When building a bridge over a large body of water, how do they get the pillars so deep into the water and how is the foundation secured?
In: Engineering
[check it out](https://en.m.wikipedia.org/wiki/Cofferdam)
[this photo is a good visual](https://s3.ap-south-1.amazonaws.com/hansindia-bucket/5210_Classroomcofferpic.jpg)
Basically they make a little enclosed area and then suck out all the water from that area, essentially putting a dry hole in the middle of the water, then in that hole they build the bridge support.
There are numerous techniques.
One common one that is often used in waters that are sufficiently shallow is cofferdams.
Basically they pick where they want to build and erect a temporary dam around the place and pump out the water creating a sort of island below the sea level for them to work on.
These days, there are a few techniques. But the most common is it drive steel pipes into the river or ocean bed, then use an auger – a big soil drill – and pumps to extract the soil from inside the pipe. Don’t worry about the water. They extract soil below the bottom of the pipe, too, so that there is a void underneath the base of the pipe. Then they drop steel reinforcing into the pipe, and fill the pipe with heavy concrete. The concrete is heavier than the water is, so it stays at the bottom and the water is pushed out of the top.
You then end up with a concrete pile – the steel rusts away, but it’s only there to help form the concrete pile – sitting on a concrete plug that is down beneath the river bed. The finished construction looks like an upside-down mushroom, deep down in the soil. Each one of those piles can support a lot of weight, and as there aren’t expensive to put in, they put down a lot. Then a headstock is cast on top of them, and the bridge (or other construction) built on the headstock.
You use something called a cofferdam or caisson. Basically you build a temporary barrier around the area where you want to build, which you then pump out all the water. Cofferdamns extend the existing shoreline while caissons are freestanding in the water. You then build the bridge pier up inside them. Once it’s above the water, and the concrete is cured enough, you can flood the dam and remove it.
Latest Answers