Why don't underground tunnels collapse on themselves?

Question

I understand that underground tunnels don't collapse because the forces acting on it must be in equilibrium but I don't know which forces are present and I don't know how they are built.

score 2 · Accepted Answer · answered Oct 08 '17 at 19:30

Look at the shape of the tunnel, they are circular to spread the load, the same idea as deep sea diving vessels have spherical living areas. There are no sharp corners.

The surrounding rock is so strong (and there is so much of it), the pressure on the tunnel roof is easily spread to the sides instead.

Look at it slightly differently, how does a soap bubble keep it's shape, against the atmospheric pressure. It spreads the load to avoid pressure build up in one place.

Sorry for the short answer, this is more an Engineering SE question, imo.

Thanks, the bubble analogy helped me picture how it works – Marcio Oct 08 '17 at 21:53 — Marcio, Oct 08 '17 at 21:53

Why don't underground tunnels collapse on themselves?

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