ABSTRACT
In the planned deep geological repository at Nördlich Lägern in Switzerland, high-level radioactive waste will be emplaced in 3.5 m diameter drifts embedded in Opalinus Clay at depths up to 900 m, which will be excavated with a shield TBM. Due to the anticipated squeezing conditions, shield jamming during construction constitutes a prominent potential hazard. The authors have previously conducted a detailed assessment of shield jamming for the expected scenario of a practically homogeneous and intact Opalinus Clay formation, concluding that it can be avoided with the use of a moderate overcut around the shield. The present paper extends these investigations, to consider the adverse scenario of encountering fault zones. Based on 3D coupled hydro-mechanical simulations, the paper demonstrates that in fault zones the risk of the TBM becoming trapped cannot always be alleviated by only providing an overcut; it is also important to install a sufficiently high thrust force. A preliminary estimation of the required thrust indicated values of up to 30 MN, which are feasible with a suitable machine design. Besides verifying the feasibility of mechanised construction and providing practical guidance for the design of the drifts, the presented investigations also offer valuable insights of a more general nature into the effect of fault zones on mechanised tunnelling.
