As part of the rock and tunnel construction lecture in the winter semester 2022/23, the students and staff of the Institute of Geotechnics were invited by the company Keller Grundbau Ges.mbH to visit the underground construction site in the area of the town hall. During the excursion, the excavation injection and the excavation of the U2 could be viewed.

After the reception by DI. Andreas Kalcsics and DI. Andreas Kratochvill, all participants were first given a safety briefing and then the extensive construction measures of the U2xU5 line intersection were presented. Keller Grundbau, as a subcontractor of the U2xU5 Rathaus/Franhkplatz consortium, is entrusted with the construction of several DSV structures as well as with the settlement compensation of surrounding existing buildings. The necessary personal safety equipment was handed out and the group set off to the construction site.

The inspected existing building is located above one of the U2 station tubes to be constructed. Due to the small distance of the access structure and the remaining measures, settlements of the building are expected. However, in order to keep the settlements as low as possible, sleeve pipes for settlement compensation were drilled underneath, and the preliminary injections for this have already been completed. This construction measure is intended to keep the maximum permissible angular twist on the existing buildings, as well as to keep total settlements to a minimum. The group had the opportunity to enter the containers in which all the equipment for mixing and pumping the cement suspension is operated, and received an explanation of the exact work sequence for heave injections.

Afterwards, the Schottentor drop-off structure was visited. At this point, the future U2 leaves the existing U2 underground tunnel in a westerly direction and descends further into the subsoil. For this purpose, the existing diaphragm walls had to be removed in a partial area and replaced by massive bored piles with a lower edge of up to 62m below ground level. The floor beams also had to be reinforced in this area. After the diversion, the excursion participants were able to view the mining advance in the direction of the town hall. The tunnelling in this area is carried out under the protection of a DSV shield.

A big thank you goes to DI Kalcsics and DI Kratochvill, who answered the questions of the excursion participants in detail and gave us a deeper insight into this highly complex construction project.