travelling through the continent this summer we went through some amazingly long tunnels under some seriously high mountains. It got me thinking - how does a tunnel strength scale? Could one put a tunnel under Everest (ignoring continental drift etc)? How does a tunnel support the miles of rock above it in the first place?
The L.A. metro runs, for the most part, about 60 feet underground, and much deeper where it goes under the Hollywood Hills to reach North Hollywood. One of the stations has an escalator that goes through the entire 60 feet in one flight; it’s said to be the longest escalator west of the Mississippi.
Hehe. The London Underground has one 109 feet long!
Whoops - 197 feet long. :smack:
IANAStructural Engineer, but I believe there is no maximum depth. As long as the substance you are boring through is sufficiently strong (stone) it doesn’t really matter how deep it is. Just intuitively, in order for a tunnel twice as deep to collapse, more than twice as much rock (an n squared relationship) must move above the tunnel. In order for it to move, it will all have to crumble. That isn’t going to happen.
The physically limiting factor is tracing a navigatable path through the substance that isn’t compromised by some natural instability.
The financially limiting factor is much more important – it is extremely expensive to drill a tunnel. If there is a decent alternative route – over, or around – it will be cheaper. Tunnels are only built through the shortest pass.
I meant, the one in L.A. goes through about 60 vertical feet. The length of the actual escalator, from end-to-end, would be much more.
Ahh dammit!
Maybe 90 feet? 
Anyway, the one legion mentioned always amuses me, because four of them end-to-end would almost reach from Leicester Square to Covent Garden stations.
tx cillasi, everything a engineering virgin needed to know bout tunnels