Short-term Permeability Evolution Within the Excavation Damage Zone Around a Deep Tunnel in the Rotondo Granite

Brittle failure phenomena such as spalling occur around deep underground excavations in crystalline rock, leading to the formation of excavation damage zones (EDZ). The EDZ is characterized by stress-induced micro- and macrocracks that significantly increase the permeability of the rock mass surrounding the underground opening. In the context of deep geological repositories, such permeability increases pose a safety concern by creating preferential pathways for radionuclide migration around the underground excavation. The PRECODE project (“Progressive Excavation Disturbance Zone Evolution during and Post Mine-by Tunneling”) at the Bedretto Underground Laboratory in Switzerland investigates the temporal and spatial evolution of the EDZ in the Rotondo Granite at ~1300 m overburden depth. The tunnel section investigated in this study was excavated using a non-explosive mechanical excavation method to avoid blast-induced damage in the surrounding rock. Continuous pore pressure monitoring before, during, and after excavation, combined with repeated hydraulic and pneumatic testing using modular multiple packer systems (MMPS) in dedicated boreholes, contributes to a better understanding of transient hydro-mechanical processes in the near-field rock. Analysis of pressure responses from hydraulic and pneumatic testing reveals the impact of stress redistribution on the adjacent rock mass. The spatial extent of the EDZ varies locally between 0.6 and 1.1 m from the tunnel’s sidewall into the rock mass, and the hydraulic conductivity of the borehole intervals within the EDZ increased progressively by up to five orders of magnitude with time after excavation. The evolving EDZ interacts with pre-existing fractures, developing combined natural and induced fracture-controlled flow paths. These results offer new insights into the short-term evolution and hydraulic behavior of EDZs in crystalline rocks, supporting improved assessment and modeling of repository stability and long-term safety.