Short-Term Evolution of Excavation Damage Zone during PRECODE Mine-by Tunneling at BedrettoLab

Brittle damage commonly develops around tunnels in massive rocks under high-stress conditions. Understanding the shape and extent of the Excavation Damage Zone (EDZ) is crucial, particularly for deep geological repositories (DGRs) designed for nuclear waste storage. Within the EDZ, permeability often increases, which, in the context of nuclear waste disposal, could create preferential pathways for radionuclide migration. A comprehensive evaluation of brittle fracture formation over space and time necessitates the use of multidisciplinary monitoring systems. To facilitate such studies, a new Underground Research Laboratory (URL) has been recently developed in the Bedretto Underground lab for Geoenergies and Geosciences (BULGG) in southern Switzerland. This facility focuses on investigating the evolution of the Excavation Damage Zone (EDZ) in crystalline rocks. The experimental tunnel, which has been extended as a new branch tangential to the existing Bedretto tunnel, is equipped with a dense array of sensors installed prior to excavation. The main objectives of the PRECODE experiment are to understand: (1) short-term rock mass behavior and EDZ formation during tunneling; (2) long-term fracture propagation within the EDZ associated with environmental conditions (fluctuations in humidity and temperature); (3) permeability evolution around an open excavation and (4) the impact of tunneling on potential dislocations of nearby fault zones. In-situ data and a series of comprehensive laboratory tests provide a hydro-seismo-mechanically coupled reference data set for numerical simulations with the aim to further improve predictive models. This paper outlines the current status of the PRECODE tunnel and the short-term response of the tunnel to excavation. The development of stress-induced fractures was detected through acoustic emission (AE) monitoring during and after excavation. Brittle fracturing, in the form of spalling has been observed in the sidewall, where AE counts were concentrated. Borehole hydraulic and gas testing indicated a permeability enhancement in the close vicinity of the tunnel, attributed to the creation of new stress-induced fractures. The development of these fractures was further evidenced by borehole deformation monitoring using fiber optics.