A cofferdam is a type of watertight construction designed to facilitate construction projects in areas that are normally submerged, such as bridges and piers. One is installed in the work area and water is pumped out to expose the bed of the body of water so that workers can construct structural supports, enact repairs, or perform other types of work in a dry environment. In some regions of the world, this construction is better known as a caisson. Working inside a cofferdam can be hazardous if it is installed improperly or not safely pressurized, but advances in engineering have led to increased safety for workers using this unique work environment.
A variety of materials can be used to construct a cofferdam, which is truly a feat of engineering. Although it is a temporary structure, it must reliably hold water back from the work area and also withstand very high pressures in order to be safe, and the construction is often used as a project for engineers learning their craft. The most basic type uses sheet metal, which is pounded into the bed of the body of water to create a watertight wall. Next, pumps are used to pull water out of the enclosure so that it will be dry. Some are built from wood or concrete, while others use a double walled mechanism, with filler made from aggregate materials in between the two walls.
The walls of a cofferdam can extend all the way to the surface of the water, leaving it open at the top, or it can be built as an enclosed structure. In very deep water, enclosed and pressurized ones are used for worker safety, while in shallower bodies of water, an open structure can be used. Workers access a closed cofferdam through hatches and tubes, and care is taken to make sure that the air supply is consistent and the pressure is kept at a normal level.
Shipwrights and repair yards also use a portable version, which can be attached to the side of a ship to enact repairs below the waterline. At sea, this can be a useful way to quickly address potential problems until the ship is taken into dry dock for more long-term repair. Minor repairs can also be undertaken with a portable one in a shipyard to avoid the expense of hauling the ship into dry dock for the work to be completed.