Bürgerwaldtunnel, A98 Waldshut-Tiengen

The rock can be divided up into two sections in morphological terms. The roughly 70 m high dome of the Bürgerwald in the east comprises bedded, strongly fissured and karstified banks of dolomite of the Upper Shell Lime period. The terraced plateau in the west, which is located at a height of approx. 40 m above the valley bottom of the Wutach, is formed by a succession of Ice Age loose rocks comprising gravel sand with locally restricted sand lentils and thick nagelfluh beds, which are almost impossible to classify geologically. In the deeper underground below the water permeable quaternary loose rocks and the leached zone, there are gypsum rocks from the Middle Shell Lime period. Leaching continues here and as a result, an irregular leaching front is formed in the Underground, which is constantly changing. The outcome is laminar settlements or narrowly restricted doline thrusts, which can extend to the surface. Doline thrusts are visible on the surface over a distance of roughly 120 m and were encountered in the pilot tunnel. The execution of the tunnel completed by mining means took place in several working stages. It was executed in accordance with the NATM so that it was possible to react flexibly to the changing rock conditions. The overall cross-section was excavated in two part excavations, crown and bench in loose rock, a rounded base was additionally created. In the trickiest zone in the loose rock, a cross-section with base invert was completed. The job was carried out with the aid of an hydraulic excavator. In the zone in the solid rock, bore and blasting was applied to remove the rock. The vault was fitted with an underlain non-woven material and a 2 mm thick plastic membrane to protect it against underground water. The 40 cm thick concrete tunnel inner vault, which was only reinforced in the vicinity of the loose rock and in fault zones was placed on this seal. The chosen supporting structure concept can compensate for the irregular settlements of the tunnel caused by the gypsum table being constantly leached, by designing the inner shell along the entire distance of the leaching zone as a link chain. The links permit the blocks to become distorted to a limited degree given simultaneous transference of the shearing force. The shearing force transference is catered for through a groove and tongue construction. In the roof of each joint, its movements are checked by position sensors (inductive transducers) in order to make sure that any changes in the position of the tunnel tube is recognised at an early stage.

Country: Germany
Region: Baden-Württemberg
Tunnel utilization: Traffic
Type of utilization: Motorway tunnel
Client: BRD Landesamt für Straßenwesen Baden-Württemberg
Main construction method: Trenchless
Type of excavation: Drill-and-blast
No. of tubes: 1
Tunnel total length: 1435 m, incl. 74 m cut-and-cover
Cross-section: 88-103 m², emergency bay 112-159 m²
Contract Volume: 65 mill. DM
Construction start/end: ~ 3 years
Opening: ~ December 1996