Difference between Caissons and Cofferdams, Their types and engineering mechanismArticles > Difference between Caissons and Cofferdams, Their types and engineering mechanism
Differences between Caissons and Cofferdams
Both Cofferdams and Caissons are the structures used to build water submerged constructions. The function of both the structures is same and the mechanism of use is also same. The main difference is that Caissons are the used as the frame of the structure which is required to be built. After construction the caissons are not removed and dismantled, but they remained as the part of the constructed structure. While on the other hand cofferdams are removed/ dismantled when construction complete. They only act as the frame of the structure. When concrete is compacted, cofferdams are removed.
Cofferdams are as water-tight as possible, because the aim of a cofferdam is to provide a dry work environment. In a dry and clean environment the concrete work is carried out without the leakage of water from outside. When a cofferdam is installed the muck is sucked by means of a muck tube or muck vessel. The muck inside a caisson is also evacuated in the same way.
A caisson is typically a box-like structure made of materials such as timber, steel, masonry, steel, masonry and reinforced concrete. It may be constructed onshore then floated to the required location, where it is sunk into place, enabling access to the bed to undertake works.
Which one is most suitable: A Caisson or a Cofferdam?
It depends on the type of structure being constructed whether the caisson or the cofferdam is suitable. The ground conditions and the depth where a structure is to be constructed are the main factors to decide whether a caisson or the cofferdam is suitable. In low depth, the cofferdams are used, while depths more than 20 feet caissons are more suitable.
Features and Types of Caissons:
Caisson is a word borrowed from French caisson, derived from Italian Caisone, which means a very large box. It is very important technique of geotechnical engineering to build underwater structures. For example the columns of a bridge, underwater foundations, repairing ships and for the construction of water dams. A Caisson is a very large box which is watertight with a muck tube. This large box is place in the water and then, the debris and muck is evacuated by means of the muck tube. After getting the caisson empties, the workers and engineers enter the caisson and start the work with ease. Thus, the function of a caisson is to keep the work environment dry.
What are the Caissons made of?
Caissons are generally made up of concrete, steel or wood. The material used in a caisson depends on the function of a caisson or the purpose for which a caisson is to be used, and the types of construction to be done by the help of a caisson. In modern engineering tools, a caisson is made up of steel or concrete. But the steel made caissons are frequently used because of their durability. Steel made caissons can easily be shifted from one place to another as compared to concrete made caissons. Another advantage of steel made caissons is they can be dismantled into pieces and at the time need it can again be jointed with help of nut bolts.
Types of Caissons:
There are four main types of Caissons viz. Box Caisson, Open Caissons, Monolithic Caissons and Pneumatic Caissons. The details all these four types is given our previous article. Four main types of Caissons
Pros of Caisson:
Caissons are used in engineering field due to the following benefits:
- Caissons are economic.
- Without caissons access to the underwater earth surface is impossible.
- Caissons are environment friendly, because of less noisy and vibratory.
- Being very high in height, caissons are very heavy to pierce deep in the mud.
- Less handling equipments are required to place a caisson in the water.
- Due to heavy weight caissons can be placed very deep.
- Quality control of pneumatic caisson is good, as it is built on dry conditions.
Cons of Caissons:
- Qualified workers and engineering staff are required for caisson constructions.
- As the concrete work is done underwater, therefore, lack of satisfaction exists.
- It is not suitable at polluted sites.
- As it is highly technical work, therefore the risk of lives is likely in works associated with caissons.
- Skillful labour is needed for caisson construction, which is not easily available.
Types of Cofferdams:
A cofferdam is a temporary barrier in a water-body which stops or diverts the water to enter in a particular part of the ground in order to conduct a construction work in a dry environment. It allows a project work to continue in water submerged area by removing or diverting water. There are several types of Cofferdams which are as follows:
- Cantilever Sheet Pile Cofferdam
- Braced Cofferdam
- Earth Embankment Cofferdam
- Rock fill Cofferdam
- Double Wall Cofferdam
- Cellular Cofferdam
Cantilever Sheet Piles
Where there the heights of water or soil are to be retained are small, a cantilever sheet piles are used. The various forces acting on a cantilever sheet pile wall are the active earth pressure on the back of the wall and the passive earth pressure on the front of the wall. Cantilever Sheet Piles are used a Cofferdam which is susceptible to leakage and flood damage, making these forms of damming better suited for smaller depths of water, up to 18 feet. They can be constructed of wood, concrete, or steel, each with their own size limitations in what they can handle. Wooden Sheets are piled up to 9 feet height, while the steel sheet may be piled up to 20 ft. The stronger is the material, the larger would be the height of piled sheets. It is also used to avoid noise and vibration.
When there is high head of water with low velocity, such types of cofferdams are used. It is not necessary for a cofferdam to be watertight and minor seepage of water is tolerated in a successful cofferdam. The little seepage of water helps compacting the embankment. The seeping water is evacuated simultaneously. The embankment is provided with a free board minimum of 3 feet to prevent overtopping by waves. This type of cofferdam requires large base area and is adopted when an area of excavation is very large. Clay soil is appropriate for the construction in dry season. If constructed in wet season, sand fill is the best material.
A rock and debris filled cofferdams is usually considered better than an earth filled cofferdam. The compaction and durability of a rock filled embankment is more than the other types of embankments.
Double-Wall sheet piles
When water up to the height of 36 feet is needed to be removed, this type of cofferdam is used. It is a structure of two parallel vertical walls of about 40 feet height. These walls of made up of sheet piling. Both the walls are tied to each other, which strengthens the sustainability of walls. The space between the walls is filled with soil and debris. The material of walls does not allow water to cross the embankment, while the soil and debris protects walls from damage and to fall down.
Cellular Cofferdam is made up of a series of cells. Such type of cofferdam is constructed by driving sheet piles of special shape. All the cells of the structure are strongly interconnected which form a watertight wall. These cells are filled with soil. The soil filled cells, then, provide stability against the lateral forces. There are further two types of Cellular Cofferdams; Diaphragm and Circular Type Cofferdams. Diaphragm types are cofferdams with circular arcs at the sides, connected to straight diaphragm walls. Circular types are large circle-shaped cells, connected to one another by slightly smaller circular cells.
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