Deep Tunnels in Clay Shales: Evaluation of Key Properties for Short and Long Term Support

Opalinus Clay shale, which is part of the mesozoic Dogger formation, underlies many parts of Northern and Western Switzerland. The rock has been encountered in many rail and road tunnels and it is also under consideration to be the host formation for a radioactive waste repository. This is a soft rock with relatively high deformability and medium to low strength. Excavation and support installation of a tunnel deep in the Opalinus clay shale formation are simulated using three-dimensional Finite Element analyses. The rock behavior is approximated using a material model obtained from laboratory tests. The model represents the elastic -, plastic, -failure -, and creep response of clay shales. The numerical analyses simulate the excavation and liner installation process, and thus capture the effects of advance rate on the behavior of rock and support. Both short term and long term conditions are investigated. The numerical simulations also explore the effects of material properties such as compressibility, dilation, and strength of the rock. Results from the analyses illustrate the inter-dependency between rock and support stresses, excavation process, and properties of the rock.