U5 Europaviertel
Frankfurt am Main

The contract comprises construction of the city centre underground rail extension and the underground connection to the existing underground rail network.

The tunnel is being constructed using the cut-and-cover method for a section of around 186m length, and bored for a section of around 2 x 830m, using an 80m long tunnel boring machine (TBM). The contract also includes a ramp structure around 137m long, plus the construction of the Güterplatz station, located around halfway along the new route section. The underground connection to the existing network is being carried out using conventional construction methods, aided by ground freezing, injection and compressed air.

In the eastern part of the Europa Allee, Stump has already completed the work on the slurry walls of the 18m deep launch pit from which the TBM will start excavations. Slurry walls are also being constructed at the site of the future Güterplatz underground station, in preparation for the erection of the construction pit walls – but excavation work there cannot commence until both tunnel tubes have been bored.

The geology in Frankfurt's city centre is notable for its cohesive soil with high proportions of clay and silt, and low permeability. The Frankfurt clay, through which a large part of the underground route runs, is of particular note. The clay layer is overlaid with several quaternary layers of sandstone and gravel up to 2-10m thick, which must be bored through from the launch pit to reach the clay layer. In some places, sandstone formations extend into the clay layer and appear at other points along the planned route.

The water table level in Frankfurt lies between 3 and 7m below the surface. The sandy covering layer has a high pore volume which acts as a ground water conductor with a high degree of permeability. The clay layer, however, has the opposite function and generally acts as a groundwater barrier, which means that the tunnel can be bored here without water pressure building up.

During earlier construction projects, underground branches for the new U5 line were created at the Platz der Republik near the main railway station; the two new tunnels will be connected to these. Getting the tunnel boring machines to arrive precisely at these existing constructions, given the geological situation and existing urban development, already presents one challenge; another consists of sealing the junction between the new and the existing structures. Initially, the plan was to achieve this using what are known as “sealing blocks” directly in front of theexisting construction. These blocks are comprised of numerous jet-grouted columns, where a rotating jet mixes the surrounding soil with a binding suspension containing cement to form a soil-cement structure. These sealing blocks were to have been manufactured on the surface, resulting in significant disruption to traffic, as the connection to the existing structure is located directly below one of Frankfurt's major junctions (Düsseldorfer Straße – Mainzer Landstraße).