On March 19, 2025, the staff of the Institute of Geotechnics had the special opportunity to view a tunnel boring machine (TBM) up close. The excursion began at the Pilgramgasse subway station, in the construction area Hofmühlgasse, where we were welcomed by Dipl.-Ing. Robert Wäger.

Already during the descent into the shaft, the various geotechnical and structural measures were vividly explained. One particular challenge was the flooding in September 2024, which inundated Pilgramgasse station and required extensive cleanup and restoration work. However, this delay had the advantage that the tunnel boring machine was still undergoing maintenance at the station at the time of our visit, making it possible to view it in detail.

The station and emergency exit shafts were constructed first, after which the TBM advances between them. Between each stage, the machine is inspected and serviced. Since station shafts, due to planned infrastructure, have a larger diameter than tunnel sections used solely for rail traffic, a comprehensive inspection of the TBM was possible. In addition to the shield with the cutterhead, the thrust cylinders, sealing system, and erector could also be observed. A particularly fascinating part was the control room, where it was demonstrated how many parameters must be managed to operate the machine. The TBM’s launch phase is especially critical, as the risk of damage is highest at this point.

Another highlight of the excursion was the visit to a tunnel section with an exceptionally tight curve radius of only 300 meters. Such small radii are rare in tunnel construction and were made possible in this project by the TBM’s articulated shield. The excursion concluded with an exit through the construction section at Bacherplatz – a particularly demanding area of the subway expansion. Here, unexpected ground conditions posed a major challenge, as they significantly deviated from the geotechnical survey forecasts.

Most of the expansion work on the U2 subway line is being carried out using the tunnel boring machine “Debohra,” an earth pressure balance shield machine. In this method, the excavated material is used to support the tunnel face. In the area of the Vienna Tegel formation, the soil must be conditioned with surfactants to create a stable earth paste. Due to the favorable soil, wear on the TBM is relatively low. All excavated material is removed via the shaft at Matzleinsdorfer Platz in order to minimize traffic disruption in the city. Currently, this transport is carried out using a diesel locomotive, which requires intensive ventilation. The construction management hopes to use an electric locomotive for the second tunnel tube to make operations more environmentally friendly and efficient.

Sincere thanks go to Dipl.-Ing. Robert Wäger, who made this exciting excursion possible and accompanied us with his expertise.