Cofferdams: Construction, Types, and Applications

Andhra Pradesh Chief Minister N Chandrababu Naidu laid stone for Polavaram project's cofferdam on Thursday. The Chief Minister says he is determined to complete the project as scheduled and release water by 2018. He also reviewed the designs of the iconic bridge across the spill channel of the project and discuss the upcoming works with the concerned officials.

Cofferdam is a watertight enclosure pumped dry to permit construction work below the waterline, as when building bridges or repairing a ship. A cofferdam also called a coffer is a temporary enclosure built within, or in pairs across, a body of water and constructed to allow the enclosed area to be pumped out. This pumping creates a dry work environment for the major work to proceed.

Enclosed coffers are commonly used for construction and repair of oil platforms, bridge piers and other support structures built within or over water. These cofferdams are usually welded steel structures, with components consisting of sheet piles, wales, and cross braces. Such structures are typically dismantled after the ultimate work is completed.

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 dam foundation area. These cofferdams are typically a conventional embankment dam of both earth- and rock-fill, but concrete or some sheet piling also may be used.

Typically, 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. Dependent upon 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, writes Wikipedia. Cofferdams are classified according to the type of construction.

The type of construction is dependent upon the depth, soil conditions,, fluctuations in the water level, availability of material etc. Cofferdams are advantageously constructed where a large area of site is to be enclosed and the hard bed is at reasonable depth, observes theconstructioncivil.org.