Schützenalleetunnel, B 31, Transitway through Freiburg

As part of the new B 31 within the city boundary of Freiburg, the 4-lane Schützenallee Tunnel is being built by the top cover method. This method was selected because on the one hand, the nearby residential areas must be accessible throughout the entire construction process and on the other, major groundwater lowering must be avoided. In order to accomplish the first of these requirements, the cover was partly created at one side over a 300 m long section. Geologically speaking, the entire tunnel is located in the gravel layers of the Dreisamtal valley. The groundwater level lies considerably above the tunnel base and fluctuates to a great extent. As a brewery well is in close proximity along with further drinking water installations, it is imperative that the groundwater is accorded full protection. The tunnel is set some 3 m deep into the groundwater. Normal lowering of the groundwater would have resulted in considerable damage being done to the adjacent buildings and was not compatible with water legislation. As a result, the bored piling walls are installed on average 6 m below the tunnel base. Then a sealing layer of cement injections is installed between them so that the groundwater can be pumped out to the available trough without any negative effects. Soft gel cannot be applied for the sealing layer for fear of influencing the brewery well. The link with the city centre creates more difficulty. This meant that this part of the tunnel roof had to be designed as a pre-stressed plate beam roof with span width of up to 40 m. ln order to reduce the flexural differences between the individual blocks with different supports, a detailed programme for the sequence of loads resulting from fills was worked out ln order to guarantee that the block joints are tight in these sections, watertight carriageway crossovers, capable of sustaining vertical movements of up to 700 mm are installed. Altogether, around 2,500 bored piles with a length of approx. 14 m and a sealing layer of 14,000 m² is to be created with the aid of around 8,000 injection bores. The block length amounts to 12.50 m. A pure longitudinal ventilation system is to be installed. A special feature here is that should the determined value laid down in the plan assessment procedure be exceeded then the air from the north tube will be diverted to the south one at the western portal and returned through it. This will be carried out via the port for the crossover facility.

Country: Germany
Region: Baden-Württemberg
Tunnel utilization: Traffic
Type of utilization: Road tunnel
Client: Bundesrepublik Deutschland, Straßenbauverwaltung Baden-Württemberg
Consulting Engineer: Regierungspräsidium Freiburg
Contractor: Ed. Züblin AG, Dyckerhoff & Widmann AG, Philipp Holzmann AG
Main construction method: 85 % cut-and-cover method, 15 % Open
Type of excavation: Excavator
No. of tubes: 1
Tunnel total length: 850 m
Cross-section: 26 t
Contract Volume: 58,0 mill. DM
Construction start/end: 4 years
Opening: end of 2001