Beneath the streets of Munich
Munich's city centre is criss-crossed by a network of underground pipes and tunnels. These not only supply district heating and cooling along with power and sewer service, but also carry the existing core S-Bahn route and Munich's U-Bahn.
For the construction of the second core route, this means that the new tunnel must be built at a safe distance from the existing tunnels. The new stations will therefore lie around 35 to 40 m below ground.
Numerous transport tunnels have been built under Munich's city centre in recent decades. In addition to road tunnels and the existing core S-Bahn route, these include the various U-Bahn tunnels. The U-Bahn lines all pass beneath the existing S-Bahn core route tunnel: U1 and U2 at Munich Main Station, U4 and U5 at Karlsplatz (Stachus), U3 and U6 at Marienplatz and U5 at Munich East. The new core route tunnel therefore needs to run even deeper to maintain a safe distance.
At Munich Main Station, the existing core S-Bahn route is located beneath Arnulfstrasse at a depth of 14 m. The U4 and U5 pass under Bayerstrasse at a depth of 22 m, and the U1 and U2 are located 28 m beneath the station forecourt. The station for the second core route is therefore planned at a depth of 40 m. The new subterranean station at Marienhof will also be around 40 m below ground. In the enhanced plans for the second core route, Munich East station will now only be 16 m beneath the surface.
Lifts and escalators will provide quick and convenient surface access at all three stations. Underground walkways will give passengers access to the U-Bahn platforms and the existing core S-Bahn route.
Another benefit: thanks to the depth of the new core route tunnel, noise and vibration during construction and operation will be barely perceptible on the surface.
Station construction: the diaphragm wall construction method
The diaphragm wall/cut-and-cover construction method is being used in the construction of all three underground stations: Munich Main Station, Marienhof and Munich East. The name gives away the main elements of this method: diaphragm walls (i.e. walls concreted into the ground) and covers (ceilings cast from concrete).
For each station, a deep excavation wall made of reinforced concrete is first erected around the edges of the excavation pit. The wall is constructed by digging trenches into the ground and filling them with concrete. This diaphragm wall forms what will eventually become the outer shell of the station. From a construction point of view, the wall is needed to secure the excavation pit by retaining the surrounding earth and preventing the inflow of groundwater. The excavation pit for the station is dug in the area enclosed by the diaphragm wall. A concrete deck is then placed over it. This is the cover part of the cut-and-cover method. Further excavation work takes place underneath this concrete deck. More layers of soil are gradually dug out from under the deck and more concrete slabs installed at specific intervals. These form the mezzanine levels and also reinforce the excavation pit. The big advantage of working underneath the concrete deck is the significant reduction in noise and dust emissions for local residents. A small opening is left in the deck to supply and remove materials. Dewatering or groundwater lowering measures prevent the ingress of groundwater during the construction phase and reduce pressure on the structure.