Debohra completes both tunnel tubes beneath Vienna highlighting low-disruption urban tunnelling

Tunnel boring has been completed on both tracks of Vienna’s U2 metro extension, marking a major milestone for one of the city’s most important public transport projects. The works form part of the wider U2xU5 expansion, which includes a new U5 metro line and the southward extension of the U2.

The tunnel boring machine, named Debohra, completed the second tunnel tube between Matzleinsdorfer Platz and the Augustinplatz emergency exit almost one year after the first breakthrough at the same location. The two completed tunnel tubes will connect four new U2 stations: Matzleinsdorfer Platz, Reinprechtsdorfer Strasse, Pilgramgasse and Neubaugasse.

Across the six tunnel sections, around 2,850 precast concrete rings and approximately 17,000 lining segments were installed. The completion of both bores is an important step toward expanding Vienna’s underground transport capacity and improving sustainable mobility across the city.

The project required a tunnel boring machine specifically designed for Vienna’s ground conditions. Debohra is around 127 m long, weighs approximately 1,300 tonnes and consists of about 27,000 individual parts. After being manufactured and assembled for testing in Germany, the machine was transported in sections to Vienna and reassembled at Matzleinsdorfer Platz.

Since tunnelling began in autumn 2024, the machine has worked through Vienna Tegel, a challenging fine-grained geological formation commonly encountered in the city’s underground works. Urban tunnelling in such conditions requires careful control of face stability, settlement, groundwater behaviour and segmental lining installation.

The use of precast lining rings allowed the permanent tunnel support to be installed progressively as the TBM advanced. This is essential in dense urban environments, where ground movement must be controlled to protect buildings, utilities, streets and existing transport infrastructure above.

One of the most important construction decisions was the handling of excavated material. Rather than moving spoil through multiple city routes, material from the TBM was removed through the central shaft at Matzleinsdorfer Platz. This approach avoided around 20,000 truck journeys through Vienna and reduced carbon emissions by approximately 75 tonnes.

After the first tube was completed, parts of the TBM were removed and returned to the starting location, while the trailing sections were pulled back through the already completed tunnel. The machine was then relaunched to bore the second tube. Following the final breakthrough, dismantling is now underway before Debohra is returned for overhaul and future use.

For Vienna, the completion of both U2 tunnel tubes is a major underground milestone. For urban infrastructure more widely, it demonstrates how large metro expansions can be delivered beneath dense cities while reducing surface impacts and supporting long-term climate-focused transport planning.