High swelling pressures and heaves of anhydrite-rich marls and Opalinus Clay shale (OPA) caused substantial damage to the Belchen highway tunnels between Basel and Lucerne in Switzerland, excavated with drill-and-blast technique in the 1960s. The tubes have to be refurbished and a new tunnel tube was constructed with a tunnel boring machine between 2016 and 2018. We have installed a monitoring system in a section of OPA to explore the evolution of radial pressures on the tunnel lining and the local hydro-mechanical conditions with the aim to characterize swelling processes of OPA in-situ. For the interpretation of radial pressures, the mechanical properties of the annular gap grout were explored and the impact of thermal strains on measured radial stresses was investigated with numerical simulations. Here we discuss key results obtained from the new Belchen tunnel that can be also relevant for design considerations of nuclear waste repositories in clay rocks.
Rock swelling is a hydromechanical-chemical process that leads to volume or pressure increase in the ground caused by reduction of confining stresses, absorption and adsorption of water (i.e., inner crystalline and osmotic swelling; e.g., Madsen & Müller-Vonmoos 1989), or by a combination of these processes (e.g., Einstein & Bischoff 1975). Swelling of clay, clay rocks (e.g., Madsen & Müller-Vonmoos 1989), and anhydrite-rich rocks (e.g. Rauh & Thuro 2007 and Amann et al. 2013) can damage underground infrastructures. Rock swelling is considered a potential hazard to the built infrastructure and requires good design solutions for the construction process and the lining systems.
In the Swiss high-level waste (HLW) repository concept the great length of repository drifts and tunnels to be excavated in swellable Opalinus Clay shale (OPA) in the order of 24 km, together with the constraint to limit the damage of the geological barrier surrounding the emplacement drifts for HLW, among other factors, may suggest the use of tunnel boring machines (TBMs) for repository drifts (e.g., NAGRA 2016). Most in-situ observations of swelling phenomena come from conventional excavations (e.g., using drill-and-blast or road header technique) and less from excavations that use TBMs and segmental linings (tubbing stones) as part of the lining system. Thus, there is a need to better understand in-situ swelling around tunnels drilled with TBMs as such excavations do not require drilling fluids for blast holes and may cause smaller excavation damage zones (EDZs) with increased rock mass permeability, which should have an impact on the evolution of swelling processes, compared to conventionally excavated tunnels.
