thumb|A cofferdam on the Ohio River near Olmsted, Illinois, built for the purpose of constructing the [[Olmsted Locks and Dam|Olmsted Lock and Dam]]
thumb|right|A cofferdam during the construction of [[canal lock|locks at the Montgomery Point Lock and Dam]]
A cofferdam is an enclosure built within a body of water to allow the enclosed area to be pumped out or drained. This pumping creates a dry working environment so that the work can be carried out safely. Cofferdams are commonly used for construction or repair of permanent dams, oil platforms, bridge piers, etc., built within water.
These cofferdams are usually welded steel structures, with components consisting of sheet piles, wales, and cross braces. Such structures are usually dismantled after the construction work is completed.
The origin of the word comes from coffer (originally from Latin meaning 'basket') and dam from Proto-Germanic meaning 'barrier across a stream of water to obstruct its flow and raise its level').
The term is also used in naval architecture, to refer to a space between two watertight bulkheads or decks within a ship.
Uses
For dam construction, two cofferdams are usually built, one upstream and one downstream of the proposed dam, after an alternative diversion tunnel or channel has been provided for the river flow to bypass the foundation area of the dam. These cofferdams are typically a conventional embankment dam of both earth- and rock-fill, but concrete or some sheet piling also may be used. Usually, upon completion of the dam and associated structures, the downstream coffer is removed, and the upstream coffer is flooded as the diversion is closed and the reservoir begins to fill. Depending on the geography of a dam site, in some applications, a U-shaped cofferdam is used in the construction of one half of a dam. When complete, the cofferdam is removed and a similar one is created on the opposite side of the river for the construction of the dam's other half.
Cofferdams are used in ship husbandry to allow dry access to underwater equipment and to close underwater openings while work is done on the fittings inside the ship. This is more common in naval vessels where a cofferdam may fit several vessels of a class.
Examples
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A 100-ton open caisson that was lowered more than a mile to the sea floor in attempts to stop the flow of oil in the Deepwater Horizon oil spill has been called a cofferdam.
A cofferdam over 1 mile long was built to permit the construction of the Livingstone Channel in the Detroit River. See main article at Stony Island.
The museum battleships USS Alabama (BB-60) and USS North Carolina (BB-55) have had cofferdams since 2003 and 2018, respectively. This saves much money compared to towing and dry docking them after the tow and this also provides additional security so there is a low chance of the ships sinking and becoming impossible to repair.
Types
Several types of structure performing this function can be distinguished, depending on how they are constructed and how they are used.
Marine salvage
When all or part of the main deck of a sunken ship is submerged, flooded spaces cannot be dewatered until all openings are sealed or the effective freeboard is extended above the high-water level. One method of doing this is to build a temporary watertight extension of the entire hull of the ship, or the space to be dewatered, to the surface. This watertight extension is a cofferdam. Although they are temporary structures, cofferdams for this purpose have to be strongly built, adequately stiffened, and reinforced to withstand the hydrostatic and other loads that they will have to withstand. Large cofferdams are normally restricted to harbor operations.
See also
- Caisson (engineering)
- Causeway