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