New Laying Technique for Power Cable Installations: Premiere for KaRoSch, an innovative installation carriage from Bohlen & Doyen

In the Wahle-Mecklar pilot project by project developer TenneT, Bohlen & Doyen used the “KaRoSch” cable pipe carriage, a new laying technique for power cables, for the underground cabling section, achieving a significant increase in efficiency: up to 25% higher laying speed, increased occupational safety, and a significant reduction in resource consumption.

The energy transition requires new power lines. Therefore, the grid in central Germany is being reinforced 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 permitted to use underground cabling on partial sections. Geographically, the sixth construction lot of section C is located between Hetjershausen and Olenhusen near Göttingen. Over approximately 5.5 km, one of the two underground cable sections of the Wahle-Mecklar line is being built here, and Bohlen & Doyen deployed its new KaRoSch on the Göttingen section.

The construction of transmission networks often involves elongated sections where underground cables are pulled into protective pipe systems. Traditionally, heavy construction equipment for earthmoving, pipe handling, and dewatering is required for implementation, and large quantities of construction and auxiliary materials are consumed. Therefore, efforts are constantly being made in underground cable construction 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 installation of the protective pipes in the Wahle-Mecklar project, section C6, was to accelerate and simplify the work steps in the protective pipe installation process under the aforementioned aspects.

For this purpose, the “KaRoSch” cable pipe carriage, newly developed and manufactured in-house by Bohlen & Doyen, was used for the first time on this pilot section near Göttingen.

This installation carriage enables the consolidation of a total of eight work steps that previously had to be carried out partly without machine support, i.e., with arduous and physically demanding manual labor. The carriage completely eliminates the conventional need for the lower pipe bedding, which is traditionally installed to profile using a delta loader with a mini-grader. Thanks to this installation innovation, skilled workers in the pipe trench, who are required for laying the pipes and aligning and positioning the pipe strings along the templates in conventional laying techniques, can be deployed elsewhere.

The introduction of bedding material and compaction by vibratory plates are further work steps that are carried out in one pass with the KaRoSch. Thus, during the laying process, no personnel are in the pipe trench to perform work. This means a significant improvement in terms of both occupational safety for colleagues in the pipe trench and personnel and equipment costs.

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

In addition to the modified trench geometry, the profile stripping plates and the arrangement of the compaction unit ensure that no excess bedding material is introduced. Thanks to state-of-the-art technology, the compaction unit is powered by rechargeable batteries. The always short sequential use ensures that the devices can typically operate for a full working day. Thus, no exhaust gases are generated for the direct use of the carriage itself. In addition, the risk of environmental damage in the event of a possible accident is eliminated, as no water and soil-hazardous operating materials are required for the technology.

Overall, the impact on nature can be significantly reduced with the KaRoSch method through the reduction of excavation volumes due to the minimization of the trench cross-section, the short opening time of the trench, and the optimally coordinated technology.

With the development and use of the new KaRoSch cable and pipe carriage, an efficient laying system was successfully developed, designed, manufactured, and deployed in the project by Bohlen & Doyen Bau GmbH. Simulations of the processes and previous testing with comprehensive documentation optimized the application of the device. Its use on the construction site then showed that optimally coordinated preparation of the individual work steps is fundamental for a successful application of the KaRoSch. In particular, the functioning of the logistics chain for the delivery of bedding material plays an essential role here.

The KaRoSch offers its advantages particularly on predominantly straight route sections without crossing obstacles. Compared to conventional laying methods, the KaRoSch can achieve up to 25% higher laying speed. This can be attributed to the simultaneous laying of the six pipes, the easy convertibility, and, above all, the integration of compaction technology. It was particularly important to the project developer TenneT to be able to use the KaRoSch on slopes that made the standard trench of the open laying method impossible, or in very moist soil structures, which renders extensive dewatering, as would be necessary in an open cable trench, obsolete.

During the execution of the work, many insights were gained for a partial further optimization of the KaRoSch. This enables Bohlen & Doyen to further develop this system for future applications, e.g., also for other bedding materials and pipes.