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Here goes a recently opened underground railway station in Moscow, Russian Federation (image is from here) Cast Iron Tubing

On the left is an undecorated (just painted and it will likely remain this way) tunnel wall. The tunnel wall is lined with cast iron tubings - specially crafted pieces of cast iron connected with thick steel bolts and having the seams sealed with cement. This lining forms a huge pipe which holds the ground from collapsing and the shot is made from inside the pipe.

The cast iron is something like 30 millimeters thick.

Is there actual ground right behind that (not very thick) cast iron? What if the ground underneath is not hard enough and the loaded pipe has to bend downwards?

Does the cast iron hold all the possible mechanical loads alone or is there anything else in the structure?

sharptooth
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  • The design is chosen according to the geology - if that tunnel goes through a river, then it will probably have a different construction... – Solar Mike Jan 16 '19 at 13:07

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They have done their homework, one hopes. Many competent dense soils are basically self supporting, like all the caves. Yes there is compact or dens soil behind the cast iron panels to take advantage of soils structural strength. The cast iron is sufficient to take all the loads, even if it rarely is called in to take the full load.

They have tested the soil and it chemical and structural content and have chosen the cast iron alloy suitable for that condition.

There are methods to protect cast iron from corrosion. Here is what they have done in London Road Shunt Tunnel. link

kamran
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  • I think another way to say it is : The iron or steel s lining in tunnels is basically to prevent loose rocks, etc. from falling onto the rails or roadway. – blacksmith37 Feb 16 '19 at 01:46