Reunifying Berlin Underground

Herrenknecht TBM Mines Subway Tunnel in German Capital

Twenty-five years after reunification, the Berlin subway network is also growing together. After completion, 22 km of subway lines will run uninterrupted into the center of the capital. The 2.2-km long extension of the subway line U5 from Alexanderplatz to the Brandenburg Gate is being carried out by the municipal transport company Berliner Verkehrsbetriebe (BVG) and its subsidiary Projektrealisierungs GmbH U5 on behalf of the German Federal Government and the State of Berlin. The Swiss construction company Implenia Construction GmbH is handling the construction work. Of the 2.2-km extension, 1.6 km was built by using the Herrenknecht Mixshield S-788, christened “Bärlinde.”

After the start on June 20, 2013, the 74-m long, 700-tonne TBM tunneled from the launch shaft on Marx-Engels Forum to the Brandenburg Gate. With overburdens of 5 to 17 m, the machine with a shield diameter of 6.67 m worked its way under the river Spree, the Spree Canal and Schlossplatz through the two future stations Museumsinsel (Museum Island) and Unter den Linden along the boulevard Unter den Linden. At the same time it built the tunnel shell out of high-quality concrete segments. Currently, the TBM is awaiting disassembly and transport of the back-up and the shield components back to Marx-Engels Forum.

The slurry TBM was designed for mix soil conditions and high water pressures. The groundwater depth was just 2-3 m below the surface, while surface conditions included sand and large granite boulders.

For safe mining under the Spree and the Spree Canal, the contractor originally planned elaborate ballasting of the river and canal beds with 40 centimeter thick steel plates. Working together in a spirit of innovation, however, the owner, contractor, a research institute and the machine manufacturer developed a smarter solution, which has led to a significant improvement in the ability to use Mixshields for tunnel sections with small overburdens. The revised plan used higher density bentonite suspension (HDSM = High Density Support Medium). This meant that in comparison to conventional suspensions, with considerably higher transferable support pressure, the penetration depth into the soil ahead of the machine was significantly reduced. Thanks to the HDSM used (up to 1.4 t / m3), the TBM was able to safely cross under the Spree with very low overburden beneath the riverbed.

The new section of the U5, will save travel time for passengers between Alexanderplatz, the Brandenburg Gate and the main railway station. BVG expects up to 150,000 passengers per day on the new line and 3,000 fewer cars driving on weekdays.

Herrenknecht looks back on a successful history in Berlin. In the late 1990s, the tunnel for the U55 was also driven with a Mixshield from Herrenknecht, the S-126. Two small AVN machines from Herrenknecht were used in the construction of the subway station Brandenburger Tor with the pipe arch method. There have also been numerous smaller machines, mainly to build wastewater pipes and sewers.