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_PN_PAGE 1 _PN_OF 2 Any structure first need a substructure or foundation. Even a pipe line or road needs some foundation work like preparation of subgrade soil, laying of sand layer etc. Special foundations are needed for the structures like dams and bridges built in water.
If you throw a handful of soil into running water, it would get readily washed away. If you dive to some three or four meters deep into water, the water pressure on you is overwhelming.
Then how did all those magnificent and colossal structures like dams and great bridges are built in water by Civil Engineers? Now, let us know that interesting story.
To start construction of dams or piers and abutments of bridges in rivers, the water need to be drained out first, to expose the river bed. For constructing a dam foundation, first, a coffer dam or temporary dam is constructed to hold or divert water or it is used along with river diversion works. For bridges, besides using coffer dams to drain the bed, special foundations are used for substructure.
Coffer Dams
Coffer dam is a temporary barrier structure, constructed in water to facilitate the construction of substructure of say a dam or bridge, within it. For construction of Bhakra Dam on Sutlej River, India, the earthen and rock-fill cofferdam built upstream is as high as 215 feet.
Earthen Coffer Dam
In this simplest form, a mixture of sand and clay is dumped around the spot like an embankment and the section above water level is compacted well. Then water from inside is pumped out to start construction of foundation inside. It can not withstand more than 2 m of water depth and water currents.
Rock-fill Coffer Dam
This type is similar to earthen coffer dam but a mixture of sand, clay, gravel and boulders is used, to withstand more water pressures and moderate currents.
Sheet Pile Coffer Dam
These can be single or double walled sheet pile structures. Timber sheet piles are driven along the network of guide piles and wales (see discussion on pile foundations). The sheet piles are securely jointed together and bolted to wales. However, the sheet piles need longitudinal braces to prevent collapse.
If the water inside is pumped right away, sheets would give away easily. Therefore, the water is pumped out in stages and bracing work is carried out inside on exposed parts of sheets. When sheets are fully braced and water is pumped out, the bottom may be given a small earthen bund for good support.
In case of double walled sheet pile structure, two rows of guide piles and connecting wales is formed around the spot. Two rows of sheet piles are also driven inside the guide pile rows as usual and the two rows are further stiffened with cross beams or struts. The space between two rows is filled with a mixture of sand, clay, gravel etc., called puddle.
Crib Coffer Dam
A timber crib, in the typical form we see in timber yards, in which roughly finished timber logs are laid transversely, is made on the bank and bolted together. It is then floated to site and sunk at spot using boulders as weights. When it sank fully, it is filled with sand, mud, gravel and boulders. These can withstand currents and water depths of up to 15 m.
Cellular Coffer Dams
Whenever, the water depth is great and rocky river bed is at great depth, steel sheet piles are formed into shells on bank and these shells are driven in joined together to form coffer dams. There are two different types of cell patterns namely, the diaphragm type and circular type. In the former case, two series of arc shaped sheet piles, with their concave sides facing each other are laid with connecting cross diaphragm walls in between. In the latter case, circular shells are joined together with circular arc shaped steel piles. These cells are filled with gravel and soil.
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