Efficient under difficult conditions: Primus Line^® Rehab for inverted siphon rehabilitation

01 Apr 2025

Flexible, fabric-reinforced liners are the ideal solution for the challenging rehabilitation of pressurised pipes under waterways or traffic routes: they combine stability and flexibility, require only short rehabilitation times and protect the environment and surroundings. This is demonstrated by various projects in Germany, the Czech Republic and Canada.

They run under roads, railway tracks, rivers or lakes: pressure pipes that transport various liquids under these ‘obstacles’. These pressure pipes, known as inverted siphons, are often characterised by long lengths and a U-shape with curved fittings built into their course. In addition, they are only accessible to a limited extent, for example from the banks of a body of water or next to the track bed. Many trenchless rehabilitation methods reach their limits here. New construction is time-consuming and cost-intensive and is often associated with geotechnical and hydrogeological problems as well as construction risks.

Advantages of Primus Line^® Rehab

Primus Line^® Rehab, the technology for trenchless rehabilitation of pressurised pipelines, is both strong and flexible. Why is that? These properties result from the multi-layer liner structure in combination with the specially developed connectors.

The liner consists of three layers. The inner layer is tailored to the medium to be transported: it is made of either polyethylene (PE) or thermoplastic polyurethane (TPU) with high chemical, abrasion and corrosion resistance. The middle layer is a seamlessly woven Kevlar^® fabric. It fully absorbs the operating pressure and is up to ten times stronger than steel of the same weight. This layer in particular provides the necessary stability, but also the required flexibility when travelling through bends. The outer layer of Primus Line^® Rehab made of polyethylene (PE) protects the fabric from abrasion during insertion and operation.

Insertion lengths of up to 8,200 feet, a bendability – depending on the application – of up to 90 degrees and low machine employment make the rehabilitation of ageing pipes with Primus Line^® Rehab quick and easy. This makes the system particularly suitable for environments with high geographical, economic, architectural or ecological requirements. For inverted siphons, for example.

Whether under rivers, lakes, railway tracks or motorways: Inverted siphon rehabilitation with Primus Line^® Rehab requires minimal space, which significantly reduces construction activity and therefore the impact on the surrounding area. The liner is pulled in using a cable winch at a speed of up to ten metres per minute. For transport, the flexible pipe is folded into a U-shape and wound onto special reels. Depending on the diameter, a single reel can hold up to 13,123 feet of pipe, which can be efficiently loaded onto a single truck. This minimises both the logistics costs and the environmental impact associated with transport.

Another positive aspect: a rehabilitation with Primus Line^® Rehab extends the service life of the inverted siphon by at least another 50 years.

Primus Line^® Rehab has already been used several times for inverted siphon rehabilitation. Three examples from Germany, the Czech Republic and Canada illustrate the different requirements that the technology can fulfil:

Rehabilitation of a flooded inverted siphon in Flensburg harbour

A drinking water inverted siphon belonging to Stadtwerke Flensburg crosses the Flensburg Fjord in the harbour area. The important 12-inch supply pipeline made of PE 80 was built in the 1970s and flooded into the subsoil of the fjord like a submarine cable. It runs in a wide arc over 4,068 feet 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 fjord 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 fjord 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.

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 to 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 siphonvert renovation.

Inverted siphon recommissioning after five years in the Czech Republic

The 8-inch cast-iron water supply pipeline with a diameter reduction to 6 inches 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 161 feet long, there were two 45-degree bends, two 30-degree bends and one 15-degree bend in the pipeline.

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. An existing shaft on the right bank of the river served as the starting pit so that traffic on the main road could continue to flow unhindered. On the opposite side, only a small excavation pit on a car park was required.

The host pipe was not cleaned mechanically during this renovation, but with high-pressure water jetting technology at 1,300 bar and a flow rate of 200 litres per minute. The entire rehabilitation was successfully completed after one week.

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New life for leaking drinking water inverted siphon in Winnipeg

A leaking 10-inch steel drinking water inverted siphon crossing under a river in Winnipeg, Canada, was in urgent need of action due to the constant loss of water. The original plan to rehabilitate the pipe using the CIPP method had to be abandoned due to a large number of bends in the pipe. In addition, the pipeline was constantly filling with water due to leaks in the outer area, which jeopardised the curing process in the CIPP method.

This is why the responsible engineering office and the City of Winnipeg opted for the Primus Line^® system, which is also certified to Canadian drinking water standards. Thanks to its flexibility, it could be laid through the three 45-degree, 22.5-degree and 11.5-degree bends in the 560 feet long rehabilitation section.

Before the Primus Line^® 10-inch system was installed, a CCTV inspection was carried out to detect any deposits, incrustations or unclean weld seams. This revealed several sharp-edged indentations, which were removed with a milling robot before the liner was pulled in so as not to damage the liner during installation. During this inspection, an unmarked 45-degree bend was also discovered around 20 feet before the end of the pipeline. Despite this, the renovation was executed as planned. The pipeline was then cleaned and a second camera inspection was carried out to check the preparatory measures.

Pulling in the liner itself only took around 30 minutes with the help of a cable winch. To do this, the transport reel with the U-shaped pre-folded liner was positioned at one of the two excavation pits.

After the liner had been moulded into its round shape using compressed air, the installation team fitted the connectors at both ends. The installation was completed within a day and the pipeline went back into operation after successful pressure testing and disinfection to ensure drinking water safety.

New pipe-in-pipe construction for double safety

Primus Line^® Rehab is also an interesting alternative for new inverted siphon construction as a double pipe solution (dual containment) in combination with an HDPE pipe. In contrast to steel double pipes or cast steel pipes, this solution allows greater flexibility, requires less space and is more cost-effective. With such double-pipe solutions, the HDPE pipe is first inserted into the ground using an HDD bore and then the liner is pulled into the pipe. As Primus Line^® Rehab absorbs the entire internal pressure of the pipe, the HDPE pipe only serves as a protective jacket and only requires a small wall thickness. In addition, Primus Line^® Rehab does not require any bonding or curing. An annular space remains between the liner and the old pipe. This makes the combination of HDPE pipe and Primus Line^® Rehab a cost-effective method for double safety.

The pipe-in-pipe solution is used, for example, in the high-pressure gas sector or for pipelines under critical transport infrastructure such as railway lines or motorways, as this example from Regensburg in Germany shows. A new drinking water subway was laid under the A3 motorway using the HDD flush drilling method and subsequent insertion of Primus Line^® Rehab with 10 inches over a length of 360 feet. In this case, the jacket pipe consisted of PE-HD 100 SDR 11 (315 x 28.6 mm). The individual pipe sections were joined using heating element butt welding.

As is usual with conventional Primus Line^® Rehab installations, a CCTV inspection and cleaning preceded the installation. In addition, a milling robot removed existing weld seams that could have structurally damaged the liner during installation.

After just six working days for the liner installation, including all preparatory and follow-up work, the new pipeline was put into operation.

Even in earthquake-prone areas or in particularly sensitive environments such as water protection areas, the dual containment method can offer added safety compared to rigid pipe solutions.

Bottom line

Primus Line^® Rehab enables the efficient and sustainable rehabilitation of pressurised pipes that run under complex infrastructure such as rivers or roads. Thanks to its multi-layer structure and special connection technology, the system offers maximum stability with minimum impact on the environment. This makes Primus Line^® the ideal solution for rehabilitation projects with the highest demands on flexibility, load-bearing capacity and service life.

Efficient under difficult conditions: Primus Line^® Rehab for inverted siphon rehabilitation

Rehabilitation of a flooded inverted siphon in Flensburg harbour

Inverted siphon recommissioning after five years in the Czech Republic

New life for leaking drinking water inverted siphon in Winnipeg

New pipe-in-pipe construction for double safety

Bottom line

More articles

Valuable addition to the team

Expanded product portfolio with new solutions for trenchless pressure pipe rehabilitation

Disaster relief with Primus Line’s flexible pipe

Your product watchlist

Efficient under difficult conditions: Primus Line® Rehab for inverted siphon rehabilitation

Rehabilitation of a flooded inverted siphon in Flensburg harbour

Inverted siphon recommissioning after five years in the Czech Republic

New life for leaking drinking water inverted siphon in Winnipeg

New pipe-in-pipe construction for double safety

Bottom line

More articles

Valuable addition to the team

Expanded product portfolio with new solutions for trenchless pressure pipe rehabilitation

Disaster relief with Primus Line’s flexible pipe

Your product watchlist

Efficient under difficult conditions: Primus Line^® Rehab for inverted siphon rehabilitation