ABSTRACT
The new Boezberg tunnel is the largest and most important project of a large railway upgrade program of the Swiss railway authorities in order to further increase the transfer of transalpine traffic through Switzerland from rail to road. The tunnel was designed and built in an environmentally sensitive area with complex geological and hydrogeological conditions. One of the main challenges is swelling and complex ground conditions due to the presence of Opalinus clay, Anhydrite and Gypsum. Tunnels in comparable conditions have been exposed to long-term effects from swelling phenomena. In addition, the hydrogeological conditions with highly mineralised hydrothermal water in the catchment area of a close thermal spa imposed several measures in order to protect this zone from any impact of the tunnelling activities. In the first 180 m of tunnel, extensive exploratory drillings from the surface were performed parallel to the tunnel excavation in order to precisely determine the starting position of the TBM excavation. By means of systematic underground exploratory drillings a zone of swelling ground was identified below the tunnel with a limited extent. For this area the final lining was designed as a monolithic tube over a length of 37.5 m with continuous reinforcement in longitudinal direction in order to distribute and transfer any swelling pressure into zones with no swelling. The critical zone below the tunnel is permanently monitored by means of deformation meters.
