TRANSPORT | IMMERSED TUBE TUNNELS


full height of the concrete elements, from the lower edge of the base slab to the upper edge of the ceiling, is approximately 8.50m. Only in the section of tunnel with the emergency bays and ventilation niches is the total width greater, at approximately 28m, and the total height higher, at approximately 10m. The elements are of different lengths, varying between 115m and 125m. Each long element is itself formed of shorter lengths of the full tunnel cross-section, consisting of 12–13 individual blocks of 9.50m in length. The dry dock was initially prepared so that its floor,


in particular, could be used for the construction of the polygonal concrete elements. Special attention is paid to the subsequent need to float the heavy concrete elements, which will be sealed at the ends with bulkheads. Adhesion between the substructure and the concrete base must be avoided to guarantee a smooth flotation process. The base is constructed using permanent timber formwork, which is fixed to the concrete base slab with screws. The individual concrete tunnel blocks are manufactured


Above: Figure 7. Completion of the first two tunnel elements in the dry dock in Bremerhaven. The curvature of the route is clearly visible. Credit: Eric Shambroom Photography


in May. In the other excavation pits in the southern area the procedures are similar, albeit with a slight time lag. All necessary D-Wall works were completed in March 2026.


5 - CONSTRUCTION OF THE TUNNEL ELEMENTS IN THE DRY DOCK Since the two tunnel portals are not aligned straight, the immersed tube tunnel takes an S-shaped curve in the Weser. Each of its six elements, therefore, has an individual curvature corresponding to the alignment in all three directional axes. The motorway enters the tunnel with a 4% gradient, winds its way underwater in the S-curve and rises back (also with a 4% gradient) to the surface on the other side of the river. This creates the shortest possible route for the final section of the A281 motorway. Like the ramps and structures leading to the A281


junctions, the tunnel elements contain structures to permit separated traffic in each direction. The six concrete elements are being constructed in a dry dock in Bremerhaven (Fig. 7). The immersed tube tunnel‘s load- bearing structure is a closed, rigid, two-cell rectangular frame. Each cell is dedicated to one direction of traffic. The tunnel base slab extends slightly beyond the


Below: Figure 8. Formwork, reinforcement and concreting sequence for the construction of a tunnel element. Credit: Wayss & Freytag


outer edges of the tunnel walls, forming lateral toes. The wall-to-ceiling connection has a haunch configuration, on the inside at the corners of the frame (Fig. 8). Taking into account the polygonal design of the tunnel blocks, the width of the emergency walkway was increased to 1.02m. The clearance height is 4.7m. The width of the base slab is just over 23.00m for most of the tunnel length. The


in a semi-monolithic configuration. After the base slab has been reinforced and concreted, the central wall is erected so that the outer walls and ceiling can then be reinforced and concreted in a single step (Fig. 8). In addition, the sand injection pipe for subsequent underflushing is laid on the outer toe. Once afloat, the freeboard of the individual elements


is only 30–50cm. In addition to the defined weights of reinforcement and built-in components, the weight of the concrete is also a decisive factor. Therefore, constant monitoring of the concrete density of each individual concrete pour is of utmost importance. The target and actual concrete density are continuously compared and, if necessary, corrected in consultation with the concrete manufacturer. All visible concrete surfaces are finished with smooth,


level formwork as exposed concrete. The construction process was planned in-depth due to the tight time frame. The direction of work was determined from the centre outwards, so that four formworks could be used in parallel. Two 120m-long tunnel elements, each weighing a total of 22,000t, are constructed simultaneously, one behind the other, in the dry dock. In addition to the concrete construction work, special


focus is placed on the temporary equipment for the towing and lowering processes. In addition to the ballast tanks (Fig. 9), a piping system for filling and draining the ballast tanks is installed, as well as the associated control system, lighting, ventilation and access points. Furthermore, the pre-stressing tendons must be installed and tensioned. Before flooding the dock, the tunnel elements are


sealed at both ends with watertight concrete bulkheads equipped with doors and pipe connections.


5.1 Current status of the work By October 2025, the preparatory work for the transport fittings – namely the bulkheads on both sides of each element, the steel end-frame, the Gina sealing gaskets, bollards and fenders, as well as winches, ballast tanks and much more – had been completed on the first two concrete


14 | July 2026


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