Building underground

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Germany has over 1,300 km of rail tunnels, including 650 km of S-Bahn and U-Bahn tunnels alone. Different construction methods and technologies are used to build tunnels underground.

When choosing a construction method, conditions of the site play a decisive role: what is the structure of the subsoil and rock? Are there utility connections below the surface? Are there other rail tunnels in place which need to be negotiated? The aim is always to choose the safest and most cost-effective option for building the tunnel.

Tunnelling methods

There are two basic approaches to tunnel construction: cut-and-cover tunnelling and tunnel mining. In the cut-and-cover construction method, the tunnel is built by digging down from surface level. Tunnel mining, by contrast, involves boring the tunnel through the earth from one or more starting points. This can be achieved using the sprayed concrete lining method or with tunnel boring machines.

Both cut-and-cover and tunnel mining methods are being used to build the second core route. The two S-Bahn tunnels for the second core route and the escape and rescue tunnel in between them are each seven kilometres long. This means that a total of 21 km of tunnels has to be made.

Proportion of each construction method per tunnel:

  • Tunnel boring machines: 6.4 km
  • Cut-and-cover: 600 m in the vicinity of the tunnel portals
  • Sprayed concrete lining method: total length of approximately 800 m near the three underground stations

Tunnelling with tunnel boring machines

Due to Munich's ground conditions, most of the tunnels for the second core route will be built using tunnel boring machines.

At the tip of a tunnel boring machine is a cutter head with a cutting wheel that slowly eats its way through the rock as pressure is applied from the rear. The loosened rock and earth passes through openings in the cutting wheel and enters an excavation chamber, where it is transported out of the tunnel using pumps or conveyor belts.

While the front part of the tunnel boring machine excavates the tunnel, the rear part installs concrete segments, which line the inside of the tunnel tube and create a watertight seal. The ring-shaped concrete segments, which form the final tunnel tube, have an inside diameter of 7.5 m and an outside diameter of 8.4 m. The tunnel rings must withstand the pressure of the surrounding earth and seal the tunnel to prevent groundwater from leaking in.

The tunnel boring machines are currently scheduled to make up to 10 m of progress per day, depending on ground conditions. During the tunnelling work, as many as ten workers and engineers operate the computerised machines from the inside. Experts from Deutsche Bahn and the construction supervision team monitor the situation continuously.

Because three separate tubes are to be made, a total of six tunnel boring machines will be used in the construction of the second core route. The machines will start from the western and eastern tunnel portals. Starting pits will be located to the west of Donnersberger Bridge for the western end of the tunnel and in the vicinity of Haidenauplatz for the eastern end. The tunnel boring machines will dig their way towards Munich city centre from each end and meet at the Marienhof underground station, around the halfway point. There, they will be dismantled and removed from the tunnel.

Der Tunnelverlauf im Längsschnitt

Sprayed concrete lining method

In this method, the rock is first loosened by machine and then hauled away. This is done in short sections around one metre in length. After or even during excavation, the tunnel wall is immediately stabilised with steel mesh and steel arches along with a sprayed concrete lining. These materials form the outer shell of the tunnel. Once the entire tunnel tube has been bored, a formwork carriage is used to line the inside of the tube with reinforced concrete. This is the part of the tunnel that will be visible after its completion.

During construction, water in the rock must either be diverted away or kept out using groundwater lowering. In special cases, the underground structures are pressurised. The increased air pressure ensures that groundwater cannot enter. The tubes for the platforms in the underground stations are being constructed from an open starting pit using the sprayed concrete lining method. This method is also being used to build the passageways between the rail tunnel tubes and the escape and rescue tunnel.

Cut-and-cover construction/open trench

The conventional cut-and-cover construction method involves digging a trench that remains open during the entire construction period. Once construction is complete, it is filled in again. The starting pits for the tunnel boring machines will be dug out using the cut-and-cover method.