Mixshield Bärlinde closes east-west gap in Berlin Mitte

The undercutting of the river Spree and the Spree Canal with overburdens of sometimes only five meters was a challenge. With a high density suspension, as little pressure as possible and as much supporting effect as necessary, Bärlinde bored her way safely beneath the river bed. Thanks to modifications to the machine, elaborate ballasting of the river and canal beds with steel plates was not required. Shipping remained unimpaired, a success of modern tunnelling technology from Herrenknecht.

With the closing of the gap by the Berlin metro line U5, 25 years after reunification the Berlin subway network is also growing together. After completion, 22 kilometers of subway lines will run uninterrupted into the center of the capital. The 2.2 kilometer 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. 1.6 kilometers of the 2.2 kilometer long new line were built by mechanized tunnelling using the Herrenknecht Mixshield S-788, christened "Bärlinde".

After the start on June 20, 2013 the 74 meter long, 700 tonne Bärlinde tunnelled her way from the launch shaft on Marx-Engels Forum to the Brandenburg Gate. With overburdens of 5 to 17 meters, the machine with a shield diameter of 6.67 meters 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 Bärlinde is awaiting disassembly and transport of the back-up and the shield components back to Marx-Engels Forum. The connection to the subway station Brandenburger Tor planned for 2016 is being created manually.

Designed especially for heterogeneous soils and high water pressures, on the shield drive through the center of Berlin the slurry-supported TBM Bärlinde mastered complex geological challenges: groundwater at a depth of only two to three meters below the surface, sandy and in the Spree area sludge laced soils as well as large granite boulders.

For safe undercutting of the Spree and the Spree Canal the construction company originally planned elaborate ballasting of the river and canal beds with 40 centimeter thick steel plates. Working together in a spirit of innovation, however, client, construction company, 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: in some stages the machine was driven using a 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), Bärlinde was able to safely cross under the Spree with very low overburden beneath the riverbed. Big Bags (large sandbags) lowered into the water near the banks provided sufficient weight. Shipping remained unimpaired. The modification of the machine for the HDSM drive was already taken into consideration during design, factory assembly and jobsite assembly. During the advance no further alterations were necessary.

With the new section of the U5, in future passengers between Alexanderplatz, the Brandenburg Gate and the main railway station will save travel time. Exhaust fumes and noise will be reduced – according to BVG, after the scheduled completion in 2020 on Unter den Linden alone there will be at least 3,000 fewer cars driving on weekdays. BVG expects up to 150,000 passengers per day on the new line.

The entire 2.2 kilometer long new line runs from the future subway station Berliner Rathaus past the Rotes Rathaus (Red City Hall), then crosses under the Marx-Engels Forum and the Humboldt Forum as well as the river Spree and the Spree Canal. It then passes under the boulevard Unter den Linden to the metro station Brandenburger Tor at the Brandenburg Gate. Once the new line has been connected with the existing 1.5 kilometer long section of the U55 and the 18.3 kilometer long current U5, from Hönow to the main railway station the new U5 will have a total length of 22 kilometers.

Herrenknecht looks back on a successful history in Berlin. In the late 90s 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 waste water pipes and sewers.

 

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