Cofferdams are enclosures constructed inside water bodies like the lake, river, etc. for facilitating a dry working space throughout the construction period. Cofferdams serve as a temporary dyke wall built across the water body. They allow the water to be pumped outside and ensures a hindrance-free and dry construction area.
Significance of cofferdams
Construction in water is the most tedious activity faced in civil engineering. A dry working and safe environment is necessary to maintain the quality of the construction and safety of the project. A lot of methods are used for construction of structures in water and maintaining the area dry. One of the most preferred methods is cofferdams.
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What is a Cofferdams
Cofferdams are created by building water tight enclosures around the construction area and exposing it by pumping the water. Cofferdams are preferred for bridge construction sites, Bridge piers, boat jetties, ports etc. The type of cofferdams and the design depends on the size of structure, water depth, water flow velocity etc.
Types of cofferdams
Coffer dams are classified into differently types based on the the design requirements, depth of water, soil conditions, type of material used etc.
- Earth cofferdam
- Rock fill cofferdam
- Single sheet pile cofferdam
- Double-wall sheet piling cofferdam
- Braced cofferdam
- Cellular cofferdam
Earthen cofferdams are the popular/simplest type of cofferdams. They are suitable for areas where the water depth and water current are low. For constructing earthen cofferdams locally available sand, soil, clay, rocks materials are used.
The height of the earthen cofferdam has to be at least 1 mtr above the maximum water level. Earthen cofferdams require a large base area and are adopted when an area of excavation is very large.
Impervious clay core or sheet piles are driven in the middle to resist water pressure and seepage. The upstream side is stone pitched to protect from scouring and subsequent dam failures. However, these systems do not provide a hundred percent waterproof areas. Waterproofing systems and pumps have to be installed to drain the water periodically.
Rock fill coffer dams
Rockfill cofferdams are superior compared to earthen cofferdams. The choice of rockfill dams is linked with the availability of rocks locally and economically. The maximum height of the rockfill dam should be kept under 10 ft.
The rockfill area is pervious and to be lined with impervious clay layer to prevent seepage and subsequent failure of dams.
Single walled cofferdams
Preffered when depth of water is below 6 mtr and the area of construction is localized like that of smaller sites like bridge pier etc.
Mainly constructed by driving wooden sheets as guide piles around the perimeter followed by a support layer of steel sheets in the inside portion. Guide piles may be a steel section in case of large depth of water. After having driven the guide piles, wales or runners of timber logs are bolted to the guide piles at suitable vertical intervals.
Inside sheets should be placed at a specific distance from the wooden planks with the help of wales. These wales are bolted to the sheets from both sides as shown in the fig.
The inside sheet piles are heavily braced. For further improving stability sand bags are placed on both sides of the walls.
The depth of penetration should be around 1 mtr for clay 0.5-0.75 mtr for sand and 0.25-0.5 for gravels etc.
The water inside is pumped out and construction can be started.
Double walled cofferdams
Preferred when the depth of water is more than 6 mtr and the construction area is big and areas where the single walled cofferdams seems uneconomical.
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 piles has to strutted as shown in the fig if the height is more than 3 mtr.
The filling materials to be selected carefully considering the co-efficient of friction to give stability to the cofferdam.To prevent the leakage from the ground below, the sheet piles are driven to a good depth in the bed in upstream area.