Author: Nicole de Jong

The energy transition is forging its path beneath the Elbe. With a 5.2-kilometre tunnel, the SuedLink megaproject will transport wind power from northern Germany to Bavaria and Baden-Württemberg. What might at first glance seem to be a mere technical detail is in reality a pivotal bottleneck of the energy transition – and an engineering feat of the highest order.

Since December 2024, the tunnel boring machine (TBM) ‘Elsa’ has been carving its way underground. It has already covered 2.2 kilometres, advancing around ten metres each day. ‘We are on schedule and delighted with the steady progress,’ says Lars Bayer, Project Manager for Tunnel Construction at the ARGE Tunnel ElbX joint venture. ‘Elsa’ is 190 metres long, designed by Herrenknecht AG and specially adapted to the geological challenges beneath the Elbe: the nearly five-metre-wide cutterhead has to penetrate clay, silt, peat, sand, and even large boulders.

Work is proceeding simultaneously from both sides of the Elbe. In Wewelsfleth (Schleswig-Holstein), the launch shaft for the tunnel is already complete. It is 25 metres deep and sealed with an underwater concrete base slab to keep out groundwater. ‘We are working with specialist divers who supervise the concreting and meticulously inspect every surface before we move on to the next construction phase,’ explains Damian Pikos, Team Leader for Heavy Special Deep Construction in the joint venture. Over on the Lower Saxony side, at Wischhafen, PORR Spezialtiefbau has just completed the diaphragm walls for the reception shaft – up to 60 metres deep and more than a metre thick. A construction challenge that demands both structural precision and logistical excellence. ‘Cutting at these depths requires absolute accuracy, as every joint must be completely watertight,’ he adds.

For the tunnel builders, 4 December is a special day. In honour of Saint Barbara, patron saint of miners, underground sites around the world celebrate St. Barbara’s Festival – Wewelsfleth is no exception. At the SuedLink Elb crossing site, PORR and the client TenneT, together with political representatives, marked the centuries-old tradition by christening both the tunnel and the tunnel boring machine ‘Elsa’. The celebration, which blends tradition and modern engineering, is seen by workers to offer symbolic protection for the hazardous work underground.

'Elsa'

The tunnel boring machine ‘Elsa’ is 190 metres long and weighs around 700 tonnes. The TBM manufacturer Herrenknecht designed ‘Elsa’ specifically for the geological conditions beneath the Elbe. Since December 2024, its 4.9-metrediameter cutterhead has advanced underground at a rate of roughly ten meters per day to excavate the 5.2-kilometre tunnel between Wewelsfleth in Schleswig-Holstein and Wischhafen in Lower Saxony. Completion is scheduled for 2027.

SuedLink, operated by the grid operators TenneT and TransnetBW, is hailed as the ‘electric highway of the energy transition’. Over more than 700 kilometres, high-voltage DC cables will carry wind energy from northern Germany to the main load centres in the south. The Elb crossing, ElbX, is one of the most demanding sections. The tunnel under the Elbe forms a key part of the entire SuedLink project. Without it, a continuous route from the North Sea to Bavaria would be impossible.

The ARGE Tunnel ElbX joint venture – comprising PORR and Wayss & Freytag – has pooled its extensive expertise in tunnel and specialist deep construction. Massive steel structures secure the shafts, underwater concrete slabs prevent groundwater intrusion, and micropiles stabilise the structure against buoyancy. An additional excavation, just five metres deep, is being prepared for the cable jointing facility, which will later house the cable connections.

In the coming months, the team will install the head beams, construct the wells, and carry out the preliminary excavation. The reception shaft is expected to be ready this summer, in time to receive ‘Elsa’ on the Lower Saxony side. The tunnel is scheduled for completion by 2027. It is only then that the cables, which will literally carry the energy transition under the Elbe, will be laid.

This progress is as much symbolic as it is technical. The project demonstrates that complex megaprojects can be successfully executed under strict environmental regulations. Construction is already on schedule. ‘This is truly a once-in-a-century project,’ emphasises Bayer. ‘Seeing the cutterhead advance little by little at the end of the day feels like a small triumph, both for us and for the energy transition.’