Correct Answer - Option 2 : Cantilever sheet pile type
Concepts:
Coffer dam is defined as a temporary structure around a water body which allows the process of de-watering and diversion within an enclosed area.
The main purpose of it is to hold back upcoming waters and to create a dry work environment.
Types of Coffer Dam:
1. Cantilever Sheet Piles:
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This type of cofferdam is susceptible to leakage and flood damage, making these forms of damming better suited for smaller depths of water.
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They can be constructed of wood, concrete, or steel.
2. Rockfill dam:
- 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 up to 3m.
- The side slope can be maintained at 1:1.5 to 1:125.
3. Earthen Cofferdam
- These are constructed at the place where the height of the water is less and the current velocity is low.
- These dams are built using the local available material such as clay and fine sand.
- The height of the dam is kept 1m more than that of max water level.
- 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.
4. Single Walled Coffer Dam
- This type of cofferdam is preferred when the depth of the water is more than 6 m and area of construction is less.
- Usually, this is used in the construction of bridges.
- Wooden or timber sheets are driven into the river bed on the perimeter of the area of construction. The water from the inside is pumped out and the construction process is undertaken.
5. Double Walled Cofferdam
- These 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 in case of the latter two piles are driven inside the water bed.
6. Braced Coffer Dam
- 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.
7. Cellular Dam
- When the water layer is more than 20m, then cellular cofferdams are used.
- This type of dam is used in construction of dams, locks, weirs etc.
- It is made by driving straight steel sheet piles and arranged to form a series of interconnected cells.
