Herrenknecht TBM Completes Elbe Underpass

As one of several ultra-high-voltage direct-current (UHDC) transmission lines planned in Germany, SuedLink is a key project in the expansion of Germany’s energy infrastructure. With a length of approximately 700 km, this “electricity highway” operated by the two transmission system operators TenneT Germany and TransnetBW will connect northern Germany’s offshore and onshore wind power regions with industrial sites in southern Germany, specifically in Bavaria and Baden-Württemberg.

With a transmission capacity of 4 gigawatts, SuedLink will transport electricity from Schleswig-Holstein to Southern Germany. To do so, the 525-kilovolt direct-current lines will pass under the Elbe River through a tunnel approximately 5.2 km long, running from Wewelsfleth (Schleswig-Holstein) to Wischhafen (Lower Saxony). With the breakthrough into the target shaft on June 22, 2026, the tunnel boring machine (TBM) – christened “Elsa” – completed the tunnelling work that began in February 2025. An average of 233 tunnel rings made of concrete segments were installed each month, corresponding to 303 m of tunnel length per month.

PORR is constructing the cable tunnel for the SuedLink Elbe crossing on behalf of the transmission system operator TenneT. PORR has commissioned the ARGE Tunnel ElbX – a joint venture between PORR and Wayss & Freytag Ingenieurbau AG – to build the tunnel. In addition to the six underground cables, the tunnel will house the associated building, safety, control, and monitoring systems, as well as rails for the tunnel vehicles required for maintenance and repairs.

Mixshield for highly variable geology and high water pressure

For the construction of the tunnel, the ARGE Tunnel ElbX consortium ordered a tunnel boring machine (TBM) from Herrenknecht AG that was specifically tailored to the project’s requirements. The Mixshield has a diameter of 4,900 mm, is 190 m long, and weighs a total of 700 metric tons. The main challenges for tunnel advancement at ElbX were, in particular, the geology, the high water permeability of the ground, the water pressure beneath the Elbe River, and the relatively compact diameter for a Mixshield.

“The tunnelling route beneath the Elbe River features highly variable geology consisting of clay, silt, peat, sand, gravel, as well as stones and boulders. To cope with the extremely sticky Lauenburg clay on this geologically very heterogeneous drilling section, the TBM is equipped with a powerful center flushing system at the cutterhead with a capacity of 500 cubic meters per hour,” explains Herrenknecht project manager Johannes Faißt. A total of 1,200 cubic meters of bentonite slurry flowed through the machine’s circulation system every hour. This prevented the cutterhead and the mining tools mounted on it from clogging.

The Mixshield is sealed against water pressure of up to 3.8 bar – which prevails 20 m below the riverbed – using a multi-layer sealing system. The TBM not only excavates the tunnel but also lines it simultaneously with prefabricated concrete segments. The tunnel will have an inner diameter of 4 m and an outer diameter of 4.60 m. To navigate the ElbX route – which includes curves, descents, and ascents – the TBM is equipped with a navigation system from Herrenknecht’s subsidiary VMT.

“The engineering challenge was to incorporate the technology of a highly complex Mixshield into a machine with a diameter of just under 5 m: the conveyor system, electrical systems, hydraulics, ventilation systems, the transport system for the segments, and walkways for the crew on the machine. The systems supplying the machine are therefore distributed across 12 gantries,” emphasizes Project Manager Faißt, who regularly visited the machine during the construction period.

Comprehensive separation technology

Herrenknecht Separations also supplied a custom-designed separation system with chamber filter presses for dewatering fine solids, as well as two water treatment plants at the launch and reception shafts.

The separation plant ensured a continuous slurry cycle, thereby enabling stable and high-performance tunnelling by the tunnel boring machine. “The filter presses ensured efficient dewatering of even the finest particles, thereby reducing disposal effort and costs,” explains Gino Vogt, Head of Herrenknecht Separations. The two water treatment plants at the start and end shafts ensured the resource-efficient provision of process water as well as its safe treatment and discharge back into the Elbe River in compliance with strict environmental regulations.

Precision-fitted formwork for 24,000 tunnel segments

Herrenknecht Formwork manufactured the segment formwork – including 3D surveying – for the production of prefabricated concrete segments for the tunnel construction, as well as the handling equipment for demolding, turning, and transporting the segments. The 5.2-km-long tunnel structure consists of approximately 4,000 segment rings, each 1,300 mm wide and composed of six elements. The formwork ensured the highest level of dimensional accuracy throughout the entire production process for the 24,000 tunnel segments – a crucial prerequisite for the precise fit of the segments and, consequently, the quality of the tunnel structure.

Herrenknecht technology for key sections of grid expansion

ElbX is an outstanding example of how Herrenknecht’s mechanical tunneling technology supports the expansion of the power grid to facilitate decarbonization. Tunnels for high-voltage cables are being built with Herrenknecht TBMs in many countries and also in urban centers such as London and Berlin.

For underground crossings of existing infrastructure or topographical obstacles, Herrenknecht offers not only tunneling solutions but also various drilling and laying technologies for protective casing and pipeline construction, as well as for gas or hydrogen pipelines. In Germany alone, an estimated 4,500 crossings are expected as a result of the numerous grid expansion projects.

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