Stadtbahn Bielefeld, Baulos 2312, Jöllenbecker Str.

Taking into account the special geological marginal conditions (location in solid clay stone, favourable primary stress behaviour), the high overburden (> 1 d) and urban constructional considerations, it seemed well worth the effort to undertake a monocoque means of construction. After consultations with the financial backers and the experts involved, a 104 m long section of tunnel was chosen, the corresponding requirements formulated and subjected to competition as part of the tendering procedure. A "monocoque construction" comprises two layers, with the outer layer of the shotcrete outer shell corresponding to the composite construction method. A jointless supplementary shell was added to this through wet-spraying. 70 kg of steel fibres (Type - Wirecon, Ø 0.6 mm, l = 24 mm) was added to this B 25 class shotcrete per m³ of concrete. Two different procedures were undertaken, each applying to one half of the test section in connection with accelerating the setting of the concrete and improving the watertightness. In the one case, synthetic' silicon dioxide (14 kg/m³) was added to the concrete, in the other, liquid aluminate (19 kg/m³) as accelerator and micro silica (20 kg/m³) in the form of a suspension additionally added. In order to ensure that a jointless shell was produced, the steel fibre shotcrete was continuously placed. This work was accompanied by a large number of suitability and quality tests. The test section was provided with a reinforced concrete base, as a precautionary measure should an in situ concrete inner shell have to be added at some later stage.

Country: Germany
Region: Nordrhein-Westfalen
Tunnel utilization: Traffic
Type of utilization: Underground, urban and rapid transit
Client: Stadt Bielefeld, Tiefbauamt
Main construction method: Trenchless
Type of excavation: Drill-and-blast
Lining: Shotcrete witch stell fibres
No. of tubes: 2
Tunnel total length: 123 + 478 = 601 m
Cross-section: underground 2 x 38 m²
Contract Volume: 24.5 mill. DM
Construction start/end: 29. 01. '87 till 28. 11. '89
Opening: April 1991