TRANSPORT | IMMERSED TUBE TUNNELS
4.4 Current status of construction work on the deep excavation pits 4.4.1 Northern excavation pits Work on excavation pit N4, which is closest to the river (see also Fig. 3), is currently at the most advanced stage. Following wet excavation from summer to autumn 2025, around 500 micropiles were installed in a tight grid pattern from pontoons (Fig. 6). The piles, which are between 14m and 20m in length, serve to anchor and secure the approximately 1.50m-thick, unreinforced underwater concrete base slab against buoyancy. The slab was cast in the last week before Christmas 2025. Operating continuously, the project’s two concrete batching plants supplied around 7,000m3 of concrete in 72 hours. Approximately 100 m3 of concrete was placed per hour. The pit was pumped dry from early January to late
February 2026. The D-Walls and bored pile walls appeared to be in perfect condition. This was followed by the strengthening of the bulkhead towards the immersion channel, the installation of the drainage concrete and the levelling layer. At the end of March, concrete works for the tunnel using the C&C method began. Following the first base slabs and middle walls, work began in April 2026 on erecting the formwork carriages for the outer walls and deck, running up to the bulkhead separating the open pit from the immersion channel. Once this work is complete, the section of the tunnel constructed using the open-cut method can be backfilled around the end of 2026. In parallel with the work in excavation pit N4, wet
Above top: Figure 4. Dredging of the excavation pit in section N4.
Credit: Eric Shambroom Photography
Above bottom: Figure 5. Southern construction site in Seehausen with the sheet pile wall to the Weser river in the foreground. Credit: Wayss & Freytag
tunnel elements is completed on time. The C&C section is followed by the trough structure on both sides. The protective cover structure extends a further 565m to the north and is designed to protect against flying sparks from the slag dump.
4.2 Excavation pit south of the Weser In contrast to the northern section, which is used for industrial purposes, the route in the south passes through the village of Seehausen, crossing the Hasenbürener Landstraße road. To maintain traffic flow, a diversion with a temporary bridge over the northernmost excavation pit on the south side will be set up (Fig. 5).
Right: Figure 6. Micropile installation from pontoon in excavation pit N2. Credit: Eric Shambroom Photography
12 | July 2026
4.3 Construction process The construction processes in the north and south are similar. First, vertical shoring is installed using diaphragm walls (D-Walls), bored pile or sheet pile walls. After preliminary excavation, up to three anchor layers are installed, depending on the depth of the excavation pit. The subsequent dry excavation is carried out under the protection of groundwater lowering. This is followed by dredging in the flooded excavation pit. After cleaning the excavation base, threadbar micropiles are drilled from pontoons to secure against buoyancy and the pile heads are installed by divers. Subsequently, the underwater concrete is placed using the tremie method. Once the concrete has reached sufficient strength, the excavation pit is drained.
excavation took place in N2 and N1, which are located north of the track crossing. Conditions here are challenging, partly due to the high proportion of compacted slag and partly due to the work being carried out between the closely spaced struts stabilising the walls of the excavation pits. Works for stabilisation of the base using micropiles has begun, to be followed with around 8,000m3 of concrete poured in June to form the underwater concrete bases for the two pits. Farther north, piling works for the protective gallery started in May.
4.4.2 Southern excavation pits In excavation pit S1, the deepest pit directly behind the southern immersion channel, the wet excavation works and micropile works began in autumn 2025 and have been completed; the excavation base was cleaned of mud and around 5,000m3 of underwater concrete base was poured
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