Offenbau Tunnel

A 1,331 m long two-track tunnel was produced close to the village of Offenbau between km 40.536 and 41 .867 as part of the contract section North on the new route between Nuremberg and lngolstadt. The excavated cross-section amounts to roughly 150 m². The tunnel is located relatively close to the surface with a maximum overburden of some 4 m. It penetrates quaternary clays and sands as well as the clays of Middle Brown Jura. On account of the hydrological conditions (confined groundwater) and planning approval stipulations, the Offenbau Tunnel had to be produced by the top cover method with the excavation taking place under compressed air. Towards this end, first of all, overlapping concrete piles (1.20 m diameter) were drilled down to a depth of 30 m. Then the cover (107 blocks each 12.50 m) was produced by a back-step sequence and linked flexibly to the piles. Once the compressed air plant was installed in the first blocks produced under atmospheric pressure, the excavation commenced under compressed air; in the first phase, 50% of the cross-section (approx. 2.00 m above the rail upper edge) was removed. In the process, the maximum overpressure amounted to 0.70 bar. Once the pressure bulkhead was reached, the residual cross-section was excavated by retreating. ln order to restrict the excess pressure in the working chamber to max. 1.0 bar, wells were installed in the working chamber by means of which the groundwater was partially relieved. A temporary concrete floor was installed after the base was excavated, which took over the hydrostatic pressure after the compressed air was discharged. Two electric excavators were used to remove the ground, which loaded the excavated material on to trains. The structural shell in the given excavation phase was produced by means of the wet spraying method. The reinforced permanent lining was produced under atmospheric pressure, divided up into floor and walls once the compressed air had been discharged. A 30 cm thick buffer layer was installed beneath the floor to accept heaves.

Country: Germany
Region: Nuremberg
Tunnel utilization: Traffic
Type of utilization: Main-line rail
Client: Deutsche Bahn AG
Consulting Engineer: Schmidt Stumpf Frühauf & Partner
Contractor: Bilfinger Berger AG, Max Bögl Bauunternehmung GmbH & Co. KG
Main construction method: Cut-and-cover
Type of excavation: Excavator
Lining: Concrete formwork
No. of tubes: 1
Tunnel total length: 1,331 m
Cross-section: approx. 150 m²
Contract Volume: approx. € 100 million
Construction start/end: June 2002 to December 2004
Opening: 2006