Types of Cofferdams
There are many types of cofferdams. A cofferdam is defined as a temporary barrier in or around a body of water which allows the process of de-watering, diversion, or damming of water within an enclosed area. The major purpose of any cofferdam type is to hold back overwhelming or inconvenient waters and create a dry work environment. This allows a project to proceed with as little resistance and as much safety as possible.
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
- Rock fill Cofferdam
- Double Wall Cofferdam
- Cellular Cofferdam
Cantilever Sheet Piles
This type of cofferdam is susceptible to leakage and flood damage, making these forms of damming better suited for smaller depths of water, up to 18 feet. They can be constructed of wood, concrete, or steel, each with their own size limitations in what they can handle. Wooden sheet pile cofferdam is suitable for up to 9 feet, steel sheet pile is suited up to 18 feet head of water, similar to concrete, which suitable when headroom is limited. They are bored and cast in place and are to be used to avoid noise and vibration.
This type of cofferdam is suitable for high heads of water with low velocity. A successful cofferdam does not need to be completely watertight because some seepage of water into the excavation is usually well-tolerated. The water collected is pumped out of the excavation afterwards. The embankment is provided with a free board minimum of 3 feet to prevent overtopping by waves. This type of cofferdam requires large base area and is adopted when an area of excavation is very large. Clay soil is appropriate for the construction in dry season. If constructed in wet season, sand fill is the best material.
This cofferdam type is made of rock-fill. A typical section rock-fill 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.
Double-Wall sheet piles
This type of cofferdam is suitable when it is required to exclude water over 36 feet. 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 7.5 feet should be strutted. Sometimes an inside berm is provided. Consequently, this helps to keep the phreatic surface within the berm.
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.