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The provincial capital of Bregenz is securing its sewer system against flooding. To this end, the 900-meter-long emergency overflow of the wastewater treatment plant is being relocated. In addition, a new flood pumping station is being built, which will pump 4000 liters per second when completed. The large-scale project requires extensive civil engineering and structural engineering work, which is being carried out by the Lauterach-based company i+R. The company AGRU Kunststofftechnik GmbH supplied 280 linear meters of large-diameter pipes with an outside diameter of 1800 mm in SDR 26.
The flood protection project in Bregenz has taken an important step: The lake section of the drainage pipe supplied by AGRU Kunststofftechnik was floated into Lake Constance from the shore on the right bank of the Rhine by train ship on April 8 and sunk the next day by professional divers to a depth of up to 16 meters in a previously excavated trench at the bottom of the lake. In the two weeks beforehand, i+R welded 22 prefabricated polyethylene (PE 100-RC) pipe rods with a diameter of 1.8 meters together and attached concrete half-shells to the seams of the total 280-meter-long pipe. These served as ballast during floating and as pipe supports after lowering. This means that 360 tons of material lie on the bottom of the lake, which was additionally covered with two meters of gravel taken from the trench beforehand and thus firmly anchored.
Construction on land and in the lake
The drain, 900 meters long in total, leads from the Bregenz wastewater treatment plant into the lake. On land, i+R laid a 650 meter long reinforced concrete pipe to the shore, which runs up to 5 meters below ground. This is where the 280-meter-long PE 100-RC pipe from AGRU Kunststofftechnik comes in. "On land, the pipe runs below the existing high-pressure gas lines, separation lines and wastewater lines, among others," describes Ernst Stemer, head of i+R Tiefbau.
Special framework conditions
Additional challenges include construction in a nature reserve, the generally high groundwater level in the area near the riverbank, the location in a traffic and recreation area, and ensuring the ongoing operation of the wastewater treatment plant. "In February, we also had a 100-year flood event," adds Ernst Stemer. To ensure smooth construction, i+R secured the trench with 12-meter-long braced sheet piles.
Flexible and durable
There are several reasons why the lake portion of the pipe was realized in plastic: "PE has a much longer service life than reinforced concrete or fiberglass, is corrosion-resistant, less expensive and easier to install," explains Markus Ebster, head of the "XXL Pipe Systems" business unit at AGRU Kunststofftechnik GmbH. "In addition, it is more flexible and can thus withstand a swell during installation and any vibrations or settlements at the bottom of the lake later on. AGRU manufactures large-diameter pipes and fittings made of PE 100-RC with outside diameters of up to 3,500 mm at the customer's request. That is a world record", adds Markus Ebster. In order to connect the materials on the shore without any problems, i+R erected a sheet pile box and installed a special construction made of plastic (PE), which can withstand many tons of load.