How are bridges built over rivers? I’m talking about a series of arches rather than a suspension bridge.
I presume the engineers must dig a hole in the riverbed for the foundations, then lay the bricks on top of that. Doing this under, say about 20 feet of water must be a bit of a challenge. How do they do it?
In particular, how did they do it a few hundred years ago without modern machinery to help them?
My understanding is that you build a caisson, which is in effect a tube, from the surface to the bottom. You can then pump out the interior, and this allows you to work on the river bottom, in effect with a cylindrical dam around you.
Princhester has nailed it for the in river part of the construction.
Once the caisson is in place the piers can be constructed. These need to be sufficiently thick to resist the forces that will be imparted by the arch.
For the rest of the bridge an arch former is used, which is essentially a (usually) wooden frame supported from below that is the same profile as the arch. The brickwork is then laid on the arch former. When the mortar is set the arch former is removed and can be re-used if multiple arches are required.
Read up on the construction of the Brooklyn Bridge for a description of how they did it a hundred plus years ago. Fascinating stuff. They didn’t have modern engineering techniques for calculating stresses so they just overbuilt. And they didn’t know about working under pressure so lots of workers got the bends and died.
David McCullough’s The Great Bridge does an excellent job of describing the construction techniques, as well as political background behind the building of the Brooklyn Bridge. It includes a fascinating description of some of the first recognized cases of the bends (also known as caisson disease).
LaCrosse, WI recently got a new bridge across the Mississippi. How the footings were built is already answered. For the main span, they built it on barges parallell to the shore. When it was done they rotated it and put it place (IIRC they built it higher than it needed it to be. once it was in place they flooded the barges to lower the main span in place. then they removed the scaffolding)
Here is a picture of Bixby Creek Bridge (on California Highway One). You can see the wooden framework used to support the bridge during construction. Actually, I think any arch (on a bridge or in a building) would need a temporary framework for support until the arch is complete.
basically, you sink open concrete or wood cylinders on top of each other until you reach the water surface. The joints between these cylinders are watertight. Then you pump out the water inside the big cylinder you have built. Voila. Caisson.
Any bridge that requires a deep water support–whether it be two (or one) for a suspension bridge or mutiple for an arch bridge or one or more for a beam bridge–will require the use of something to keep the water off the workers as they build the piers. If the water is shallow, the structure need be no more than a breakwater to allow men to dig on the bottom of the riverbed. It the water is deep, then a caisson is required, regardless of the type of bridge or whether the water crossed is a river (Brooklyn Bridge) or ocean or lake straits (Golden Gate or Mackinac Bridges).
Most bridges built before the Brooklyn Bridge were built in shallow rivers. If the river was too deep, then the bridge piers might have been built by filling barges with stone and dumping them or sinking them where the piers were needed. Such piers were probably adequate for many smaller bridges. Another way to build piers, prior to caissons, was to drive pilings into the river bed and then fill the area surrounded by the pilings with stone. (This was the method used for the 12th-13th century London Bridge.) Once the caisson technology was available (and constantly improved), then engineers began building larger bridges that needed deeper piers for support.
One significant point that hasn’t been directly addressed is the issue of pressure. Some caissons, such as the one used for the Brooklyn bridge had to be pressurized or the river water would have erupted through the muddy bottom into the caisson. They had to build the tube, cap it, pump out the water and pressurize the caisson. Hence the problem with the bends. shallower caissons and/or harder bottoms allow open caissons where you don’t have this serious and expensive problem.