Types of Cofferdam

Single-walled cofferdam:

This type of cofferdam is used in places where the area to be enclosed is very small and the depth of water is more, say 4.5 to 6 m Timber piles known as guide piles are first driven deep into the firm ground below the river bed. Depending upon the velocity of the current of the water in the river, the centre to centre spacing of the piles may vary between 1.8 to 4 m. Longitudinal runners called wales are then bolted to the guide piles at suitable distance apart. Steel or wooden sheet piles are then driven into the river bed along the wales and are secured to the wales by bolts. The sheets on the two faces arc braced by trussed arrangement of struts. This helps in increasing the stability of walls against the water pressure. Half-filled bags of sand stacked on the inside and the outside faces of the sheets help in increasing the stability of cofferdam. After the cofferdam is constructed, the water in the enclosed area is pumped out and the construction work is taken up.

Double-walled cofferdam:

For cofferdams required to enclose larger areas in deep water, single wall type becomes uneconomical as larger sections of trussed struts would be necessary to resist the water pressure. Double-walled cofferdam is provided in such situations. Its construction is essentially the same as that of a single-walled cofferdam except that in place of one wall, a pair of walls with a gap in between is used all along the boundary of the space to be enclosed. This type of cofferdam can be used in depth of water up to 12 m. As the depth of water increases, the wall should be made wider in order to make it stable against over4urning and sliding. The distance between the two walls depends upon the depth of water. The thickness of wall should be equal to the depth of water up to 3 m. For greater depths of water, the thickness of wall should be 3 m. plus ½ the depth of water in excess of 3 m. At their top, the two faces of the walls are connected by steel rods spaced at close intervals. To prevent the leakage from the ground below, the sheet piles are driven to a good depth in the bed.

Crib cofferdam:

In deep waters where it is difficult to penetrate the guide piles or sheet piles into the hard bed below, crib cofferdam is used. In this type of construction, the sheet piles are supported by a series of wooden cribs. A crib is a framework of horizontal timbers installed in alternate courses to form pockets which can be filled with earth or stones. The length and breadth of each crib depend upon the depth of water and the current of flow. The framework of the cofferdam (made from, logs of wood) is prepared on ground and then floated to the site where the cofferdam is to be constructed. The layers of sand and the other loose material overlying the impervious hard bed is dredged out. Crib is then sunk to the position, the bottom of each crib is given a shape to fit in the variation in the surface of bed rock. The space inside the crib is then filled with stone or any other material, so as to make it stable against sliding and overturning. Timber or steel sheet piles are then driven around the crib.

This article comes from theconstructioncivil.org edit released