How do you build a bridge on water?

[Dischord]

I've always wondered this and have searched online about the construction of 2 local bridges but haven't found anything that really answers my questions.

How do you build the foundation for bridge 'towers' or supports over deep, moving water?

The bridges in question are Menai and Britannia over the Menai Straits in North Wales?
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[Hetzer]

Put big tubes in the water, pump out the water, fill with concrete.
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[Suntan Sid]

Piles!

Put simply, modern structures, built on unstable material are more often than not supported by concrete piles.
Drilling rigs bore out cores and inject concrete into them, to form a stable base for the foundations, the depth and number of piles depends on the loads and the ground conditions involved.
I remember seeing a program on Discovery about one of the old New York bridges where they tunneled down underneath the river, to build the foundations, the water was kept out of the tunnel using compressed air. Unfortunately they didn't know about the "Bends" at the time!

I'm currently working on a new "marina" project, all the structures, offshore, are supported by piles!

Start watching Discovery Science, sooner or later they repeat everything and you'll find out everything you need to know!
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[Suntan Sid]

Here's some info on the Brooklyn bridge, I mentioned.

https://www.eyewitnesstohistory.com/brooklynbridge.htm
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[fatpies]

You could always divert the water during construction as well.

They did this for the Hoover dam and the massive dam in China.
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[Paulington]

As Hetzer said, put huge steel/some kind of material tubes in the ground through the water, drive piles down into the ground at the bottom and then fill it with reinforced concrete of some type.

As Fatpies said, a few dams in America they diverted all the running water around the construction site and then when they'd finished they blocked up the diversion so it flowed through it's normal old route.

Take a look at Discovery/Discovery Science/Turbo some time and try to catch "Extreme Engineering", "Megastructures" or something similar. A lot of bridges on these programmes that show you exactly how they are done.

Where there's a will, there's a way! !
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[Frost]

The Clifton suspension bridge looks like it was built by some men in a boat taking a thin line across, then that being used to pull a thicker one across before eventually the final iron chains. But that bridge is so high up that supports in the middle would have been impossible.

Generally rivers can either be done in a single span, diverted or pylons put in. Sea bridges are a bit harder as you've got to work with the tide, and build it strong enough not to get destroyed by it.

[Pie-Roe]

Check out the King Fahd causeway. It's pretty cool.
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[R6 Wilson]

Same way you would go about building Oil thingy's
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[Dischord]

Would these techniques have been used in the 1820's?

Cheers for the info so far guys, very useful!
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[fatpies]