Cofferdam can be defined as an enclosure built within a body of water. They are constructed to make it possible for that enclosed region to be pumped out. The area that is pumped out creates a dry environment which allows for work to kick off. These enclosed cofferdams are commonly used in the construction or repair of bridges, oil platforms, pipelines, river crossings, flooding, etc. and other structures built over or within the water.
Cofferdams often are steel structures that are properly welded with components that consist of wales, cross braces, and sheet piles. The structures are often dismantled following the completion of the works. Due to the fact that cofferdams are built within water bodies, these sheet piles will be installed using templates and are constructed prior to construction of the cofferdam. This helps in accurately positioning each of the sheet piles by help of a barge.
In the construction of dams, it is often likely that two cofferdams will be built. One of the cofferdams will be downstream and another one upstream at the location of the proposed dam. This will be done after a diversion channel or tunnel is constructed to allow the free flow of the river past the area where the dam is to be constructed. Such cofferdams often are a traditional embankment dam consisting both of rock fill and earth although some sheet piles. Concrete might be used as well. Upon the completion of the entire structure, the cofferdam constructed downstream is removed and thereafter the upstream cofferdam flooded. This happens even as the previously built diversion points are closed hence allowing the reservoir to fill up with water.
Depending on the geographical features of the dam location, in certain instances, a cofferdam with a ‘U’ shape is built in the construction of one side of the dam. The cofferdam is thereafter removed and another similar cofferdam built on the other side of this river for the purpose of construction of the other half of the dam.
Cofferdams may also be used occasionally in the building of ship and in their repair. This happens in instances where it would not be reasonable to take that ship into a dry dock for repairs. Cofferdams may, in instances of an operation for lengthening a ship, be used. Some instances even involve the cutting of the ship into two parts while at sea and another new section inserted for the purpose of lengthening it.
There was an open caisson weighing 100 tons that was lowered into the sea floor. The sea floor was in excess of one mile deep. The caisson was lowered in the attempt of stopping oil from flowing in the oil spill that occurred in the deepwater horizon. The caisson was referred to as a cofferdam. This cofferdam failed to work. The reason was that the methane hydrates in upper caisson levels froze, therefore, preventing the process of oil spill containment.
Shapes of Cofferdams
The structure of a cofferdam can take any shape that is so desired. For practical purposes, however, and due to the economy, many cofferdams will either take a circular or rectangular shape. In general, the circular shaped cofferdam provides advantage since they offer an open and wider excavation area across the whole planning area when the bracing process is done using ring beams alone. Sheet piles installation inside circular cofferdams, however, requires tougher controls in construction. The rectangular cofferdams are easily constructed although they require some advanced internal bracing.
These cofferdams are only utilized in circumstances where the size of excavation prevents cross excavation bracing utilization. In such circumstances the cofferdam has to be stable by having itself overcome the lateral forces.
These types are built from using a single sheet pile wall. This sheet pile is forced into the surface hence it forces formation of a box shape around this excavation area. These sheet piles will then be propped on their inside area and thereafter the interior dewatered. Braced cofferdams are used primarily for the bridge piers while at shallow water areas. Often the water is between 30 and 35 feet deep or 9 to 12 meters deep.
Cofferdams with two layers of sheet piles
These are cofferdams that comprise of two sheet pile rows parallel to each other. The sheet piles are forced into the surface and joined together using a tie rods system either at one or at more than one level. The area between the sheet piles is often filled with materials of granular texture such as broken rock, gravel or sand.
The Construction Procedure of Cofferdams
A typical cofferdam like the bridge pier has a construction process as the one defined herein.
The first step is pre-dredging in order to get rid of soil or the soft sediments and to level up the region of this cofferdam.
After these, temporary pillars for supporting the template are driven in. A bracing frame is erected temporarily on the templates support piles. Sheet piles made of steel are thereafter installed. The process starts on all corners and comes together at the central area of every side.
Sheet piles are then driven in order to grade the area. The area between the sheets and frame is blocked, and the sheet piles are tied as needed at the topmost area. The area slightly below or inside the grade is excavated while allowing the cofferdam to remain filled with water. The water inside the cofferdam is thereafter lowered while progressively installing the internal bracing as necessary depending on the cofferdam design. Piles are thereafter driven inside the cofferdam as would be necessary. Rock fill is placed afterwards as a way of supporting and leveling. A concrete seal may be placed using tremie (to spread the concrete under the water).
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