COFFERDAMS
A cofferdam is a temporary structure which is built to remove water from an area and make it possible to carry on the construction work under reasonably dry conditions. Cofferdams are usually required for projects such as dams, locks and construction of bridge piers and abutments.
REQUIREMENTS OF A COFFERDAM
1) The cofferdam should be reasonably water-tight.
2) The design and layout of a cofferdam should be such that the total cost of construction, maintenance and pumping should is minimum.
3) It should be sufficiently stable against bursting, overturning, and sliding, under the floods, waves and anticipated loads.
4) It should be generally constructed at site of work.
5) It should be so planned as to facilitate easy dismantling and reuse of materials
TYPES OF COFFERDAMS
(1) Earth fill cofferdam. (2) Rock fill cofferdam.
(3) Rock fill crib cofferdam. (4) Single wall cofferdam.
(5) Double wall cofferdam. (6) Cellular cofferdam.
1. EARTH FILL COFFERDAM:- This is the simplest form of cofferdam. Its use is limited in the conditions where impervious earth is available and water depth is shallow with low velocity of flow. It should never be used where there is danger of overtopping of water.
2. ROCK FILL COFFERDAM:- They are constructed by placing rock along stream. They can be used for depths of water up to about 3 meter and are suitable even in case of swift water. They can withstand the overtopping of water without any serious damage.
3. ROCK FILL CRIB COFFERDAM:- Rock fill crib cofferdam is comprised of timber cribs. A crib is a framework of wooden horizontal cross beams laid in alternate courses. The cribs are open at the bottom and are filled with rock or gravel or earth. This gives stability to the crib against overturning or sliding.
4.SINGLE WALL COFFERDAM:- This type of cofferdam is suitable when available working space is limited and area to be enclosed is small. It may be used up to the maximum depth of water of 25 meter. The walls of this cofferdam are generally made of steel, sheet, pile. Reinforced and prestressed concrete sheet piles are also used.
5. DOUBLE WALL COFFERDAM:- These cofferdams are provided to enclose a large area. The double wall gives stability to the cofferdam. This type is useful where scour problems and space limitations are prevalent.
6. CELLULAR COFFERDAM:- They are made of steel sheet piles and are suitable for dewatering large areas. They are comprised of diaphragm cells or circular cells or modified circular cells. It can with stand overtopping of water. These types of cofferdams are quite expensive and should be used only in the case of long span bridge piers across wide rivers or in back water areas subjected to tidal waves.
The selection of a type of cofferdam depend upon the following factor :-
1) The extent of an area to be protected by a cofferdam.
2) The depth of water to be dealt with.
3) The possibility of overtopping by floods, tides, etc.
4) Velocity of flowing water.
5) The natural of bed on which the cofferdam is to rest.
6) The possibility of scour due to reduction of waterway caused by the construction of cofferdam.
7) The availability of construction materials in the vicinity of site of work.
8) Transporting facilities available.
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