New laying technology for power cable laying: Premiere for innovative laying sledge

In the Wahle-Mecklar pilot project of the project-executing agency TenneT, Bohlen & Doyen used the "KaRoSch" (cable conduit sledge) for the underground cabling section, the simultaneous laying of the six pipes, the easy convertibility and, above all, the integration of compaction technology, a new laying technique for power cables, and thus achieved a significant increase in efficiency: a laying speed that is up to 25 % higher, increased work safety and a significant reduction in the consumption of resources.

The energy transition requires new power lines. For this reason, the grid in the centre of Germany is being strengthened from north to south. The almost 230 km long connection runs from the Wahle substation, between Peine and Braunschweig, to the Mecklar substation near Bad Hersfeld in Hesse. One of the special features of this project: Wahle-Mecklar is one of four pilot projects in the AC sector that are legally allowed to use underground cabling on sections. On the sixth construction lot of section C between Hetjershausen and Olenhusen near Göttingen, one of the two underground cable sections of the Wahle-Mecklar line is being built over a distance of approx. 5.5 km; on the Göttingen section, Bohlen & Doyen brought its new cable conduit sledge into use.

The construction of transmission networks often includes long sections where the underground cables are pulled into protective pipe systems. For the implementation, heavy construction equipment is classically required for earthmoving, pipe handling and dewatering, and large quantities of construction and auxiliary materials are consumed. Therefore, in cable line civil engineering, too, efforts are constantly being made to make the construction process more efficient through new and further developments of construction machinery and techniques.

In addition to efficiency, Bohlen & Doyen also focused on developing and implementing economical and sustainable construction methods for the construction of transmission networks. The goal for the implementation of the protective pipe laying in the Wahle-Mecklar project, section C6, was to accelerate and simplify the work steps in the protective pipe laying process under the aforementioned aspects.

Newly developed cable conduit sledge in use

This laying sledge makes it possible to combine a total of eight work steps, some of which previously had to be carried out without machine support, i.e. with laborious and physically demanding manual work. The sledge completely eliminates the need for the lower pipe bedding, which is conventionally laid first and is installed in the correct profile using a delta loader with a minigrader. The skilled workers in the pipe trench, who are required with the conventional laying technique for laying the pipes and aligning and positioning the pipe strings along the templates, can also be deployed elsewhere thanks to the laying innovation. The placement of the bedding material and the compaction using vibratory plates are further work steps that are carried out in one work step through the use of the "KaRoSch" - and without people in the pipe trench.

The cable conduit sled also offers significant advantages in terms of environmental compatibility and resource savings. Compared to the usual open construction method, the period during which the pipe trench is open is reduced to a few minutes. Depending on the soil conditions, dewatering can be minimised or eliminated altogether. By adapting the trench profile to the cable conduit sled geometry and the technology installed in the sled, considerable amounts of bedding material (up to 20 %) can be saved.

In addition to the modified trench geometry, the profile discharge plates and the arrangement of the compaction unit ensure that no excess bedding material is placed. Thanks to state-of-the-art technology, the compaction unit is powered by rechargeable batteries. The short sequential operation ensures that the machines can usually be operated for a full working day. This means that no exhaust gases are produced for the direct carriage operation itself. In addition, there is no risk of environmental damage in the event of an accident, as the technology does not require any operating materials that are hazardous to water and soil.

The device offers its advantages particularly on mainly straight sections of pipeline without crossing obstacles. Compared to conventional laying methods, the „KaRoSch“ can achieve up to 25 % higher laying speeds thanks to the simultaneous laying of the six pipes, the ease of conversion and, above all, the integration of the compaction technology. It was particularly important to the project developer TenneT to be able to use the cable conduit sledge on slopes that made the regular trench of the open laying method impossible, or in very moist soil structures, which makes extensive dewatering, as would be necessary in the open cable trench, obsolete.

In the course of the execution work, many insights were gained for further partial optimisation. This enables Bohlen & Doyen to further develop this system for future applications, e. g. for other bedding materials and pipes.