Backup pipeline for sewer at 66 m water depth

Concrete collars are fixed at intervals of 3 metres in order to lower the pipeline to the bottom of the lake.

Lake pressure pipelines for wastewater were built in several lakes in Austria since the beginning of the 1970s. This also applies to the Fuschlsee in Salzburg, where a PE sewer pipeline has been in operation on the lake bed since 1975. Since the calculated service life of 50 years will soon be reached, it is necessary to construct a parallel 4,200 m long sewage pressure pipeline. It serves as a reserve line that can be put into operation immediately in the event of a problem with the old line. AGRU Kunststofftechnik supplies the components necessary for the lake pressure pipeline from the stress crack resistant high-tech plastic PE 100-RC.

Lake pressure pipelines score points for their rapid erection time and leave the shore areas, which are often protected by nature conservation laws, untouched during installation. In addition, due to the direct routing of the pipes across the lake, several pipe lengths can be saved. This means lower investment costs compared to a pipe laid on land, but since accessibility in the event of damage is difficult due to deep water, it is necessary to have a replacement pipe available at all times. For this reason, the water-treatment association Fuschlsee-Thalgau is investing in a new parallel lake pressure pipeline made of PE 100-RC plastic. It will cross the entire length of the crystal-clear alpine lake with drinking water quality and transport the wastewater of the municipality of Fuschl am See, between 600 and 900 cubic metres per day, from one shore to the other, where it will be led via a collector to the wastewater treatment plant.

AGRU Kunststofftechnik had previously supplied 4,248 linear metres of polyethylene pipes in OD 280 mm and SDR 17 (16 mm wall thickness, maximum 10-bar pressure) as well as fittings and a CNC-controlled butt-welding machine. The black PE 100-RC pipes with axially running, brown colour strips for clear identification of the application area wastewater are characterised by enormous resistance to point loads and stress cracks. The piping system thus ensures additional safety during decades of operation on the rocky bottom of the lake at a depth of 66 metres, where a temperature of only 4 °C prevails all year round due to the anomaly of the water. In addition, the material also offers very good resistance to chemicals that are transported in household wastewater (e.g. alkalis and surfactants).

The company PEER WASSERBAU GmbH. & Co. KG. is a recognised specialist company in the fields of hydraulic engineering, structural engineering and civil engineering and is very familiar with the professional installation of lake pressure pipelines. Mr. Erich Peer and his team have only about two months to weld the more than 4 km long lake pressure pipeline from 18 m long pipe rods, to float it into the pipeline route and to sink it. The AGRULINE pipes are welded with the AGRU ST CNC 2.0 butt-welding machine. Thanks to state-of-the-art control technology, all parameters such as pressure, temperature and time are always under control. The machine documents the welding parameters of each individual weld and even the name of the welder. Only an experienced, certified welder is allowed to produce the welded joints. Since the individual pipe rods are joined exclusively with the pipe material, a monolithic, continuously corrosion-resistant piping system is created.

Weighting represents another important element in lake pressure pipelines. Concrete weights must permanently prevent the piping system from buoyancy, which is intensified by possible digester gas formation in the pipeline. In addition, it has the task of stabilizing the lowered pipeline at the bottom of the lake so that it is still in place decades later. PEER uses precast concrete elements consisting of two halves, which are placed around the plastic pipe like rings. The halves of each ring are then permanently connected with stainless steel screws. In order to protect the pipe and ensure additional safety against slipping, a foam rubber insert is clamped between the pipe and the concrete. Thanks to these concrete collars, which are fastened at intervals of 3 metres, about 50 % of the buoyancy is compensated. If the pipe is filled with water, it sinks to the ground due to the concrete weights.

Approx. 200 linear metres of ready-to-use pipeline are pushed into the lake from the shore every day. The kilometre-long floating pipe strings must be protected from drifting by wind. PEER anchors and therefore tensions the pipe strings every day. After about two months of work in any weather, the time has come to lower the 4.2 km long lake pressure pipeline to the lake bed. Before the sinking process, some basic conditions have to be taken into account. The detailed exploration of the lake bed in the area of the pipe route and the exact positioning on the basis of GPS coordinates are important. Then the actual lowering process can start. For this purpose, the pipe string is carefully filled with lake water at one end and with pressurized air at the other end. The lowering speed is critical, since a certain angle of curvature of the pipeline must not be exceeded. After all, the future sewer crosses the entire length of the lake and thus also the deepest point of 66 m. The complete lowering process takes several hours. After successful lowering, a pressure test of the complete pipeline is carried out.

The Steinbacher + Steinbacher ZT GMBH office in Thalgau has many years of experience in hydraulic engineering and has already successfully implemented several projects with the water-treatment association Fuschlsee-Thalgau. For this reason, DI Martin Roither was commissioned by the Steinbacher office to plan the overall concept. This included a variant study, the planning, tendering and local construction supervision of the upstream retention basin, the pumping station and the lake pipeline II through the Fuschlsee.

Christian Winkler is managing director of the Reinhalteverband Fuschlsee-Thalgau: “The Fuschlsee has drinking water quality and that should remain so. The new lake pressure pipeline serves as a backup and provides us with additional safety, as it can go into operation within minutes in an emergency. Together with Mr. Martin Roither (planning and construction supervision), we have therefore also decided in favour of a plastic pipeline made of AGRU’s PE 100-RC material, as studies show that a PE pipeline of this quality can last up to 80 years. We are satisfied with the planning and delivery as well as with the quality of the AGRU pipeline. I would also like to thank PEER for the professional and careful installation”.


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