Introduction

• A cofferdam is a temporary structure designed to keep water and/or soil out of the excavation in which a bridge pier or other structure is built.
• Cofferdam are temporary enclose to keep out water and soil so as to permit dewatering and construction of the permanent facility in the dry.
• A cofferdam is a temporary structure designed to keep water and/or soil out of the excavation in which a bridge pier or other structure is built.

Requirements for a Cofferdam

• Cofferdam should be reasonably watertight.
• Absolute water tightness is not desired in a cofferdam.
• Designed in maximum water level.
• Ground water or water lying above ground level should be excluded.
• Constructed at site of water.

Purpose to use cofferdam structure

• Pile driving operation.
• To retain soil & water.
• Can be used as temporary or permanent.
• Main purpose is to provide dry working area for workers.
• It is used to place grillage as well as the raft foundations.
• Enclose space for removal.
• Constructing without disturbance.

Factors for choice of cofferdams

• Area to be protected by a cofferdam.
• Depth of water to be dealt.
• Possibility of over topping.
• Nature of velocity of flow.
• Availability of materials at site of work.

Types of Cofferdams

1. Earth-fill cofferdam
2. Rock-fill cofferdam
3. Single-walled cofferdam
4. Double-walled cofferdam
5. Crib cofferdam
6. Cellular cofferdam
7. Concrete cofferdam
8. Suspended cofferdam or movable cofferdam
9. Sandbag dike cofferdam

Earth-Fill 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.

Rock-Fill Cofferdam

• If the depth of water to be retained by the embankment of cofferdam is of order of 3m.
• Stone or rubble is used for the embankment.
• This construction is adopted only if the stone is easily available in the nearby areas.
• On the waterside, the rock fill is to be provided with an impervious layer of earth.

Single-Walled Cofferdam

• Area to be enclosed is small and the depth of water is more 4.5 to 6m.
• 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 4.5m.
• Longitudinal runners called walls 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.

Double-Walled Cofferdam

• Double-Walled Cofferdam is provided in such situation.
• 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 12m.
• The distance between the two walls depends upon the depth of water.

Crib Cofferdam

• A crib is a framework of wooden horizontal & cross beams.
• This cribs are open at the bottom and are filled with rock, gravel or earth.
• This type of cofferdam favorable where the bed of stream is hard, depth of water is high, water is swift.

Cellular Cofferdam

• Used for de-watering large areas in places where the depth of water may be of the order of 18 to 21m.
• Cellular cofferdams are used only in those circumstances where the excavation size precludes the use of cross-excavation bracing.
• Mostly used during the construction of marine structures like dams, locks, whales, etc.

Concrete Cofferdam

• A cofferdam 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, creating a dry work environment for the major work to proceed.
• 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.

Suspended Cofferdam or Movable Cofferdam

• Where there are numbers of repetition work in under water foundations, such as in the piers of multi span river bridges, it is economical to design the cofferdam to be moved as a single unit from one foundation to another.

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.