The extension of the U5 urban railway line in Frankfurt a.M. will connect the Europaviertel to the city's underground network. The ARGE U5 Europaviertel, consisting of the PORR teams of civil engineering, tunnelling and the subsidiary Stump-Franki Spezialtiefbau, is working shoulder to shoulder to construct the 1,160 metre long section. An important project step has now been achieved: at the end of March 2022, the mining excavation was safely completed - the existing wall has been exposed in both tunnel tubes.
A new urban quarter is being built on the former site of the Frankfurt freight station: the Europaviertel. The new quarter will be connected by the extension of the U5 light rail line. The extension of the line beyond the main station will provide the Europaviertel with an efficient connection to the city's public transport network. The existing line will be extended by four stops. Around 850 metres of the line, the Güterplatz station structure and the connection to the existing structure are underground. In 2016, Stadtbahn Europaviertel Projektbaugesellschaft mbH commissioned ARGE U5 Europaviertel, consisting of the PORR teams of civil engineering, tunnel construction and the subsidiary Stump-Franki Spezialtiefbau, with the construction of the 1,160-metre-long section of the U5 light rail line.
Tunnelling method under the skyline
The difficult conditions of inner-city tunnelling posed great challenges for all those involved. For the first time, a tunnel was constructed in Frankfurt a. M. using mechanised tunnelling. In September 2019, the 580-tonne tunnel boring machine (TBM) had begun to work its way through the soil in the south tube. At the end of May 2021, the TBM reached its final position in the north tube. As the TBM moved forward, the tunnel construction team had simultaneously installed a total of 8,358 tubbings for the supporting concrete tubes. The last metres to the existing wall at the underground tunnel under the Platz der Republik were mastered by excavation.
Tunnel work safely completed under overpressure
The groundwater is about 3 to 5 metres below ground level, so that the tunnel is completely in groundwater in the area of the connection. So that the work could be carried out, the mining excavation was carried out using the overpressure method and under the protection of the ice body. Ground freezing, carried out by Stump-Franki Spezialtiefbau, is a modern construction method that significantly reduces the interference with the building ground and the surrounding area. In the sensitive, densely built-up inner city location, it was important not to endanger the existing buildings and above-ground traffic during the tunnel construction work. In the course of the overpressure procedure, all personnel and work equipment had to be brought in and out in a complex manner, which made the work all the more complex. The work was carried out in 3 shifts of 6 hours each. In addition, 1.5 hours were needed to lock and unlock in the pressure chamber in order to gradually acclimatise and deacclimatise the body to the pressure. At the end of March 2022, the mining excavation under the earth of the Frankfurt skyline was safely completed. The existing wall behind which the U5 currently turns is exposed in both the north and south tubes. After completion of the excavation, the next step will be the construction of the so-called inner tunnel shell. This involves installing a second lining in the outer tunnel lining that has already been constructed. For this, formwork made of wood and reinforcing steel is installed in sections and then concreted in stages.
Hand in hand: special civil engineering work while the tunnel is being driven
Parallel to the tunnelling work, the special civil engineering work progressed above ground on the construction site in Europa-Allee. A total of 18 wells were drilled by Stump-Franki Spezialtiefbau in late summer 2021 for dewatering to relieve and monitor groundwater levels using a rotary drilling rig. In order to relieve the groundwater within the excavation pit in all areas, an additional 92 relief lances were constructed.
One of the deepest excavation pits in the city
The new underground station "Güterplatz" is being built using the cut-and-cover method. With a depth of about 25 metres, the excavation pit will be one of the deepest inner-city excavation pits in Frankfurt. Civil engineering is in charge of this part of the project. Civil engineering has already reached important milestones in the course of the project. This includes the completion of the shell for the emergency exit at the Platz der Republik in autumn 2021, a challenging task because the existing structure from the 1970s had to be connected to the new structure in such a way that it is safely sealed against the groundwater.
