Shallow Berlin link drives Mixshield innovation 04 Nov 2015

Breakthrough into the reception shaft at Brandenburg Gate Station in Berlin completes mechanised tunnelling on the 2.2km connection between the German capital’s U5 and U55 lines.

Launch shaft near Berliner Rathaus

Contractor Implenia of Switzerland procured a 6.67m diameter Mixshield from Herrenknecht to complete twin running drives of 1,620m each from a launch shaft site in Max Engels Forum near the existing U5 Berliner Rathaus Station, through to a reception shaft at Brandenburg Gate Station on the U55. Excavation of the final connection at Brandenburg Gate is scheduled for completion next year (2016).

Launched on 20 June 2013, TBM Bärlinde faced a number of challenges, including shallow overburdens of as little as 5m as she crossed under the River Spree and the Spree Canal.

Designed especially for heterogeneous soils and high water pressures, the slurry-supported Mixshield was presented from the outset with a complex range of geological challenges: groundwater at a depth of only 2–3m below the surface, sandy – and in the Spree area sludge-laced – soils, as well as large granite boulders.

For safe underpassing of the Spree River and Spree Canal, Implenia originally planned an elaborate operation to ballast the river and canal beds with 40cm thick steel plates. However, working collaboratively, the client (Berlin Transport Authority), construction company, a research institute and the machine manufacturer together developed a smarter solution, and one which has led to a significant improvement in the ability of Mixshield machines to complete tunnel sections with small overburdens.

Using a higher density bentonite suspension for the particularly shallow sections of the alignment, including the water crossings, 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 high density support medium (mixed at up to 1.4tonne/m³) the Mixshield was able to safely cross under the Spree. Large sandbags lowered into the water near the banks provided extra weight above the drive path.

Attention now turns to the construction of the two new subway stations along the 2.2km U5–U55 link, which are scheduled for completion – also by Implenia as part of its €525 million construction contract – in 2020. These are located at Museum Island (MUI) and Unter den Linden (UDI).

6.67m diameter cutterhead of the Herrenknecht Mixshield

At both new station locations the 350mm thick reinforced concrete segmental lining of the running tunnels is being removed. The platform hall at MUI Station near the Spree Canal is being built with mining-grade shotcrete protected with ground freezing. The running tunnels are being connected to the existing Berlin Rathaus Station with the help of ground freezing, a plain concrete diaphragm wall, as well as jet grouting and sealing injections.

Once the new line has connected the existing 1.5km long section of the U55 and the 18.3km long U5, the newly named U5 will have a total length of 22km.

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. Two small microtunnelling machines from Herrenknecht were also used during the construction of Brandenburg Gate subway station, using the pipe arch method.