On 11 September 2023, the ground-breaking ceremony for the tunneling of the Elbe between Wewelsfleth in Schleswig-Holstein and Wischhafen in Lower Saxony took place. It marked the start of construction for one of the largest special structures in the major SuedLink project, which is being realised by PORR on behalf of the transmission system operator TenneT TSO. To successfully realize this challenging project, PORR’s entire expertise is being brought to bear — from structural engineering, special civil engineering and overall BIM coordination by pde Integrale Planung. The diaphragm wall work is currently in full swing so that the construction of the underwater construction pits can begin on schedule from March 2024.
The powerful HVDC cables, which will be used to transport wind power to southern Germany from 2028, will be laid through the 5.2 km long crossing structure. The planned start date for tunneling is the end of 2024. The tunnel with an inner diameter of 4.0 m and an outer diameter of 4.6 m will be constructed at a depth of around 20 m using mechanized tunneling with segmental lining. The concrete segments for the ring-shaped bracing of the tunnel tube are known as tubbings. The enormous tunneling machine (TBM) is individually designed for the geology that it has to overcome on its way. Clay, clay, peat, sand, gravel, stones and boulders alternate in the ground. The individual parts of the TBM are finally assembled in the construction pits of the launch shaft near Wewelsfleth.
The cables are led under the Elbe via 21 meter deep shaft buildings
Parallel to the tunneling, 21 m deep and 8 m high shaft buildings will be erected on both sides of the Elbe. They will be used for the structural connection of the SuedLink line to the tunnel and enable quick access during ongoing operations. The cables will be connected in neighbouring sleeve buildings. Operating buildings with storage, ventilation and recreation rooms will also be erected above the shaft buildings.
Diaphragm wall work under challenging conditions
23 slots, each around 6 meters long and 36 meters deep, have already been completed for the launch pit in Schleswig-Holstein. To ensure that the cable storage area can withstand the loads caused by the enormous cable drums, each of which weighs around 230 tonnes, an additional 549 displacement piles were constructed. The wintry ground and weather conditions made the work on the Schleswig-Holstein side a challenge. “High and sudden wind loads jeopardize crane operation. The rain-soaked ground is making soil removal, transport and construction site logistics more difficult,” explains Robert Krause, Project Manager at PORR Germany. The construction site team is coping with the challenging conditions with routine and a cool head. Less predictable things in the subsoil also awaited the team: the recovery of a granite-gneiss boulder at a depth of 27 meters shortly before Christmas 2023 is not only interesting from a geological point of view. It had travelled from Sweden to the Elbe marshes around 15,000 years ago as a result of the glacial drift of the Ice Age. But large boulders rarely come alone. “That can be exciting during tunneling,” continues Krause.
Creation of the construction pits and DSV floor with the support of industrial divers
Once the diaphragm walls have been completed, dry excavation is scheduled for March 2024. Wet excavation, which will involve the use of industrial divers, will begin shortly afterwards. The construction of the construction pits will be completed with the technically demanding production of the underwater concrete base from the end of June and the pumping out of the shaft later in the summer.
Photo courtesy of Porr