The word coffer means a casket, chest or trunk. A coffer dam is a temporary structure built to enclose an area for excavation of foundation. Coffer dams are used when the size of excavations is very large and sheeting and bracing system becomes difficult, inconvenient or uneconomical. Coffer dams are generally required for foundations of structures, such as bridge piers, docks, locks, and dams, which are built in open water. These are also used for laying foundations on open land where there is a high ground water table. A coffer dam generally consists of a relatively impervious wall built around the periphery of the proposed excavation to prevent the flow of water into the excavation so that the foundation may be laid in dry condition.
Types of Coffer Dams –
The following are different types of coffer dams commonly used in practice.
1. Earth Coffer dams –
These are the simplest type of coffer dams well-adapted to depths of water upto 3 m. Earth embankments are constructed around the area to be dewatered. The earth coffer dams are built of local soils, preferably fine sand. These usually have a clay core or a vertically driven sheet piling in the middle. The upstream slope of the bank is covered with a rip rap. A successful coffer dam need not be completely watertight.
For reasons of economy, it is not possible to make it watertight and hence some seepage of water into the excavation is usually tolerated. The water collected is pumped out of the excavation. The embankment should be provided with a minimum free board of 1 m to prevent over-topping by waves.Sand-bag coffer dams are used in an emergency.
2. Rockfill Coffer dams –
Rockfill coffer dams made of rockfill are sometimes used to enclose the site to be dewatered. These are permeable and are usually provided with an impervious membrane of soil to reduce seepage. The crest and the upper part of the impervious membrane are provided with rip rap to provide protection against wave action. Overtopping does not cause serious damage in case of rockfill coffer dams. The slopes of a rockfill coffer dam can be made as steep as 1 horizontal to 1.5 vertical.
3. Single-Sheet Pile Coffer dams –
Single-sheet piling coffer dams are generally used to enclose small foundation sites in water for bridges at a relatively shallow depth. In this type of coffer dams, there is a single row of cantilever sheet piles. The piles are sometimes heavily braced. Joints in the sheet piles are properly sealed. This type of coffer dams are suitable for moderate flow velocities of water and for depths upto 4 m. The depth of penetration below ground surface is about 0.25 h for coarse sand and gravels, 0.50 for fine sand and 0.85 h for silts, where h is the depth of water. Sometimes, single sheet pile coffer dams are provided with earth fills on one or both sides to increase the lateral stability.
4. Double-Wall Sheet Piling Coffer dams –
A double-wall sheet piling coffer dam consists of two straight, parallel vertical walls of sheet piling, tied to each other and the space between walls filled with soil. The width between the parallel piles is empirically set as (h/2 + 1.50 m), where h is height of water. Double-wall sheet piling coffer dams higher than 2.5 m should be strutted. Sometimes, an inside berm is provided to keep the phreatic line within the berm.
The fill material should have a high coefficient of friction and unit weight so that it performs as a massive body to give the coffer dam. stability against sliding and overturning. Suitable measures should be adopted to reduce the uplift on the coffer dam. This is generally done by driving the sheet piling on the upstream as deep as possible.
The double wall sheet piling coffer dam has the advantage of having less leakage than that in a single wall coffer dam. These coffer dams are suitable upto a height of 10 m.
5. Braced Coffer dams –
A braced coffer dam is formed by driving two rows of vertical sheeting and bracing with wales and struts.These are similar to sheeting and bracing system,with one basic difference that braced cuts are required for excavations in dry areas whereas braced coffer dams are used to isolate a working area surrounded by water. The braced coffer dams are susceptible
~ Land Coffer dams –
Braced coffer dams are sometimes used as land coffer dams to prevent ground water from entering the foundation pit on land and to support the soil so as to prevent cave in. After the pit is dewatered, the structure is then concreted. When concreting has been completed above the water level, the coffer dam is removed.
6. Cellular Coffer dams –
A cellular coffer dam 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 cells are filled with soil to provide stabilising force against lateral pressure. Basically, there are two types of cellular coffer dams that are commonly used.
(a) Diaphragm type –
This type of cellular coffer dam consists of circular arcs on the inner and outer sides which are connected by straight diaphragm walls. The connection between the curved and the diaphragms are made by means of a specially fabricated parts Y-element. The coffer dam is thus made from inter-connected steel sheet piles. The cells are filled with coarse-grained soils which increase the weight of the coffer dam and its stability. The leakage through the coffer dam is also reduced. To avoid rupture of diaphragms due to unequal pressure on the two sides; it is essential to fill all the cells at approximately the same rate. One advantage of the diaphragm type is that the effective length of the coffer dam thus may be increased easily by lengthening the diaphragm.
(b) Circular type –
It consists of a set of large diameter main circular cells interconnected by arcs of smaller cells. The walls of the connecting cells are thus perpendicular to to 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 coffer dams are self-sustaining, and therefore independent of the adjacent circular cells. Each cell can be thus 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 large lateral pressure due to high heads is limited.