With its diameter of 12.75 meters, on February 2, 2018 a Multi-mode TBM chewed through the last few centimeters of concrete in the target shaft. 2.6 kilometers of tunnelling through challenging subsoil for the Eppenberg railway tunnel between Olten (Canton of Solothurn) and Aarau (Canton of Aargau) now lay behind man and machine. After just over a year of work – and thus three months earlier than planned – they were greeted at the destination by representatives of Swiss transport and local politics as well as the client, SBB.
The approximately 115 meter long and around 2,400 tonne tunnel boring machine had been designed and built by the Herrenknecht engineers in close cooperation with the contractor's specialists. The requirement was safe and rapid excavation through changing ground conditions with both hard and unconsolidated rock. A machine concept was implemented allowing the first 1,800 meters to be driven in open hard rock mode before conversion inside the tunnel to cover the remaining distance of around 760 meters in closed Mixshield mode. Thanks to the multi-mode concept, in the planning phase mechanized tunnelling was found to be the most economical approach and also reduced the impact on both the environment and local residents compared to the conventional methods which were also considered.
The Eppenberg Tunnel is the centerpiece of the four-track expansion of the rail link between Olten and Aarau on one of Switzerland's busiest railway axes. The expansion after the planned commissioning in 2020 will widen one of the largest, two-track only bottlenecks in the country's central plateau area between Bern and Zurich.