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.
Bärlinde begins her journey
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.”
Berlin's underground is teeming with challenges
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?
These cover the section between the former Bärlinde launch shaft at the Marx-Engels-Forum and the Alexanderplatz underground station, i.e. the connection to the new Rotes Rathaus underground station on both sides.
Towards the east, between Rotes Rathaus and Alexanderplatz underground stations, a tunnel has been in existence since 1930, which was used as a setting-up and sweeping facility for the U5 until 2018. From 2016 - 2019 this tunnel section was completely renovated, reinforced and converted for through traffic. At the moment the lines are being laid here as well.
Towards the west, at the Marx-Engels-Forum, a track changing facility was built. Here the trains can change track once more or for the first time before they enter the two separate tunnels to the main station or before they continue towards Alexanderplatz.
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
The tracks are laid along the entire route. The conductor rail is scheduled to be put into operation at the end of April. The "lightening" is taking place above the Alex tunnel.
The Alex-Tunnel has to lose weight
The first train between Rotes Rathaus Station and Brandenburg Gate Station
The "Lightening" above the Alex-Tunnel
Rail welding between Rotes Rathaus und Museumsinsel station