A cofferdam is defined as a temporary structure which is constructed so as to remove water and/ or soil from area and make it possible to carry on the construction work under reasonably dry condition.
Requirements of a cofferdam
v The cofferdams should be reasonably watertight. It may either rest on impervious soil or may be extended impervious soil or may be extended to impervious strata through previous soils. Otherwise, a layer of concrete may be laid at the bottom of a cofferdams and this layer should be allowed to harden sufficiently before pumping of water is started.
v It should be noted that absolute water tightness is desired in a cofferdam. It is not only impracticable but expensive too. The design and layout of a cofferdams should there for be such that total cost of construction, maintenance and pumping is minimum.
v The cofferdams should be designed for the maximum water level and other destructive force so as to make it stable against bursting, overturning and sliding.
v The water to be excluded by a cofferdam may be either ground water or water lying above ground level. It may be deep or shallow and still or running and sliding.
v The materials used in the construction of a cofferdam are earth, timber, steel and concrete.
v The cofferdam is generally constructed at site of work.
v The type of construction for cofferdam is depended up on the depth, soil conditions, fluctuations in the water level, availability of material, etc.
v The cofferdams are constructed with advantage where a large area of site is to be enclosed and the hard bed is at reasonable depth.
USES OF COFFERDAMS
· To facilitate pile driving operation
· To place grillage and raft foundation
· To construct foundations for piers and abutments of bridge, dams, locks, etc;
· To enclose a space for the removal of sunken vessels
· To provide a working platform for the foundation of building when water is met with; and
· To provide space for carrying out the foundation work without disturbing or damaging the adjoining structure such as buildings, pipelines, sewers, etc.
TYPE OF COFFERDAMS
A wide variety of different types of cofferdams is available. The factors which influence the choice of a particular types of cofferdam are follows:
- The area to be protected by a cofferdam i.e. a small area or a large area.
- The depth of water to be dealt with i.e. shallow depth or deep depth.
- The possibility of over topping by foods, tides, etc.
- The nature of bed on which the cofferdam is to rest i.e. a previous layer or an impervious layer.
- The nature of velocity of flow i.e. water flawing with slow current or with swift current.
- The chance bed erosion due to reduction of waterway caused by the construction of a cofferdam.
- The materials available at suite of work the construction of a cofferdam.
- The facilities available for the transport of equipment and materials required for the construction of a cofferdam.
Following are the most common type of cofferdam:
- Single wall cofferdams
- Double wall cofferdams
- Cellular cofferdams
- Rock-filled crib cofferdams
- Concrete cofferdams
- Suspended cofferdams.
PREVENTION OF LEAKAGE IN COFFERDAMS
The water enters the cofferdam in two ways, namely, by leakage through the sheet piling and by underflow. Following. Are some of the measures which are employed to prevent the leakage of a cofferdam:
1. If the entry of water is through the fissures or cracks or cracks of the rock, it can be stopped by pumping cement ground through pipes of about 100 mm in diameter.
2. The materials such as clay, sand, ashes, etc. may be dumped all round the cofferdam to reduce the quantity of entering water.
3. The leakage in case of a double wall cofferdam occurs due to insufficient compaction after being placed. This may be prevented in the following ways;
a. Driving the piles through the filling.
b. Driving holes and filling these holes with the materials excelsior which swells quickly when wet and it also acts as a filter.
c. Forcing clay cylinders through pipes into the filling so s to have better compactness of filling.
4. The leakage in case of a single wall cofferdam may be prevented by placing a V-shaped wooden trough outside each joint and filling it with puddle.
5. A very simple method to prevent the leakage through joints in sheet piling is sometimes adopted. A box is filled with a mixture of sawdust and ashes. It is held near the joints of leakage. The water while flowing through the joints to the inside of the cofferdam takes with it the contents of the box. This leads to the sealing of the joints and consequently, the leakage is considerably reduced.
6. The interlocks of steel sheet piles may be provided with sufficient quantity of grease before driving. The fine materials carried by the water will stick to the grease and it will make the joint effective in pre venting the leakage of water.
7. If the leakage is of serious nature, it is reduced by providing tarpaulin which is a canvas coated with tar. The canvas is placed over the area of leakage and it should be see that it is not disturbed form its position by the force of water .
PUDDLE FOR COFFERDAM
The best quality of puddle for a cofferdam is composed of either clay and sand or clay and gravel. The clay to be used for making the puddle should be cohesive and impermeable.
In order to test the cohesive quality of clay, a simple test is carried out. A cylinder about 40 mm in diameter and 250 mm in length is prepared from clay by mixing it with a small quantity of water. This cylinder is suspended from one and while it is wet. If it does not break, the clay is said to have sufficient cohesion.
In order to test the impermeability of clay, a considerable quantity of clay is taken and it is converted into a plastic mass by the required quantity of water. This plastic mass is then spread out and the water is stored above it. If water is held for some duration of time, the clay is said to have sufficient impermeability.
The puddle should be laid in layers of about 80 mm thickness and each layer should be well-rammed before the next layer is laid over it. During ramming operations, the water should be added.
FACTORS AFFECTING DESIGN OF A COFFERDAM
The design of a cofferdam depends on various factors such as hydrostatic head of water, dimensions of the area to be covered by the cofferdam, subsoil conditions, fluctuations of outside water level, possibility of erosion, presence of ice, floating logs, etc. As it is sometimes not possible to determine accurately the forces due to these factors, the worst conditions should be assumed in the design of the cofferdam to prevent its failure.
It is thus clear that a purely theoretically designed cofferdam may fail for factors unaccounted in its design. It had therefore become necessary to combine practical knowledge or experience with the theoretical aspects in the design of a cofferdam.