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 8358 segments 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 Under Overpressure
The groundwater is about 3–5 m 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 an ice body. 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. At the end of March 2022, the mining excavation under 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 inner tunnel shell.
Special Civil Engineering Work During Ongoing Tunnel Excavation
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.