Offenburg Combined Water Sewer

In order to relieve the existing, historic combined water system in the east part of Offenburg, a new combined water collector was driven underground as a feeder for rainwater treatment and in turn to reduce the pressure on the Waldbach. The planned route ran at a depth of 7 m and a particular challenge emerged to be the undertunnelling of a more than 100 years old church, the Heilige Dreifaltigkeitskirche. This hydraulically and economically favourable route could only be chosen because the east part of Offenburg is built on a more than 10 m thick loess loam layers. Pipe jacking with an excavator roadheader from Herrenknecht was used as construction method on account of a lack of groundwater and the prevailing geology. On average 2 reinforced concrete DN 1600 pipes 3 m in length were laid. The entire excavation section of 850 m was split into three columns (286, 405 and 159 m). The altogether 2,600 m³ of soil, which was extracted along the route, was first of all transported by a short belt conveyor to a car, which then carried it to the thrust pit through the sewer. Working around the clock was only practised while undertunnelling the church when driving the second route section until the target pit was reached, in order to preclude the danger of Settlements affecting the church. On account of the depth of the sewer network a gap of 3.50 m between the crown of the pipe and the lower edge of the church foundation was adhered to. A 3 ‰ gradient allowed little scope while laying the sewer. The responsible contractor mastered this challenge extremely well. The maximum deviations from the intended position amounted to only a few mm. The slight gradient was called for on account of using the feeder as a storage sewer. A storage shield secures the cleansing and ventilating of the storage sewer and the exploitation of its entire volume. The domestic connections were renewed by drilling from the sewer to the properties concerned to complete the project. Settlement checks carried out on a regular basis after the pipe jacking scheme was completed made the owners secure in the belief that the project had been successfully completed and that no damage was to be expected.

• Country: Germany
• Region: Ortenau
• Tunnel utilization: Utilities
• Type of utilization: Combined Water Sewer
• Client: Stadtentwässerung Offenburg
• Consulting Engineer: Ingenieurbüro Siggelkow, Offenburg
• Contractor: Bramm Hoch- und Spezialtiefbau GmbH, Vaihingen
• Main construction method: Trenchless
• Type of excavation: Tunnel boring machine (TBM)/Excavator
• Lining: Reinforced concrete pipe
• No. of tubes: 1?
• Tunnel total length: 850 m
• Cross-section: 1.60 m (internal diameter), 1.98 m (external diameter)
• Contract Volume: 2.25 mill. euros
• Construction start/end: September 2005 till August 2006 (pipe jacking)
• Opening: 2007