General Question

Ltryptophan's avatar

How do engineers decide between bridges and tunnels?

Asked by Ltryptophan (10615points) December 24th, 2019

Self explanatory question, but..

I guess if there’s something to say here it would be just some layman think. I guess tunnels are closer to the ground and therefore don’t have to support the weight of heavy cars in the same way a bridge does. That might be one advantage if that’s true.

Then again, tunnels have to deal with huge volumes of water constantly pushing down on them.

Bridges seem like less trouble to possibly repair, since they are exposed, and you can kinda see what’s going on just by taking a look.

I really have no clue. I guess it comes down to cost effectiveness.

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15 Answers

SQUEEKY2's avatar

You answered it yourself, it all has to do with the cost$$$$$$$$$$.

Ltryptophan's avatar

@SQUEEKY2 I was thinking about it, and also it might have a big part to do with navigation. Maybe without a certain height for the bridge it would disturb important navigation. And, probably it just isn’t feasible to put up a massive suspension bridge.. But, oddly enough, the tunnel that made me think of this has a bridge right over it….For that one, probably traffic counts made it worth while.

janbb's avatar

It might also have to do with tidal variations. If the area spanned has too much of a variation in tidal height, a tunnel might be more cost efficient and effective. The quality of the water bed would be a factor too; if it is rock, it could be very difficult to tunnel through it.

ragingloli's avatar

If it is a river/valley: bridge.
If it is a mountain: tunnel.

janbb's avatar

@ragingloli Not always applicable. There are tunnels under the Hudson River from New Jersey to NY but no mountains.

elbanditoroso's avatar

Science has something to do with it as well. You can’t built a bridge that’s too long and too high, because of the weight involved. And you can’t build a bridge in water too deep. NYC has a mix of bridges and tunnels because the Hudson is fairly shallow and is narrow enough to do both.

You can’t build tunnels on geologically unstable land – that’s why San Francisco has the Golden Gate and the By Bridge, and the San Mateo, and so on. And they have a real risk with the BART tunnels, and they spent a lot of money trying to earthquake-proof them.

Bottom line: the specific location and terrain is what governs the decision.

stanleybmanly's avatar

There is the additional factor of advancements in the technology and engineering involved with both fields. There are probably plenty of sites where the choice between the 2 might be reversed were the decision made today.

JLeslie's avatar

Interesting question. Going across the East River in NY there is the Williamsburg Bridge, the midtown tunnel, and the Queensborough bridge that are not very far apart. Maybe a mile and a half to two miles from one to the next. Basically, the same waters. That’s only naming a few of the bridges and tunnels that cross into NYC. I don’t know which was built first, which cost more, and which gives the city the most problems.

kritiper's avatar

Let’s see…
Fear of heights and falling or…
Fear of being buried alive under tons of rock and earth.
Hmmmmmmmmmmmmmmmmm…

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Bootsiebaby's avatar

It depends on the lay of the land. If it needs to go under, they make a tunnel. If it needs to go over, they build a bridge. It’s very simple, really.

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