Progressive Excavation Disturbance Zone Evolution during and Post Mine-by Tunneling (PRECODE) – Insight into New Underground Research Laboratory

Abstract

Development of brittle damage around nuclear waste repository tunnels is a common phenomenon in massive rocks in highly-stressed conditions. The time-dependent brittle fracturing may lead to an interconnected fracture network (i.e. excavation damage zone; EDZ) and induced seismicity. Within the excavation damage zone (EDZ), the permeability is often enhanced and – in the framework of nuclear waste disposal – may provide preferential pathways for radionuclide migration. Therefore, a comprehensive understanding of the brittle fracturing requires multi- multidisciplinary monitoring systems to allow for spatial and temporal characterization of the EDZ. Recently, the Swiss Federal Institute of Technology (ETH) Zurich established a new Underground Research Laboratory (URL) in Southern Switzerland in the old Bedretto Gallery. Within the PRECODE experiment, we will establish a new, experimental tunnel as a branch from the existing tunnel, which will be densely instrumented with strain, pore pressure and acoustic emission sensors prior to the 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 changes with time around an open excavation and (4) the impact of tunneling on the nearby fault zones. This study outlines an overview of the project objectives, details of the planned monitoring systems, and some preliminary results obtained from a baseline study of characterization of the +40-year EDZ from the existing Bedretto Tunnel.Bedretto Team: The team involves more than 30 people from ETHZ and 10 research institutes and companies involved in the Bedretto Laboratory (see http://www.bedrettolab.ethz.ch/en/home/ for more details)