THE MAGAZINE FOR THE DRAINAGE, WATER & WASTEWATER INDUSTRIES
SPECIAL WORKS
Fig 5. Thanks to small excavation pits – an existing shaft on the left bank that is barely visible in the aerial photo and a slightly larger pit in a car park
on the opposite bank (centre) –, traffic in Velké Meziříčí was able to continue flowing unhindered during the rehabilitation of the inverted siphon.
Inverted siphon recommissioning after five years in the Czech Republic
Fig 4. The section to be rehabilitated under the Oslava River in the Czech Republic was only 49 metres long and yet had five bends in its course.
important DN 300 supply pipeline made of PE 80 was built in the 1970s and flooded into the subsoil of the ford like a submarine cable. It runs in a wide arc over 1,240 metres from a waterworks to a path on the north-eastern shore, making several vertical and horizontal bends along the way.
The polyethylene used at the time the inverted siphon was built was far less sophisticated than it is today, meaning that the material was becoming brittle after almost 50 years of use. In order to safeguard the drinking water supply, those responsible at Stadtwerke Flensburg decided in favour of a preventative repair of the pipeline section. Replacing the pipeline proved to be difficult for time and economic reasons. There is heavy shipping traffic on the ford with its historic harbour, the shipyard and the excursion and sailing boats. The operator therefore favoured trenchless rehabilitation with Primus Line® Rehab. In accordance with the client's requirements, the flexible pipe is able to pass through the 30-degree bends in the pipeline and withstand operating pressures of up to 10 bar.
Normally, the pipes are completely emptied for the installation of Primus Line® technology. However, the thin layer of sand covering the inverted siphon, which runs directly under the ford floor, was not heavy enough to counteract the buoyancy of an empty PE pipe. To avoid destroying the inverted siphon, the installation team therefore had to ensure that a precisely controlled amount of water was maintained at each stage of the work. For this reason, all pieces of equipment and the required cable connection were routed through a specially manufactured pig launcher. The entire system and the complete installation process were specially adapted to these conditions and the long pull-in length, which was the largest single pull-in length of Primus Line® Rehab in Germany at the time. (Fig. 3)
To avoid buoyancy of the pipe, the water-displacing air in the pipe had to be kept to a minimum. Instead of pressurised air, water was therefore used to give the liner its round, self-supporting and durable shape. As water behaves differently than compressed air when pressurised, special connectors were first installed. The subsequent pressurisation was carried out with the utmost care and precision. The installation of the actual connectors and the final pressure test completed the successful inverted siphon renovation.
linkedin.com/company/draintrader May 2025 | 59
The DN 200 cast-iron water supply pipeline with a diameter reduction to DN 150 crosses the river Oslava, which is bordered by embankments, under the riverbed in Velké Meziříčí near Brno in the Czech Republic. The inverted siphon had been taken out of service five years before due to a major pipe burst. Although the section to be rehabilitated was only 49 metres long, there were two 45-degree bends, two 30-degree bends and one 15-degree bend in the pipeline. (Fig. 4)
The local water supplier considered various rehabilitation methods, including trenchless methods such as CIPP (Cured-In-Place Pipe) and HDD (Horizontal Directional Drilling), as well as conventional, open- trench construction methods. The CIPP method was not feasible due to the bends and the transition to the smaller pipe diameter. HDD was out of the question due to the granodiorite bedrock and the tight radius. A new open-cut construction through the river and the embankment walls would have been very costly. Laying a new supply line under a neighbouring bridge seemed to be a viable solution. However, this was rejected by the owner of the bridge.
The local utility company had learnt about Primus Line® Rehab as another trenchless method at the world's leading trade fair IFAT 2022. Following the utility company's project enquiry, Primus Line technicians and a local construction company examined the project and found it to be feasible. The construction company is an installation partner of Rädlinger Primus Line. It won the tender and carried out the rehabilitation work.
As the inverted siphon to be rehabilitated in this case was also located in the city centre, traffic should not be affected. (Fig. 5) An
Existing manholes can be used as access points for
trenchless rehabilitation with Primus Line® Rehab to reduce construction costs and earthmoving.
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