Design considerations for deep caverns in Opalinus Clay

Abstract

The planned radioactive waste repository in Switzerland will be located within a ca. 900 m deep and 100 m thick layer of Opalinus Clay. Low- and intermediate-level waste will be emplaced in caverns with a diameter of 14 m. The great depth of cover and the rather low strength of Opalinus Clay (uniaxial compressive strength in the order of 3-5 MPa) are expected to result in squeezing conditions, which will be demanding to handle on account of the large dimensions of the cavern cross-sections. The paper investigates alterative concepts for the tunnel support, considering the uncertainties and peculiarities of the Opalinus Clay’s response to tunnelling – inter alia its very pronounced time-dependency. The development of relevant deformations will not cease even after several decades, while in the presence of a stiff lining the rock pressure would reach very high values within the service life of the caverns (in the order of about 3-4 MPa within a period of 20-30 years after excavation).