Cofferdams are temporary structures used where construction is being carried out in areas submerged in water. They are most commonly used to facilitate the construction or repair of dams, piers and bridges. The aim of a cofferdam is to be as watertight as possible to create a dry area in which to complete the required building works, or at least to limit water ingress to a safe level that can be pumped away.
Cofferdams must be able to withstand very high pressures and can create a hazardous situation if they are installed incorrectly. They are usually constructed using steel sheet piles driven into the ground and supported by internal braces and cross braces. Timber sheet piles, concrete or a combination of materials can also be used. A cofferdam can be any shape. Its design will be depend on the depth required, the required working area, soil conditions, fluctuations in the water level, and so on.
 Types of cofferdam
There are several different types of cofferdam. The most common are as follows:
 Typical piled cofferdam
The construction sequence for a typical piled cofferdam is as follows:
- Pre-dredge the proposed area of the cofferdam.
- Drive temporary support piles into place.
- Install bracing frames to support piles.
- Drive sheet piles into place.
- Pump out water from the inside of the cofferdam.
- Progressively install internal bracing as required.
- Excavate ditch to allow leakage to run to one place.
- Place rock fill as a leveling and support course.
This is a simple form of cofferdam suitable for areas of shallow water with low current velocity, where a sloped earthen embankment is built up around the area to be enclosed. The bank should be a combination of clay and sand or clay and gravel.
If the water to be retained is not shallow enough for an earthen cofferdam, stone or rubble can be used instead. The stones are built up in the required shape of the cofferdam, and voids filled with earth, gravel or stone-chips.
This type of cofferdam is most suitable when the area to be enclosed is particularly small and the water is of a deeper level, roughly 4-6 m. Guide piles made of timber are driven into the ground and bracing constructed before sheet piles are driven into place and secured to the bracing using bolts.
Single-walled cofferdams become unfeasible as larger areas are required in deeper water, and so double-walled cofferdams are sometimes necessary. Two walls are built with a gap between them, the thickness of which is dependent upon the depth of water. The general rule is for the thickness of the wall to equal the depth of water up to 3 m, greater depths require 3 m plus half the excess depth. The two wall faces are connected at the top using steel rods placed at close intervals.
This is commonly used in deep waters where the ground is particularly hard. The sheet piles are supported by a horizontal framework of timbers, known as cribs, which form pockets to be filled with earth or stones. This is constructed on land prior to being moved into position in the water. The crib is sunk into position, the internal space filled with stone or other material and timber or steel piles are then driven around the boundary of the crib.
This type of cofferdam is most suitable for deep water areas, most commonly for during marine construction such as dams and wharves. Straight web steel sheet piles are driven into the ground to form a series of inter-connected cells of specific shapes which are filled with clay, sand or gravel to enable the structure to withstand the lateral forces of the water pressure. There are two common shapes:
- Circular type: Each cell acts as a self-supporting independent unit so each cell can be completely filled before construction of the next cell begins.
- Diaphragm type: Made from interconnected steel sheet piles using circular arcs at the inner and outer sides. The cells are then filled with coarse-grained soils, all at the same rate to avoid rupturing.
 Related articles on Designing Buildings Wiki
- Bridge construction.
- Coastal defences.
- Driven piles.
- Geothermal pile foundations.
- Groundwater control in urban areas.
- Grouting in civil engineering.
- Pile foundations.
- Pumps and dewatering equipment.
- Retaining walls.
- River engineering.
- Temporary works.
- Screw piles.
- Sheet piles.
- Water engineering.
 External references
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