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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Just taking the two extremes to keep it simple:
If you dig through a single solid rock, your tunnel’s structure is the rock. It’s basically a bridge made by a single gigantic stone that carries the weight of… the single gigantic stone. Being a single gigantic stone it has enough strenght to tolerate your “few meters wide” hole.
In liquid terrain, aka very wet sand, you need to surround the tunnel with a cilindrical structure which act like a submarine’s pressure hull, but it’s made of reinforced concrete (instead of steel or titanium) to save money.
Luckily, there are several cases in the middle, that are way better than the second scenario.
Most commonly, you find a scenario 1 with some areas that require to put very long bolts in the rock to make sure the different rock pieces act like a single gigantic rock. In some point, if you need to cross a “wet sand” area, you make the pressure hull with all the costs that it comes with. But basically you plan the tunnel by doing many surveys and small test tunnels, looking for a path that cross areas with the least troubled terrain. And steer the tunnel around and away the places that would require extreme “pressure hulls”.
The mountain holds up pretty much all of the mountain. The concrete just helps direct any pressure into the tunnel floor – so into the rest of the mountain.
The tunnel isn’t holding the whole weight of the mountain. Only certain portions.
It’s similar to 10 people can lift a car
The power or arches! Engineers have been using arches for thousands of years to support way more weight than you would think is possible. Materials like concrete and stone are really good at handling compression. You can squeeze rocks with way more weight than the rock itself before it cracks. These same materials are also pretty bad at handling bending or pulling forces, and the explanation for that goes way beyond ELI5, but it has to do with how the molecules bond together on a microscopic scale.
What arches do is they redirect the weight of all the dirt above from bending into compression. The concrete at the top of the tunnel pushes against the concrete sloping down the side, which then pushes into the soil below the tunnel, resulting in the tunnel being evenly compressed. Since concrete is good at handling compression, it’s able to hold a lot of weight while remaining hollow inside.
[Quick reference pic](https://i.ytimg.com/vi/CdNYTjXJPKE/maxresdefault.jpg)
The weight of the mountain isn’t only applied straight down, it’s applied to the sides diagonally as well. Like a bridge, the mountain spans over the tunnel just fine.
It’s sort of like jenga, you can remove some of the structure from below without the whole tower falling down.
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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