I suppose you’re all going to say that tunnels always bend so that when you are driving through them the light from the outside world at the exit (and the light from oncoming cars) is minimised, so as not to blind you, but I am hoping are other more scientific reasons why tunnels always seem to be curved, rather than saving the poor workmen some time by allowing them to dig the shortest path through a mountain or under a river.
Does the curve add structural strength? Does it minimise caving in? Does it look better? Do chicks dig it more? Did simon say they must bend? Does anyone care?
First off, I don’t think it’s true to say that all tunnels bend; I’ve driven through some tunnels (particularly the short ones on the way to Holyhead, N Wales) that seemed perfectly straight.
Tunnels that go under rivers bend for the reason that they have to go down then back up again (or it would be a ford, not a tunnel), but I’m thinking that surely this isn’t what you’re talking about.
One reason that many do is because they are in some pretty rugged terrain and the end of the tunnel has to point the same way the road on the other side needs to go.
Many tunnels go through a hill and then the road on the other side is dug into that same hill. So somehow the road needs to curve 90 deg. (or so) from the beginning of the tunnel to the road on the other side. It just makes sense to build some of the turn inside the tunnel and make it gradual rather than have a stop and a 90 deg. turn on the far side.
There are certainly tunnels that don’t bend at all. A railway tunnel built by I.K. Brunel let the sun shine straight through… on his birthday!
Although I’d like to think that women would flock if I chose to drive curved tunnels, maybe a recasting of the question would help: Why do tunnels bend at all? The shortest distance between two points, and all…
As it happens, I just read a book on the history of tunneling… obviously some Dopers have too much time on their hands…
The major reason for a curving tunnel is pretty much what you guessed: it minimizes cave-ins while the tunnel is being built, and avoids geology where it would be expensive to construct a tunnel that didn’t collapse.
Tunnels under rivers, for example, are easier when there’s bedrock that leaks very little water, and very nasty when there’s sand, mud and clay that leaks water like a sieve.
Also, answering your other suppositions, curves in the tunnel direction add no strength whatsoever. The “look” of the direction of a tunnel is not a concern: there’s far too much money at stake in tunnels. Companies have frequently gone broke miscalculating. And as for attracting the opposite sex, well, amusement parks do have special purpose tunnels for that, true enough… but I’ve never been able to get anywhere in them…