New use for river water pipeline after trenchless rehabilitation

25 May 2023

In the future, a river water pipeline in Kassel, Germany, will fill a cistern in the city centre for cleaning the sewer system and for watering urban green spaces. However, the pipeline first had to be rehabilitated. The course of the pipeline with several bends through partly inaccessible and partly protected terrain required a special – preferably trenchless – solution. To find and implement this solution, the operator KASSELWASSER needed a lot of patience.

The so-called Fulda water pressure pipeline from 1920 had to be rehabilitated due to its structural condition and the intended new use in the city centre. Originally, the pipeline was used to supply water to steam locomotives at the main railway station and to supply service water to a Deutsche Bahn repair plant. For this purpose, the water was taken from the river Fulda and transported to the railway station by means of pumps via an approximately 1.5-kilometre-long pressure pipe. The route of the pipeline begins behind the historic Orangerie in the Fulda meadows, crosses the landscape conservation area created after the 1981 German Federal Garden Show, continues across the documenta site and finally for several hundred metres across the pedestrian zone of the city centre to the main railway station.

The new utilisation concept envisages the construction of a cistern in the city centre. In the future, KASSELWASSER, as the operator of the sewer network, will be able to draw water from it for cleaning purposes. The same applies to the environment and garden department for watering the trees and green spaces in the city centre. For this, the water is to be pumped from the Fulda into the cistern with about twelve cubic metres in size within 30 minutes by means of an electronic circuit. The difference in height between the Fulda and the location of the planned cistern is about 27 metres. In the first construction phase in 2022, KASSELWASSER first rehabilitated around 750 metres of pipelines. The construction of the cistern will follow in the second construction phase in 2023.

Complex preparatory work and planning

To plan the entire construction project, operator KASSELWASSER first had to determine the location of the pipeline as well as its condition. There were only a few accessible old structures and historical site plans were also only partially and incompletely available. Therefore, the pipeline was uncovered by means of search shafts. Cameras were also used at these points for visual inspection.

KASSELWASSER quickly discovered that the existing pipeline includes a considerable number of bends and large parts run in inaccessible and protected areas. Furthermore, the operator identified two culverts at a water body and a crossing combined sewer. Nevertheless, KASSELWASSER succeeded with great effort in inspecting the old grey cast iron pipeline with a nominal width of DN 300 almost completely. In addition to the bends, the inspection team also found considerable incrustations and deposits up to 70 % of the nominal width. Extensive sewer cleaning removed the deposits and incrustations to such an extent that a visual inspection was possible.

After this preparatory work, it was clear to KASSELWASSER that an open construction method was out of the question due to the location of the pipeline in a landscape conservation area and a listed staircase or in the city centre with an avenue of trees. For the rehabilitation planning, the operator faced the following challenges:

Which rehabilitation system is suitable for the culverts and bends up to 90 degrees with the incrustations and residual deposits currently still present?
How can open construction be avoided or minimised?
Is the accessibility for possible rehabilitation vehicles granted?
How can the residual incrustations and deposits be removed from the pipeline and be disposed of?

During the planning phase, KASSELWASSER rapidly recognised that common methods for pressure pipe rehabilitation, such as PE pipes of close-fit liners, could not be installed without extensive accompanying open construction. Reason for this are the conditions, particularly the 90-degree bends. The preparatory work for inventory and the rehabilitation planning took nearly two years altogether.

Primus Line^® exclusive option for the rehabilitation

A comprehensive market analysis revealed that the exclusive option for rehabilitation was the installation of the self-supporting Primus Line^® system by Rädlinger primus line GmbH. This system consists of a flexible fabric hose made of aramid fibres and coated with plastics. But also the installation of Primus Line^® consumed another year of planning and preliminary discussions, since the use of the system for such a rehabilitation was a first.

KASSELWASSER and Rädlinger primus line GmbH elaborated a rehabilitation concept together that answered the following questions comprehensively:

How does the new pipe get into the existing pipeline with its numerous bends?
Can the existing diameter be kept?
Which pump pressure can be absorbed?
How can the locations for probably required insertion pits be realised?
Can or must a smaller pipe be inserted to pass the bends?
If so, can or must the annular space be backfilled?

Since the system is self-supporting, it does not need any connection to the host pipe. The pressure rating can be selected freely according to requirement and has been adapted to the existing pump pressure. Bends and culverts could be passed without excavation pits, just four were required for Primus Liner insertion. These and the existing structures made if finally possible to insert a liner DN 150 into the host pipe DN 300. The smaller nominal width was chosen because of the existing bends. The locations of the excavation pits were determined in close consultation between all parties involved and the realisation period was postponed until after the end of documenta 2022.

The greatest challenge was the cleaning with steel scrapers and rubber discs as well as the recovery and disposal of the residue. Bends and culverts were very difficult to pass with the special equipment and the debris kept collecting in front of the culverts. After three weeks of precision work, cleaning was completed: The pipeline was free of deposits and the incrustations almost completely eliminated.

Now, the actual rehabilitation could start. The liner which was delivered pre-folded in U-shape on reels was inserted into the pipeline sections to be rehabilitated by means of pulling winches. Then the Primus Liner was inflated to its circular shape with the aid of compressed air of about 0.5 bar. The longest insertion distance amounted to a bit over 200 metres. The assembly team of Rädlinger primus line GmbH mounted the specially developed connectors that couple the liner with the host pipe at the ends.

The corresponding spacers in the excavation pits were later made on site with weldable PE-HD pipes and finally a pressure test was performed. The installation time for the liner with all preparatory work amounted to four weeks, the overall construction measure covered a period of eight weeks.

Conclusion

The rehabilitation of the Fulda water pressure pipeline is an exceptional project which must certainly be considered as an individual solution due to the general conditions. In rehabilitation planning, both KASSELWASSER and Rädlinger primus line GmbH have broken new ground.

Already during the damage assessment as part of the initial inspection, it became obvious that classic rehabilitation methods were out of the question due to the incrustations, deposits, culverts and 90-degree bends. The difficult accessibility and the local conditions excluded further options. After a long planning process, KASSELWASSER found a partner for this exceptional project in Rädlinger primus line GmbH who was ready to test their product for this novel rehabilitation method. KASSELWASSER as client took a certain risk and Rädlinger primus line GmbH as contractor exploited the special features of their product to the full. Thus, an extremely satisfactory result was achieved for all involved. The joy was all the greater when, after completion of the rehabilitation work and commissioning of the pipeline, at the other end – where the cistern will be located in the future – the first drops of Fulda water flowed out.