HLW disposal projects under deep geological conditions, such as those that exist in Japan, which are dominated by sedimentary rocks, including mudstone.