I get that they’re layered with reinforced concrete but how is it that strong considering the walls aren’t even that thick?
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Well they can sometimes collapse
But a lot of it isn’t just the reinforcement of the walls itself, a lot of it is also geometry.
If you consider a mineshaft, those are usually somewhat square. If you take a cardboard box and press down on it.. it will squish quite easily.
If you look at older mineshafts you see they get reinforced typically with wooden beams which makes it more of a triangular shape.. press down on a triangle and it won’t crumble nearly as easily.
You can test this out yourself, this is an experiment a lot of schools love to do.. eggs and soda cans are pretty fragile from the side but on the top of the shape of an egg.. even an empty eggshell you can put a suprising amount of weight onto, you can press down quite hard even.. and a few eggs in the right orientation next to eachother can even hold up a child and it balances out the load quite well.
This principle in general is insane theres similar experiments where seemingly fragile objects can hold the weight of a car just because the shape is reinforcing itself from crumbling.
A modern tunnel meanwhile is usually rounded, fully encased in reinforced concrete… so you basically have an already somewhat geometrically strong shape, a half circle.. reinforcing itself with thicker walls and in addition the length of the tunnel.. they are also often built in a way that if one segment fails theres still a second layer / the rest of the tunnel is kinda reinforcing and balancing the pressure out… pretty much the same as a child being able to stand on a few eggshells without them cracking even though they are just flimsy egg shells.. the weight gets distributed well enough that nothing breaks.
This rounded shape is also why you see windows in research submarjnes be rounded, and also why aquariums with tunnels in them will almost always be this rounded shape, it just holds up to pressure well.
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