Fourteen kilometres of station and platform tunnels, passenger walkways and other spaces are being created through another mining technique known as sprayed concrete lining.
Sprayed concrete lining is used where the underground spaces required are bigger, smaller or less uniform than the tunnels created by the tunnel boring machines.
These tunnels are excavated in short sections, using a variety of machines operated by highly skilled engineers.
The sprayed concrete lining method has been used extensively on Crossrail to form each of the station platform tunnels, cross-passages between the platform tunnels, ventilation tunnels, concourse tunnels, cross-over caverns and ventilation, escape and intervention shafts across the central section of the route.
The technique involves spraying a quick setting form of concrete strengthened with steel fibres onto freshly excavated ground to seal the new tunnels. Further layers are added to make the surface watertight creating a permanent structure some 65cm thick.
This process enables huge underground spaces and curved walls to be created. Crossrail’s passengers will see the result of all this work in the stations when the railway is open in 2018.
Five of Crossrail’s central London stations are being created using a well established technique known as ‘Sprayed Concrete Lining’ to create new station tunnels – Bond Street, Tottenham Court Road, Farringdon, Liverpool Street and Whitechapel.
The stations are typically 250 to 300m in length and have been constructed at depths to "tunnel crown" up to approximately 30m below street level. The stations are located in the central London area in areas of dense urban development.