This two lane road tunnel, at the N20.1.4 National Highway, was excavated through the typical geology of the Zurich area. The tunnel is part of the Birmensdorf bypass and serves as a critical access route for its construction.
Pöyry provided services both in the design phase (1996-2000) and during construction (1998-2006. The project required special expertise of underground excavations in difficult geological conditions. The geology consists of glacial moraines and quaternary material consisting of silty sands gravels and molasse, a soft sedimentary rock of predominately marls, clayey marls, sandstones and siltstones. The groundwater table is near the surface.
Pöyry's services included design, procurement support and supervision. For the 235 m long loose material section, a heading and bench excavation sequence with shotcrete support was used. Horizontal jet grouting was carried out to improve the ground ahead of the advancing tunnel face. The weak rock section was excavated with drill and blast, using anchors, lattice girders and steel fibre reinforced shotcrete as primary support.
The final lining is cast insitu concrete with a waterproof membrane.