New tram tracks above Basel’s underground river

There was little time and space available for implementation: within just over two months, the entire section of track between Fischmarkt and Gerbergasse had to be renewed – in the heart of Basel city centre and directly above the historic Birsig vault. “The particular challenge was to construct a modern, heavily used track system on top of a historic structure,” says Franzisca Marti, civil engineer at Gruner and the project’s local construction manager. “We need to understand exactly how the forces generated by the tracks and tram operations are transferred into the existing structure and how we can distribute them as gently as possible. The new construction and the existing supporting structure have to be considered as one overall system.”

Gruner rail projects: track construction above the historic Birsig vault in Basel

The historic Birsig vault lies just below the tram tracks. This means that the loads generated by the track system and tram operations act on a historic underground structure that was not originally designed for today’s loads. The new track system therefore had to be designed so that these forces are transferred into the existing structure as gently as possible.

Gruner designed and dimensioned a new load-distribution slab as the solution. It absorbs the loads generated by the tracks and tram operations and distributes them over a larger area. This reduces concentrated loads on the vault below and helps protect it over the long term.

Dismantling the existing structure presented another challenge. As the Birsig vault comes very close to the existing track structure in some places, removal had to be carried out in a particularly controlled and careful manner. Maximum precision was required, especially where the Birsig changes direction beneath the Market Square.

New tram tracks with a modern Infundo track system

The planning process focused not only on safely integrating the new tracks into the structure, but also on minimising future interventions. Gruner therefore designed an Infundo track system in which the rails are installed in special steel troughs and fixed in place with a grouting compound.

Due to the extremely high loads caused by tram traffic on seven inner-city lines, the rails have to be replaced regularly. Thanks to the Infundo track system, these interventions can largely be restricted to the trough area. The load-distribution slab underneath does not need to be opened and the road surface does not need to be replaced. This protects the structure, reduces the work required for future renewals, and saves time, materials and costs.

The installation comprises two 115-metre-long troughs in the load-distribution slab as well as two 30-metre-long pre-grouted trough elements.

Track construction at Basel Market Square while the city remains in operation

Construction in the middle of Basel’s Market Square does not take place on an isolated building site. City life continues all around it. Shops remain open, people cross the square, and numerous construction activities have to be coordinated within a very confined space.

To minimise disruption, Gruner planned and coordinated the construction processes as efficiently as possible. The individual work stages were closely coordinated to ensure rapid progress while maintaining access and safe pedestrian routes. Close coordination with BVB Basler Verkehrs-Betriebe, IWB, the Canton of Basel-Stadt and the contractors carrying out the work was essential.

Accessible tram stops for Basel Market Square

The stop areas are also scheduled for completion by mid-October. The surfaces will be renewed and the stops equipped with the necessary infrastructure. At the same time, they are being upgraded to provide barrier-free access. The areas are being designed to make accessing, boarding and leaving the trams easier and safer for people with reduced mobility.

The project at Basel Market Square demonstrates how modern infrastructure can be developed within a challenging historic environment. The focus was on a solution that protects the historic Birsig vault while meeting the requirements of a modern track system. Minimising the impact of the construction work on everyday city life was equally important.

Find out more about how Gruner plans rail projects and prepares existing infrastructure for the future.

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