Hamburg
Renovation of the Mönkedamm tunnel trough
Hamburg, Germany / 2021
Retrofitting of historic tunnel foundations
The tunnel trough at Mönkedamm is situated in the area of the tunnel exit leading to the Rödingsmarkt above-ground station. The structure was originally founded on oak piles, which had lain in the water of the Mönkedammfleet for many decades. As part of the renovation works, the existing foundation elements were supplemented by a new pile foundation. A total of 105 micro piles were used to support the loads of the newly constructed reinforced concrete tunnel trough base.
Micro piles in confined inner-city conditions
The micro piles were installed under challenging conditions. Before work began, the water level of the Mönkedammfleet was lowered. At the same time, the construction site was located beneath the HKIC building of the Hamburg Chamber of Commerce, which has stood over the tunnel exit since 2014. Space was severely limited. In some cases, a clear height of just 5.8 m was available for the installation of the micro piles, which were up to 12 m long. Due to the ground conditions, the piles were constructed using drill pipes to ensure buckling resistance.
Special installation technique for limited headroom
To install the micro piles, the steel pipes were fitted with connectors and driven into the ground section by section using the drilling rig. As there was no accessible ramp to the work area, the drilling rigs had to be lowered into the trench between the buildings using a 250-metric-tonne mobile crane. The micro piles were arranged between the existing timber piles in both the longitudinal and transverse directions. The foundation solution was adapted to the existing building structure and the spatial conditions.
Work in the vicinity of a water-bearing canal
During the installation of the micro piles, measures had to be taken to prevent construction materials from entering the watercourse, which remained in flow. The excess cement slurry was therefore collected and discharged in a controlled manner. The carrying out of the load tests also placed special demands on the construction process. Due to the limited height and spatial constraints, the measuring equipment had to be specially adapted to the on-site conditions.
The foundation for the renewal of the tunnel trough
The installation of 105 micro piles laid the foundation for the renewal of the tunnel trough at Mönkedamm. The retrofitting foundations enable the new reinforced concrete structure to bear the load and support the rehabilitation of a section of Hamburg’s underground railway infrastructure that is over 100 years old.
Facts & Figures
Company
PORR Spezialtiefbau GmbH
Type
Retrofitting foundations, Rehabilitation
Runtime
2021
105 micro piles are replacing the oak piles dating from the 1920s
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.
