Power plant pipeline for the Bristen power plant in the canton of Uri

Figure 1: Transporting the DN 1000 ductile iron pipes in the Maderaner valley [Source: Werner Volkart]

1 Planning and approval phase

In 2008 the planning phase commenced for the Bristen power plant in the Swiss canton of Uri. The planning and official negotiations dragged on for eight years. Eight years during which a detailed utilisation and implementation plan was drawn up for a hydropower plant.

After all these efforts, the Bristen hydropower plant in the Maderaner valley is one of the most environmentally friendly ones in the whole of Europe. In March 2015, the time finally came: the positive report from the Swiss Nature and Homeland Security Commission (ENHK) was submitted in which it is unequivocally stated that the project at the Chärstelenbach meets the requirements for maximum possible environmental protection [1].

2 Choice of pipe material

Three months later the district of Silenen issued the construction permit. To be considered as the largest structural challenge, in addition to the completion of the water catchment, is the installation of the pressure pipeline in the upper section of the route as far as the catchment area, a difficult task in both topographical and geological terms. In total, the route of the pipeline extends over 1,800 m, for the main part through impassable terrain. For the pipe material, the operator opted for DN 1000 ductile iron pipes in the thrust and tractive restraint design. These were supplied by the Swiss pipe specialist TMH Hagenbucher AG from Zürich.

Ductile iron pipes are perfectly suited to these conditions. This is not merely to do with the high degree of resistance and the long working life of the pipes, but also their simplicity of installation: the trench is excavated and the pipe is aligned on the bottom of the trench. With the help of the restrained push-in joint, the next pipe is assembled and the trench is ready for backfilling again. This means that the construction site remains very manageable. A further advantage of ductile iron pipes lies in the fact that they can be laid in rocky ground without any special bedding material [2].

3 Logistics

Because of the topography and the roughness of the terrain, the Epp construction company from Bristen (Switzerland), the Arnold AG installation company from Schattdorf (Switzerland), the pipe manufacturer Duktus (Wetzlar) GmbH & Co. KG from Wetzlar (Germany) and the pipe supplier TMH Hagenbucher AG from Zürich (Switzerland) were facing a challenging logistical task. This was further aggravated by the fact that a total of 50 special adapter pipes had to be made to measure at short notice so as to be able to adjust the pipeline to its route. At this point the experience of the pipe supplier and their technical equipment for working with the ductile iron pipes were of the highest value.

4 Construction of the pipeline

The construction of the pipeline began in April 2016. In the initial phase the DN 1000 ductile iron pipes had to be transported from their storage location at Amsteg im Reuss valley along 3 km of mountain road in the Maderaner valley to Bristen. A tractor with trailer was used for this which, loaded with up to three pipes, was manoeuvred along the narrow, winding road (Figure 1).

From this interim storage area onwards, a crawler excavator then carried each pipe individually to the pipe trench. The power plant pipeline was built uphill, against the direction of flow, so that the pipes and fittings had to be assembled “upside-down” (Figure 2).

Figure 2: Assembling a ductile iron pipe “upside-down” [Source: Werner Volkart]

Figure 3: Installation of a 22° double socket bend (MMK 22) and an adapter pipe for a change of direction [Source: Werner Volkart]

Figure 4: An adapter pipe being put together at the Hagenbucher centre of competence in Eglisau [Source: Werner Volkart]

In the normal case, to do this the pipe trench was excavated over a length of around 6 m, the pipe aligned and the trench backfilled directly afterwards. An excavator with a screening bucket was used for this, so that the excavated material could be broken up and put back straightaway. Thanks to the efficient chain of transport, the reliable BLS^® restrained push-in joint, which can be safely and rapidly assembled even under these difficult installation conditions, and the robust exterior protection of the pipes with cement mortar coating, the experienced assembly engineers from Arnold AG (Rohreinbau) were able to work at a rate of up to 60 m a day.

The special pipes, such as the adapter pieces before changes of direction (Figure 3), were made to measure at the Hagenbucher centre of competence in Eglisau, about 120 km away, within 2 to 3 working days (Figure 4), ready to be transported to the construction site for installation. This work was able to be done at the centre of competence regardless of weather conditions. In detail, the pipes were cut to length, the cement mortar coating was removed to the appropriate length, the pipe ends were chamfered and a welding bead was applied around the circumference by means of an automatic welder (Figure 5). Finally, a corrosion protection coating was applied to the pipe ends, consisting of a zinc coating and a bitumen finishing layer.

Figure 5: Welding beads on the spigot ends of two adapter pipes [Source: Werner Volkart]

Figure 6: Backfilling the pipe trench and restoring the surface in difficult Alpine terrain [Source: Werner Volkart]

Figure 7: Steel construction of the footbridge with pre-assembled DN 1000 cast iron pipes underneath the bridge [Source: Werner Volkart]

While this was being done, the construction company was backfilling those areas of the pipeline which were still open (Figure 6), meaning that at no point was the construction work delayed on account of the preparation of the special pipes.

5 Power plant building and power plant pipeline

In order for the power plant – consisting of the water catchment in the “Lägni”, the 1,800 m long power plant pipeline and the power plant building at the valley station of the Bristen-Golzern cableway – to blend harmoniously into the landscape, the water catchment has been concealed behind a rock, the power plant building has been designed as a showpiece power plant with a gable roof and the power plant pipeline has been installed underground. Only in one section did the pipeline have to be run above ground, suspended beneath a pipe bridge over the Chärstelen stream. With the deflection capability of the BLS^® restrained push-in joints, the pipeline is able to follow the curved shape of the bridge without the use of additional fittings (Figure 7).

After completion of the construction work, the pipe bridge will be upgraded into a footbridge and the pressurised water pipeline beneath it will not be visible from above (Figure 8). The entire power plant pipeline was completed within just seven months. In the 1,800 m long pipeline there are 40 fittings (double socket bends MMK 11 to MMK 45), three fittings with manholes (Figure 9) and 50 special pipes installed.

As the main building with the turbines is located directly next to the valley station of the Golzern cableway so popular with hikers and mountain bikers, the Bristen power plant will also act as a showpiece power plant. In this way, as well as the actual production of energy, it can contribute added value for tourism in the Maderaner valley.

Figure 8: A view under the footbridge with the curved steel girders – the ductile iron pipes follow the contour of the curve [Source: Werner Volkart]

Figure 9: One of three manholes along the route of the power plant pipeline [Source: Werner Volkart]

6 Data on the project

Bibliography

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