IMMERSED TUBE TUNNELS | TRANSPORT
tunnel elements. At the end of October, the ballast tanks were filled for the first time, the dock was flooded and the watertightness was checked. Subsequently, the ballast tanks were emptied, the dock re-flooded and both concrete elements became afloat in the dock. Finally, the long tunnel elements were
GINA sealing profile
Ballast tanks
pulled out of the dock. The pre-tensioning channels, intended for towed transport, were grouted whilst the elements were still on a quay in Bremerhaven. Three days apart in November, the two elements were towed upstream, each by three tugs, to Bremen, 60km away. The transport window used the rising tide and the convoy passed through two locks. In the meantime, preparatory work got under way back at the dry dock for construction of the next two elements (Fig. 10), which are due to be completed in autumn 2026 and then also transported to Bremen.
Omega sealing profile
6 - IMMERSED TUBE TUNNEL IN RIVER WESER 6.1 Dredging the trench Before the concrete tunnel elements are immersed, a trench with a slope of 1:5 is constructed by dredging between the north and south road channels on the banks of the Weser. At its deepest point, the base of the excavated trench is approximately 23m below the average water level of the river. On both ends of the long trench, transition areas connect to the respective C&C construction sites onshore. In the riverbank areas, the shoring consisting of combined sheet pile walls is mainly installed from floating equipment. To secure the Weser bank in relation to the future slope line of the dredged trench, sheet pile walls are constructed parallel to the river’s fairway and secured with horizontal anchors.
6.2 Transport of the concrete elements The concrete elements must be towed from the dry dock in Bremerhaven to the construction site in Bremen. The ballast tanks are flooded and their tightness checked. The ballast tanks are then drained, the dry dock is flooded until it reaches and equalises the external water level, and the elements are coming afloat in a controlled manner. As soon as the elements are afloat, they are pulled
out of the dry dock with winches, taken over by tugs and manoeuvred through the Kaiserschleuse lock in Bremerhaven to the Weser shipping channel. With the tide, they are pulled by two tugs at the primary end and the transport work, in towing under tension, is supported by a third tug located at the rear of the floating concrete element. A fourth tugboat accompanies the convoy so that a replacement is available if one of the tugs were to fail. The convoy heads for Bremen at a speed of approximately six knots. For passage through the locks, the towing operation is reduced to one tugboat at the front and maintaining the brake tugboat at the rear. After passing through the narrow Oslebshausen lock, the elements are parked in three different harbour basins in the industrial harbour until they are ready to be lowered into the riverbed trench.
6.3 Preparations for the lowering process As preparatory work at the river, underwater explosive ordnance detection and dredging to construct the trench will take place while shipping traffic continues, as this section of the Weser is a maritime shipping route. The shipping channel will be restricted at times to allow the underwater work to be carried out. The explosive ordnance detection and any subsequent underwater clearance pose particular challenges for those involved. First, any objects will be cleared from the riverbed, then the area will be checked by a scanner system to detect any metallic objects and, if necessary, cleared of explosive ordnance using floating equipment and with the assistance of divers. Large floating machinery is then used to dredge the
tunnel trench to a depth of up to 23m below the average water level. The excavated soil is loaded onto barges and dumped at a specified location in the Weser delta. Approximately 350,000m3 of soil must be excavated to achieve the required depth profile of the trench. This work will take place shortly before the lowering process of the concrete elements to minimise sedimentation from the river. The slopes of the trench are planned with a gradient of 1:5 without additional scour protection against crossing ships. The concrete elements, parked in the industrial harbour,
are to be prepared in a two-week cycle by setting up on each the survey tower, access shaft and installing the control system. Then each element is to be towed to the trench site and submerged, in sequence, in a south-to- north work direction.
6.4 Lowering the elements In the south, two elements will be laid inland, within the immersion channel, then three elements including the emergency bay will be lowered into the Weser, and then on the other side of the river one element will be lowered in the northern immersion channel. It is particularly challenging to lower the last element in the north, as it must be floated over the fifth element before it can be immersed. The lowering process is controlled by an immersion
pontoon, which is moved both longitudinally and transversely by means of winches and tugs to ensure precise control. Once the elements are in position, the ballast water tanks are slowly filled until the weight of the tunnel element is slightly higher than its buoyancy. The first sealing level is already in place on the element
using the GINA profile, which was previously installed in the dock. At this point, the element is connected to the
July 2026 | 15
Above: Figure 9. Schematics showing overview of the ballast tanks in a tunnel element and a detailed cross-section of the GINA and Omega sealing profiles. Credit: Wayss & Freytag
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