With around 120,000 passing vehicles daily, the Engelberg base tunnel is one of Baden-Wurttemberg’s most frequented motorway tunnels. The structure, opened in 1999, successor of the Engelberg Tunnel built in 1938, is located around 60 metres underneath the original structure. The geological conditions around the tunnel highly impact the structure: it is located in an anhydrite environment, a mineral that transforms into gypsum when it comes in contact with water and can expand up to 50 percent. This exercises constant pressure on the tunnel tubes. The resulting damages are currently repaired during a comprehensive rehabilitation.
The carriageway slabs are reinforced, and a new inner shell is fitted in both tubes. It consists of welded steel girders of class S460, installed at a distance of 0.5 metres and subsequently concreted. Above the tunnel’s clearance, an additional pressure bracing is implemented by a 3-joint intermediate ceiling made of prefabricated VFT® girders with cast in-situ concrete as well as a steel pressure column towards the tunnel roof. It is probably a worldwide unique to secure the tunnel tubes situated in the swell area.During the rehabilitation, the safety and operational technology of the tunnel is renewed and modernised as well.