I know there can be cave-ins where a slice of rock is sheared off of the inside of the tunnel, but as far as I know tunnels don’t just outright crumble and collapse. Why not?
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Tunnels are designed to distribute the weight of the mountain above them evenly, which minimizes stress concentrations. The shape of the tunnel, its dimensions, and the materials used are carefully chosen to ensure structural integrity.
The shape of the tunnel basically deflects all the crushing power to one side or the other like sticking an umbrella under a waterfall.
This is actually really difficult. The wall of the tunnel basically forms an archway that supports the weight of the rock and soil above it, and holds back the pressure pushing in from the sides. If the tunnel is dug too deep or the wall material isn’t strong enough, the walls will shatter under the strain and the tunnel may collapse.
There are lots of techniques for improving the strength of the tunnel walls. Sometimes huge steel bolts are drilled into the walls to attach the tunnel wall rock to rock deeper inside the mountain. Sometimes grout is injected into the rock, or concrete is sprayed onto the walls to glue everything together. For really weak material like sand or soil, sometimes a whole reinforced concrete tube is built to support the load.
Tunnels do occasionally [collapse](https://en.wikipedia.org/wiki/Category:Tunnel_disasters), but not as often as mines do. Mines face many of the same engineering challenges as tunnels, but with a mine the goal is to remove as much rock as possible, whereas with a tunnel you save money by removing as little as you can.
Does the tunnel wall really need to support the weight of the entire hill/mountain above it? What if it’s rock? Isn’t there some amount of strength in the already very compacted dirt/rock itself? As a kid I dug tunnels in loose dirt all the time with no tunnel walls. It’s kind of impressive how big you could make them before the weight of the dirt would collapse the tunnel.
The weight of the stone above the tunnel is distributed to the sides, similarly to how it works with bridges. However the entire weight is not carried by the concrete walls of the tunnel, but also by the rock around them. If the tunnel goes through sand instead of solid rock, the tunnel walls would need to do most of the heavy lifting.
The round shape of the tunnel can help distribute the load evenly, but is not necessary in solid rock with enough stability. Natural caves come in all shapes and sizes if the type of rock allows it.
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