On 10 October 2023, the employees of the Institute of Geotechnics were invited by ARGE U2 17-21 to visit the U2 station Neubaugasse construction site. This construction site is part of the underground extension of the U2 line and is centrally located on Mariahilfer Straße. The existing Neubaugasse U3 station will be expanded into an interchange station for the two lines.

As a busy shopping street, Mariahilfer Strasse provides significant challenges for the construction site. The construction site area therefore had to be kept as small as possible and was mostly relocated underground. In the already partially completed shell of the new station, compressors, mixing plants and even pressure silos are now located several metres below ground level. Only the areas required for bringing material in and out are still visible on the surface.

After a short safety briefing, DI Wolfgang Hornich, DI Florian Özkoral and Ing. Robert Pfeffer took the group to the construction site. The future station is located between a dissolved bored pile wall. The individual floors are connected to each other via shafts several metres high. In the third underground floor, compensation grouting work is currently taking place. In order to minimise the settlements caused by the planned tunneling, cased boreholes up to 90 m long are being drilled under the surrounding buildings. Pipes are inserted into these boreholes, which later enable controlled injections at 50 cm intervals. In this way, the buildings will later be lifted by up to 3 mm in order to compensate the expected settlements caused by the tunnelling. As it is not possible to drill into the bored piles for structural reasons, the injection points sometimes have to be arranged up to nine times above each other in the gaps.

The drilling work itself will be completed in a few weeks, while the compensation injections will continue for several months. In order to inject as gently as possible, only a very small amount of the custom-mixed injection material can be injected into each pipe. In addition, all deformations are monitored using a comprehensive monitoring system and will be displayed directly to the person responsible on site.

We would like to thank DI Wolfgang Hornich, DI Florian Özkoral and Ing. Robert Pfeffer, for taking their time to give us an extraordinary tour of the construction site and for answering all our questions in detail afterwards in the site office.