ABSTRACT
The National Cooperative for the Disposal of Radioactive Waste (Nagra) in Switzerland has designated Opalinus clay as a host rock for a deep geological repository. The drifts for high-level waste storage (∅ 3.5 m) are to be constructed with a shielded tunnel boring machine (TBM) at depths where squeezing conditions may occur and the ground is fully saturated. This paper studies the TBM shield jamming hazard during excavation or long TBM standstills by means of 3D, transient, coupled hydraulic-mechanical Finite Element computations. An anisotropic linearly elastic and perfectly plastic constitutive model with a Mohr Coulomb yield condition is adopted for the Opalinus clay. Of practical interest is the influence of anisotropy and ground desaturation on the time-dependent ground deformations after excavation. The study concludes that shield jamming is not critical. The results offer practical guidance for the identification of critical design aspects for underground systems in pronouncedly anisotropic ground.
