An additional site has emerged out of the construction works between the underground stations Rotes Rathaus and Museuminsel: the track switch area. Up to March of 2016, this open trench excavation was the launching shaft for the tunnel boring machine Bärlinde; both of the tunnel tubes were started from here. The main material storage area, nearby at Marx-Engels Forum, has served as the logistics center for the project as well, housing the offices of Implenia and the contractor Los 1 (tunnel, track switch area, Museuminsel, Unter den Linden).
As the name says, the track switch area is where the trains can switch tracks. It is important for operations because technical disruptions can be isolated, avoiding a complete shut down of the line. The two separate tunnel tubes between here and Brandenburger Tor make a switch before or beyond this point impossible.
The track switch area also had another role as launching shaft for the tunnel boring machine. One of its first challenges in tunneling here was in achieving a great difference of depth within a short distance. The platform at the bi-level underground station Rotes Rathaus is only a few meters from the surface - a new storage track and reverse loop are on the lower level – and nearby is the deep river Spree, under which the tracks will run at a 4% grade.
Before the tunnelling work was completed, the then still open excavation pit served primarily as a starting and logistics shaft. Then the actual construction work on the track change facility began: the floor, the load-bearing walls, the intermediate ceiling and the structural ceiling were reinforced and concreted. The track construction work also took some time due to the installation of the various points for the track change. As the tunnel leading to the west is located under the Spree, a so-called defence gate is being installed at the track changing facility. It would protect the subway network in case of water ingress. The extension work on the operations room level is nearing completion and the equipment of the rainwater retention basins - a protective device for the Rotes Rathaus underground station in the event of heavy rainfall - is being completed.
A harbour basin was specially built at Marx-Engels Forum in the Spree to transport materials and the tunnel boring machine to and from the site.
In total, some 120,000 cubic meters of excavated sand and stone and other rubble accumulated during excavation – enough to fill a million bathtubs! These diggings were treated using bentonite suspension and transported by conveyor back to the Marx-Engels Forum, where solids and liquids were separated at a small plant. The sand was loaded up and barged away on the Spree and the bentonite reprocessed and reused. Water transport was the environmentally friendly option, avoiding an estimated 12,000 lorry hauls by road.
To create space for construction at Marx-Engels Forum, sculptures of Karl Marx and Friedrich Engels were also transported away in September 2010, but to a temporary home only, and not so far. The bronze gentlemen now stand a few meters northwest of the forum center, towards the Liebknecht Bridge, but contrary to their former position. The statues now face to the west, not eastward.
For now, the plans are to return Marx-Engels Forum back to its original state upon completion. So Marx and Engels will once again be at center stage and presumably rotated back. However, the Berlin senate and Mitte burrough council are still pondering subtleties in the forum's overall future look.
Construction: Reinforced concrete framework sloping east to west, with a partly overlying operations area
Length: 130 meters
Width: 13 meters
Depth below ground level: 13 to 22 meters
Height on the inside: 4.68 meters
Company in charge: Implenia
The defence gates are built in. The track construction is completed.
Defensive gates against the floods
Construction progress at track changing system January 2017 till April 2017