Opalinus Clay is the designated host rock for high-level radioactive waste in Switzerland. Its geomechanical behavior during excavation works has been extensively studied over the past two decades at the underground research laboratory (URL) at Mont Terri in Northwestern Switzerland. Tunnel convergence measurements were compiled from 17 locations at the URL, suggesting that diametral strain is strongly influenced by the rock mass anisotropy. Monitoring of pore fluid pressure indicates strong hydro-mechanical coupling and potentially a large excavation damage zone (EDZ) up to several tunnel radii from the tunnel axis. Transferring experiences gained at Mont Terri URL to candidate repository sites over a large depth range some 50 to 100 km further to the east is complicated by several factors. These include stress-dependent mechanical properties of Opalinus Clay and different tectonic overprint between the URL and the candidate regions. A pragmatic workflow was established to account for observations at the URL, and drawing on geomechanical data from cores at the URL and from wells at greater depths to evaluate engineering feasibility of repositories at different depths.
Opalinus Clay was identified as the preferred host rock for spent fuel and high-level radioactive waste (SF/HLW) and as one possible host rock for long-lived and intermediate to low level waste (L/ILW) in Switzerland. A total of five potential repository sites with Opalinus Clay as host rock are being evaluated in stage 2 of the Swiss Sectoral Plan for Deep Geological Repositories in a selection process. All these sites are located in Northeastern Switzerland, with potential storage depths in the range of approximately 400 to 900 m.
As part of the process, the different sites must be evaluated based on criteria related to safety and technical feasibility. Protection from erosional processes is considered a key criterion to specify a minimum depth for a potential repository. On the other hand engineering feasibility of underground structures is considered a key criterion to limit the maximum depth. In contrast to most other underground excavations, repository tunnels must not only meet standards related to operational safety and serviceability, but must also meet requirements stemming from considerations of long-term safety analysis of potential flow of radionuclides.