Workers cut up the old steel supporting structure
© Max Döringer

Reconstruction of the Alserbach bridge: We are renewing the supporting structure under the U4 underground line

Wiener Linien is renewing the bridge structure on which the U4 line crosses the underground Alserbach canal at FriedensbrĂĽcke. The bridge is over 125 years old and needs to be renovated. Due to its location on the canal, the construction work is particularly challenging.

Continuous service to Heiligenstadt despite construction work

Despite the extensive construction work, the U4 can continue to run continuously between HĂĽtteldorf and Heiligenstadt as we are splitting the main work into two phases. In 2024, we renewed the supporting structure under track 1, now it's the turn of track 2.

During the construction work from 13.01. to 26.02.2025 inclusive:

  • Track 2 is closed. At FriedensbrĂĽcke station, the U4 will depart from track 1 in both directions (alternating track operation). 
  • Every second train travels beyond Schwedenplatz to Heiligenstadt. This leads to a slightly longer interval in this section.

Download information sheet (PDF)

Challenging construction site above the canal

The steel construction of the Alserbach bridge above the Alserbach canal
The position of the supporting structure above the canal poses particular challenges.
© Wiener linien

The underground Alserbach river runs close to the FriedensbrĂĽcke bridge in the ninth district, where it flows into the main collector sewer. The U4 crosses the mixed water overflow of the Alserbach canal on an underground bridge (see picture), which was built in the time of Otto Wagner. This historic structure is over 125 years old and needs to be completely renovated.

Working in the canal poses a number of challenges. On the one hand, the weather: work cannot be carried out here when it rains, as the canal drains excess water from four districts (9th, 17th, 18th, 19th) into the Donaukanal. On the other hand, there is the wastewater: not only rainwater flows into the canal, but also wastewater including faeces. This not only leads to unpleasant odours, but also makes construction work more difficult. Wiener Linien has therefore built a separation level between the sewer and track levels.

This is how we are renewing the underground support structure of the U4:

🚧 Removing the old tunnel roof: As a first step, Wiener Linien used a crane to remove the tunnel roof on Roßauer Lände - the former light railway covering, which is over 125 years old. We will subsequently use the resulting hole as a transport opening.

🚧 Set up working platforms: In the U4 tunnel, we set up two working floors: one at the level of the canal and one at the level of the tracks. This step is very time-consuming. Then it's on to the main work.

🚧 Cutting up the old bridge, removing parts: We cut the existing steel bridge into small pieces. We use a crane to lift the individual parts out of the track storey through the transport opening. 

🚧 Building a new supporting structure: We then build the new bridge structure from reinforced concrete. Once the concrete has hardened, the U4 can cross the Alserbach Canal again. It will then be fit for the coming decades - another milestone in the comprehensive modernisation of the U4 (NEU4).

We cut the old steel supporting structure into individual parts.
© Max Döringer
We cut the old steel supporting structure into individual parts.
The individual parts are removed by crane through the hole in the tunnel ceiling.
© Wiener Linien
The individual parts are removed by crane through the hole in the tunnel ceiling.

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