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”.
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