There are various types of cofferdams used for construction of structures in water. Construction details of these cofferdams are provided in this article. The basic needs of human being are food, air, water, shelter and transport. To fulfill the basic needs of shelter and transport every inch of the earth land is being used for the construction of roads, building or other structures. Nowadays even structure on water are being constructed. But the construction in water is a very tedious job. As the structure is hard to build in water as concrete doesn’t set in water. Many methods are being used to overcome this problem. One the methods used for this purpose are Cofferdams. Cofferdam can be defined as the temporary structure that is built to keep the water away from the execution site, so that the structure can be built on the dry surface. The cofferdams should have walls that exclude water from building site. For this the walls must be water proof and the height of the wall must be more than the maximum water level. These types of cofferdams are preferred where the area of building site is large and the dry soil bed is at reasonable depth
Coffer dams can be classified into many types depending upon the depth, soil conditions, and fluctuations in the water level and type of material used.
Earthen cofferdams are constructed at the place where the height of the water is less say 3m and the current velocity is low. These dams are built using the local available material such as clay, fine sand or even soil. The height of the dam is kept 1m more than that of max water level. Freeboard of the dam or the top of the dam is kept 1m so that the water doesn’t enter the other side even when waves arise. The slope is usually given but 1:1 or 1:2. The slope of the water side is pitched with rubble stones so the water action doesn’t score the embankment. Even sheet piles are driven in the center of the dam to resist water seepage. After the construction of earthen cofferdam, the water from the other site is pumped out and construction is executed.
Fig: Cross-Section of an Earthen Cofferdam
Rock-fill cofferdams are better than that of earthen dams. These dams are preferred when the rock is available easily at the construction site. These dams are very pervious, to prevent water from seeping an impervious membrane of soil is provided in the dam. The height of the dam is can be up to 3m. The slope can be maintained at 1:1.5 to 1:125. The slope on the water side is pitched so as to protect dam from wave action.
Fig: Cross-Section of Rockfill Cofferdam
This type of cofferdam is preferred when the depth of the water is more than 6m and area of construction is less. Usually this is used in construction of bridges. Wooden or timber sheets are driven into the river bed on the perimeter of the area of construction. On the inside steel or iron sheets are driven into the river bed. This inside sheets are placed at equal distance with the help of wales which are bolted to both sheets for either sides. To improve the stability of this types of dam, half-filled bags of sand are placed on the both side of the walls. The water from the inside is pumped out and the construction process is undertaken.
Fig: Construction Details of Single Walled Cofferdam
Double-walled types of cofferdams are used when the area of construction site is large and depth of water is high. In this place use of single walled cofferdam becomes uneconomical as the supports are to be increased. So double walled cofferdam is used. The difference in one wall and double wall dam is that her it has two walls instead of walls for extra stability. This type of dams can hold water up to 12m high. Two piles are driven inside the water bed with a space in between and attached each other with wales with bolted connection. As the water depth increases the space between the walls increases. The space between the walls are filled with soil. To prevent the leakage from the ground below, the sheet piles are driven to a good depth in the bed.
Fig: Construction Details of Single Walled Cofferdam
When it’s difficult to drive piles inside the bed in the water, then this type of cofferdam is used. In braced cofferdam two piles are driven into the bed and they are laterally supported with the help of wooden cribs installed in alternate courses to form pockets. The empty pockets here are filled with stone and earth. The framework of the cofferdam (made from, logs of wood) is prepared on ground and then floated to the site where the cofferdam is to be constructed. The layers of sand and the other loose material overlying the impervious hard bed is dredged out. Crib is then sunk to the position, the bottom of each crib is given a shape to fit in the variation in the surface of bedrock. After the pit is dewatered, the structure is concreted. When concreting has been completed above the water level, the cofferdam is removed.
Fig: Braced Cofferdam Construction Details
When the water layer is more than 20m, common types of cofferdams are uneconomical to use. In this situations cellular cofferdams are used. This type of dam is used in construction of dams, locks, weirs etc. Cellular cofferdam is made by driving straight web steel sheet piles, arranged to form a series of interconnected cells. The cells are constructed in various shapes and styles to suit the requirements of site. Finally, the cells are filled with clay, sand or gravel to make them stable against the various forces to which they are likely to be subjected to. The two common shapes of the cellular cofferdam are, (i) Circular type cellular cofferdam. (ii) Diaphragm type cellular cofferdam.
This type of cellular cofferdam consists of circular arcs on the inner and outer sides which are connected by straight diaphragm walls. The connection between the curved parts and the diaphragms are made by means of a specially fabricated Y-element. The cofferdam is thus made from interconnected steel sheet piles. The empty spaces are filled with non pervious materials like clay or sand. Due to the filling material the self weight of the membrane increases and leakage is reduced. One advantage of the diaphragm type is that the effective length of the cofferdam may be increased easily by lengthening the diaphragm. Hence in case, from design consideration it is necessary to have effective width of the cofferdam more than 21 meter, diaphragm type of cofferdam must be used.
Fig: Plan and Section Details of Circular Type Cellular Cofferdam
It consists of a set of large diameter main circular cells interconnected by arcs of smaller cells. The walls of the connecting cells are perpendicular to the walls of the main circular cells of large diameter. The segmental arcs are joined by special T-piles to the main cells. The circular type cellular cofferdams are self-sustaining, and therefore independent of the adjacent circular cells. Each cell can be filled independently. The stability of such cells is much greater as compared with that of the diaphragm type. However, the circular cells are more expensive than the diaphragm type, as these require more sheet piles and greater skill in setting and driving the piles. Because the diameter of circular cells is limited by interlock tension, their ability to resist lateral pressure due to high heads is limited.
Fig: Plan and Section Details of Diaphragm type Cellular Cofferdam
