Tunnel trough repair, Mönkedamm, Hamburg

HOCHBAHN constructed the Hanseatic city of Hamburg’s first stretch of underground railway between 1906 and 1912. The original loop line connected 23 stations around the Alster river, and today forms the U3 line of Hamburg’s rail network. Renovation works on the stretch between the central railway station and Baumwall began in 2021 with the complete rebuilding of the tunnel trough and entrance at Mönkedamm. At this point, the railway heads away from the city hall and up a sloping ramp to the above-ground Rödingsmarkt station. The trough construction lies within the waters of the Mönkedamm canal on a foundation of more than 100-year-old oaken piles, which have been severely degraded by the effects of oxygen, water and shellfish encrustation. Under extremely challenging conditions, the northern branch of PORR Spezialtiefbau constructed 105 micropiles on the site. These piles will support the newly reinforced concrete trough base by transferring the strain into the load-bearing subsoil, replacing the old oaken piles.

Buildings above the tunnel exit presented challenges for the Team

The water level of the Mönkedamm canal was lowered before construction began.

The Hamburg Chamber of Commerce’s HKIC building, which was constructed above the tunnel exit in 2014, added to the challenges of the project by creating an extremely restricted work site in the already constrained inner-city location. Each micropile measures up to 12m long, with an additional casing pipe to prevent kinking in the saturated ground. In some places the piles had to be inserted with a maximum clearance of just 5.8m. The team used a special device for this purpose: the steel pipe was fitted with connectors and bored in by the drilling equipment. As there was no ramp suitable for vehicles leading to the drilling area, a 250t mobile crane was required to manoeuvre the drill in between the buildings and into the trough. The micropiles were inserted both lengthwise and crosswise in between the existing timber piles. The excess cement slurry had to be contained to prevent contamination of the flowing waters of the canal below. Even the assembly of the measuring equipment for the load testing was made unusually complex by the limited overhead clearance and lack of space to use a crane, forcing the team to apply their many years of experience and expertise to the task.