A Method for Estimating the Highest Potential Hydraulic Conductivity in the Excavation Damaged Zone in Mudstone

Excavation of deep underground openings induces permeable fractures around the opening due to stress redistribution. Such a zone is called excavation damaged zone (EDZ). In a high-level radioactive waste disposal project, the EDZ might provide pathways for the migration of radionuclides around the facility. Thus, this study focused on the development of a method for estimating the highest potential hydraulic conductivity in the EDZ around a gallery in the Horonobe Underground Research Laboratory, Japan. Borehole televiewer surveys, rock core observations, and hydraulic tests were undertaken to investigate the extent and magnitude of hydraulic conductivity in the EDZ around the gallery. The observed extent (sidewall ≤ 0.6 m; floor ≤ 1.6 m) of the EDZ shows good agreement with the EDZ estimated from hydro-mechanical coupling analysis. The measured hydraulic conductivities of the EDZ (3.5 × 10⁻⁶ to 1.24 × 10⁻⁸ m/s) are within the range of those based on the Mean Stress Index (MSI), which is defined as the ratio of the effective mean stress derived from numerical analyses to the tensile strength of intact rock. Given that the rock mass is relatively homogeneous and artificial damage (e.g., blasting-induced damage) can be neglected, as in the Horonobe Underground Research Laboratory, the MSI model is likely to be applicable in estimating the highest potential hydraulic conductivity in the EDZ.