COFFERDAMS

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DEFINITION

A cofferdam may be defined as a temporary structure that is constructed on a river or a lake or any other water-bearing surface for excluding water from a given site to execute the building operation to be performed on dry surface.

TYPES OF COFFERDAMS

Considering the material used in their construction, cofferdams may be divided into the following categories.

› Sandbag dike cofferdam› Earthfill cofferdam› Rock-fill cofferdam› Single-walled cofferdam› Double-walled cofferdam› Crib cofferdam› Cellular cofferdam ….(Circular or diaphragm type)

Sandbag dike

Sandbag dike cofferdam

This type of cofferdam is useful if the depth of water is small.

Cement bags are partially filled with a mixture of sand and clay and systematically laid around the area in courses

Earthfill cofferdam

EARTHEN COFFERDAM

Earthen embankment built around the area to be enclosed.

The depth of water is very low.

The velocity of the current is very low.

The side slopes may vary from 1 : 1 to 1 : 2.

A mixture of clay and sand or clay and gravel.

Side slopes should be pitched with rubble boulders.

Sometimes, Sheet piles are also provided for to make the cofferdam more watertight.

Rockfill cofferdam

ROCK-FILL COFFERDAM

If the depth of water to be retained by the embankment of cofferdam is of order of 3 m .

Stone or rubble is used for the embankment. This construction is adopted only if the stone is easily available in the nearby areas. The stones are assembled in the required shape of the embankment and the voids

are partially filled with earth and stone-chips. On the waterside,the rockfill is to be provided with an impervious layer of earth.

Single wall cofferdam

SINGLE-WALLED COFFERDAM Area to be enclosed is small and the depth of water is more 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 c/c spacing of the piles may vary between 1.8 to 4.5 m. Longitudinal runners called wales are then bolted to the guide. 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 are 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.

SINGLE-WALLED COFFERDAM

Double walled cofferdam

DOUBLE-WALLED COFFERDAM:

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 overturning and sliding. The distance between the two walls depends upon the depth of wat 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.

DOUBLE-WALLED COFFERDAM

Rockfill cribs cofferdam

ROCKFILL CRIB COFFERDAM

› If the stream has a hard bottom ,it is difficult to penetrate the guide piles or sheet piles .In such cases crib cofferdam is used.

› 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. › On the side of water ,the framework is provided with sheet pile› 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.›

ROCKFILL CRIB COFFERDAM

CELLULAR COFFERDAM

› These cofferdams are suitable for enclosing large areas.› They are used for the construction of long span bridge piers across

wide rivers or in backwater areas› These cofferdams are quite expensive

Two types› Circular cellular cofferdam› Diaphragm cellular cofferdam

CIRCULAR TYPE CELLULAR COFFERDAM

The circular type of cellular cofferdam has the advantage that each cell may be filled completely to the top before starting the construction of the next cell without causing any distortion to the shell of the cofferdam

One cell is completely filled up it can be used for the construction of other cells.

Each cell acts as a self-supporting independent unit

CIRCULAR TYPE CELLULAR COFFERDAM

Circular Cellular cofferdam

Circular Cellular coffer dam

DIAPHRAGM TYPE CELLULAR COFFERDAM

Consists of a series of diaphragm of steel sheet piles connected The straight diaphragm wails are connected to each other by steel piles arranged in

the form of arches on either sides. The radius of the connecting arcs is generally made equal to the distance between

the straight diaphragm walls. After the cells are driven to the required depth, they are filled with earth, sand, gravel or other filling material. In this type of cofferdam, as the diaphragm which separates the two cells is a straight wall, it is necessary to fill adjacent cells at approximately the same rate. If this is not done, the unbalanced pressure from the fill will distort the diaphragm (cross-walls) which may result in the failure of the interlocks. In this respect, the circular type cofferdam has the advantage over the diaphragm type cofferdam because in the former, it is not necessary to fill the adjacent cells at the same time.

DIAPHRAGM CELLULAR COFFERDAM

DIAPHRAGM TYPE CELLULAR COFFERDAM