The 1.6 kilometer tunnel section of the 2.2 km stretch between Alexanderplatz and the Brandenburger Tor was excavated by way of the shield tunneling method.
A tunnel boring machine was used for this part of the U5 Gap Closure Project. Tunnel boring machines are, basically, rolling factories that drill through the subsoil and at the same time complete the tunnel carcass. Two separate main tunnels were bored for the future underground line; first the north and then the south tunnel.
A TBM manufactured by Herrenknecht in Baden was delivered by barge on the river Spree in early 2013 and then lowered into the 20-meter-deep launching shaft at Marx-Engels Forum. 'Bärlinde', as the machine was christened, crossed under the river Spree, Humboldt Forum and Spree Canal before drilling below the boulevard Unter den Linden and then on to underground station Brandenburger Tor. Not a vibration was noticed at the surface.
At the end of its first run, Bärlinde could not change directions and so was disassembled and transported piece by piece back through the finished tunnel tube to its starting point. Back at the launching shaft, maintenance was performed; the cutting wheels were replaced as well the shield skin. From April to September 2015, Bärlinde bored the south tunnel and then was again disassembled and transported back after a job well done. At Marx-Engels Forum, the parts were lifted by crane back onto the barge and transported away.
There are many elements the tunnel construction planner has to deal with, not the least of which, the element of surprise. Although test drilling clearly outlines the geological conditions, a suspenseful air still permeates the doings of crew and machine as they tunnel into the Earth. Miners of old had a saying to express this apprehension of the many unknowns: “What lies beyond the pickaxe is darkness.”
In the sandy substrata underneath Berlin lurk glacial boulders and innumerable building blocks of centuries past; the remains of a well have been left buried since the time of the Palace of the Republic's construction. Tunneling sub-Spree digs up challenges as well in having to plan around the old Imperial Era Linden tunnel and the operating U6 line.
Prudence in planning and state-of-the-art tunnel boring techniques lent heavily to Bärlinde's mastery of these hurdles. But of course a veritable host of workers were on hand to back the machine's endeavors as well. First and foremost, the shield driver with the operations team from Implenia, then shift engineers, the machine master, the tunnel construction foreman as well as site supervision from specialized planners to test engineers – only by closely working together can a project of this magnitude be brought to a successful end.
Only 1.6 kilometers of the 2.2 kilometer stretch were excavated using shield tunneling; but what about the remaining 600 meters?
This section runs between the launching shaft at Marx-Engels Forum and Alexanderplatz and will now connect to the new underground station Rotes Rathaus on both sides respectively. Also, the process of joining the tunnel to the structural works of the track changing area has been underway at Marx-Engels Forum, since early 2016.
Between Berliner Rathaus and Alexanderplatz, a tunnel has existed since the 1930s that has been used as a storage track and reverse loop for the U5. This section must now be renovated and reinforced to handle the heavy train traffic that will pass through, so tunnel construction isn't quite finished.
Manufacturer: Herrenknecht AG
Machine type: Mixshield
Length: 74 metres
Weight: 700 tonnes
Excavation diameter: 6.7 metres
Excavation by hydraulic jacking
Length: 2 x 1,620 metres
Diameter (internal): 5.7 metres
Overburden: 5 to 17 metres
Track construction has started in all parts of the tunnel. Track construction is taking place between U-Bahnhof Museumsinsel and Unter den Linden or rather U-Bahnhof Museumsinsel and the track switch area.
Between Alexanderplatz and U-Bahnhof Rotes Rathaus: Concreting of the rail table had been finished.
Getting the cutting wheels out
Pictures of the tunnel and the TBM
Closing the Gap between BRH and ALX