ABSTRACT
The deep geological repository for Switzerland’s radioactive waste is planned to be constructed in the Nördlich Lägern siting region. The drifts, tunnels and caverns of the repository will be emplaced within a 110 m thick layer of Opalinus Clay up to 900 m below the ground surface. The repository will be accessed through three shafts in rock formations that include soft marls, poorly cemented sandstones and karstic features. The paper presents design considerations for a repository within the framework of the feasibility study for site comparison. The repository design considers the site-specific geology, the repository concept and post-closure safety requirements. Main hazards during shaft construction are caused by water ingress, ravelling and squeezing ground. Squeezing ground is expected when tunnelling in the Opalinus Clay. Based on identified hazards, the choice of construction methods and the design of the tunnel lining are detailed. Shafts are planned to be constructed in the protection of ground freezing. In the final state, the shafts will be fully sealed with a steel membrane and supported with a concrete lining. Transient hydromechanical finite difference (FD) simulations considering an anisotropic linear elastic and perfectly plastic constitutive model were carried out to analyse the feasibility of construction methods and to design the tunnel linings in the Opalinus Clay. Tunnels and caverns in the Opalinus Clay are planned to be temporarily supported with a yielding support and finally supported by a concrete lining. Emplacement drifts for high-level waste are planned to be excavated with a shielded tunnel-boring machine and supported with a segmental lining.
