Types of Cofferdams
A cofferdam (also coffer dam) is a temporary barrier in or around a body of water to allow dewatering, diversion or damming of an enclosed area. The major purposes of any cofferdam type is to hold back flood waters or create a dry environment so a project can proceed.
As such, there are only a handful of structures that truly qualify as cofferdams (earthen berms and sandbags) since they all are intended to be temporary in nature.
A Dam-It Dams cofferdam holds back water to create a temporary barrier around a project site.
Cofferdams usually fall into these categories:
- Cantilever Sheet Pile Cofferdam
- Braced Cofferdam
- Earth Embankment Cofferdam (earthen)
- Rock fill Cofferdam
- Double Wall Cofferdam
- Cellular Cofferdam
Cantilever Sheet Piles
This type of cofferdam is susceptible to large leakage and flood damage. Therefore, these are suitable for small head of water about 6 m. 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.
This type of cofferdam 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. Due to economical reasons, 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.
Rock fìll Embankment
This cofferdam type is made of rockfill. A typical section rockfill cofferdam is shown in Fig.3. This cofferdam type 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. Consequently this helps to keep the pheratic surface within the berm. A sketch of double Wall cofferdam is given in Fig.4.
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 and, as a result, provide stability against the lateral forces. There are two types of cellular cofferdams, namely diaphragm type and circular type.