A cofferdam is a temporary enclosure in a river or lake built around a working area for the purpose of excluding ground water and water above ground level during construction.
Types of coffer Dams
There are various types of cofferdams. They include:
- Cantilever Sheet Pile Cofferdam
- Braced Cofferdam
- Earth Embankment Cofferdam
- Rockfill Cofferdam
- Double Wall Cofferdam
- Cellular Cofferdam
Cantilever Sheet Piles – Cantilever sheet piles are suitable for small head of water about 6 m. They are susceptible to large leakage and flood damage. They can be constructed of wood, concrete and steel. Wooden sheet pile cofferdam is suitable for small head of water i.e. up to 3 m. Steel coffer dam is suitable up to 6 m head of water. Concrete cofferdam is suitable when headroom is limited. Bored and cast in place sheet piles are to be used to avoid noise and vibration. A typical sketch of cantilever sheet pile is shown in Fig.1 below.
Earth Embankment – Earth embankment is suitable for high head of water and low water velocity. The side slopes of the embankment should be 1:3 to 1:4. The slope of the bank is covered by a rip rap. A typical earth embankment cofferdam is shown in Fig.2. A successful cofferdam need not be completely watertight. For reasons of economy, some seepage of water into the excavation is usually tolerated. The water collected is pumped out of the excavation. The embankment is provided with a minimum of free board of 1 m to prevent overtopping by waves.
This type of cofferdam requires large base area and is adopted when area of excavation is very large. Clayey soil is suitable for the construction in dry season. If constructed in wet season, sand fill is the best material.
RockfÃ¬ll Embankment – This type of cofferdam is made of rockfill. A typical section rockfill cofferdam is shown in Fig.3.
This type of cofferdam is better than the earth fill. These are very pervious and are usually provided with an impermeable membrane of soil to reduce seepage. The crest and upper part of impermeable membrane are provided with a rap to protect against the wave action. Steep slopes can be maintained at 1:1.5 or 1:1.25.
Double wall sheet piles – This type of cofferdam is suitable when it is required to exclude water over 12 m. This consists of two straight, parallel vertical wall of sheet piling tied to each other and the space between them filled with soil. Double wall sheet pile cofferdam higher than 2.5 m should be strutted. Sometimes an inside berm is provided to keep the pheratic surface within the berm. A sketch of double Wall cofferdam is given in Fig.4.
Cellular Coffer dams – A cellular cofferdam is constructed by driving sheet piles of special shapes to form a series of cells. The cells are interconnected to form a watertight wall. These are filled with soils to provide stability against the lateral forces. There are two types of cellular cofferdams, namely diaphragm type and circular type.