The word Coffer means a trunk, casket or chest in which temporary structure built to enclose and encase an area meant for excavated for the foundation of the construction. Cofferdams are used for structural foundation in Open Water & Open Land such as bridge piers, docks, locks, dams and high ground water table. A cofferdam is a waterproof wall built around the periphery of the proposed excavation to prevent the flow of water into the excavation so that the basement may remain in dry condition.
As per the Requirement of the job-site and output need the coffer dam may be many types but some common types of cofferdam are as under:
1) Earth Coffer Dams
2) Rockfill Coffer Dams
3) Single-Sheet Pile Coffer Dams
4) Double-wall sheet piling coffer dams
5) Braced Coffer Dams
6) cellular Coffer dams
1) Earth Coffer Dams: Earth Coffer Dams are constructed in a area where the velocity of the current and depth of the water is 13 to 18 in. which is carried about 1 m above the water level. It is constructed with a mixture of clay and sand. The Side Slopes of the bank on the water side should be pitched with rubble Boulder to avoid embankment from scouring. On Completion of coffer dam the water is pumped to dry the inside surface. Sand bag can be used in an emergency.
2) Rockfill Coffer Dams: Rockfill coffer dams made of rockfill enclose the site sometimes with dewatered. To protect against wave action the crest and the upper part of the impervious membrane are provided with rip rap. The slopes of rockfill cofferdam can be made as steep as 1 horizontal to 1.5 vertical.
3) Single-Sheet Pile Coffer Dams: Single Sheet Coffer Dam constructed in a very small area and whose depth of water is more than 4.5 to 6m. At First Guide Piles better known as Timber Piles inserted into the firm Ground which is below water bed. Depending upon the velocity of the Current in water the Longitudinal runners spacing varies known as wales which are bolts to a timber piles at a necessary distance. Steel or Wooden Sheet inserted into the River Bed along with wales which are secured to the wales by bolts. To increasing the stability of walls against the water pressure the sheets on the two faces arc braced by trussed arrangement of struts and Half-filled bags of sand stacked on the inside and the outside faces of the sheets. After the cofferdam is constructed, the water in the enclosed area is poured out and the construction work begun.
4) Double-wall sheet piling cofferdams: :In Large Area Single Wall Sheet Coffer Dam is not workable as it unable to resist Water Pressure thus double wall sheet piling cofferdam is constructed with a depth of water upto 12m. The distance of the both wall depend on the depth of the water, the wall should be wider depending upon the depth of the water increase to stabilize against overflowing and sliding. The Thickness of the water depend on the depth of the water, greater the depth thickness should be excess of 3m. The Sheet piles are inserted at a reasonable depth in the Bed to prevent the leakage from the Ground Level.
5) Braced Coffer Dams : The braced cofferdams are influence to flood damage. A braced cofferdam is prepared by operating two rows of vertical sheeting and bracing with wales and struts. To prevent ground water from entering the foundation pit on land the Braced Cofferdams are sometimes used as land cofferdams to support the soil so as to prevent cave in. The structure is formed After the pit is dewatered. The cofferdam is removed after completion of concreting above the water level,.
7) Cellular Cofferdams: Cellular Cofferdams are of 2 varies known as i) Circular Type . ii) Diaphragm type
1) In Circular Type Cofferdams are constructed by filling the cell completely up-to top before the start of Construction work. The Crane and other equipment can be place requires for construction purpose without any adverse effect to the neighboring Cell due to Interlock Stress within a permissible diameter of a cell.
2) In Diaphragm type steel Sheet piles are connected Series wise. The Distance of Straight Diaphragm is equal to the radius of the Connecting Arcs. The Cell are filled with sand, gravel and other material after inserting the cell at a required depth. The adjacent cells should be filled at approximate rate to avoid the failure of interlocks. This type of Cofferdam increases the desirable limit without increasing the Interlock stress.