How does a coffer dam work, or how is it used underwater?

Answer 1

a coffer dam is a term used to describe the isolation of the area from the water.
for example, if you want to construct a bridge support in the water, you drive sheet pile around the area that is completely enclosed. You pump out the water and construct in the dry environment.
Think of a dam, you build a box without a lid, lower it down and fasten it to the face of the dam. Pump the water out and work in the dry

Answer 2

A cofferdam is a means to gain dry access to the bottom of a body of water so that the construction of support piers , usually for bridges, can be stated. A heavy, long and specially shaped metal plate is aligned and positioned onto the bottom. Then another is aligned and fitted next to it so that the special interlocking seams of the two mesh. The second is driven into place adjoining the first. This is continued until there is a closed vertical cylinder of interlocked plates. The plates are individually driven into the bottom while the top of the cylinder remains above the water surface. The water is pumped out and the bottom is removed, preferably to bedrock to begin the construction of the pier. When construction has progressed to the point where the cofferdam is no longer needed, it is removed and used elsewhere.

Answer 3

A coffer dam is just a wall used to hold back water. It is normally used whenever people need a part of a river or lake bed to be temporaily dry so that they can build something on it, like the foundation of a bridge tower.