It’s no secret that underwater construction poses a challenge to engineers around the world. Working above the waterline is quite achievable, however, when it comes to working below the waterline, innovative solutions must be created for a successful project. Many times, these solutions involve constructing cofferdams.
What Are Cofferdams?
Cofferdams are temporary structures created within a body of water to allow construction crews to completely dewater an area, allowing them to perform underwater construction such as laying foundations, pouring concrete, welding, and oil rig construction. Cofferdams can be completely customized for any given project and can be built in many different types and configurations to suit the needs of the project. Often, the area must be dredged with submersible dredge pumps to remove soil and fine silts in order to level the ground before the cofferdam construction can begin.
Types of Cofferdams
- Braced Cofferdam
- Single and Double-walled Sheet Pile Cofferdam
- Earth Embankment Cofferdam
- Rock Fill Cofferdam
- Cellular Cofferdam
Braced cofferdams are formed from a single all sheet of piling. This sheet is then driven into the ground in sections around the planned excavation site, creating a box. This box is then braced with struts and/or beams on the inside to keep water from getting inside. Once the box has been braced, construction crews will begin to dewater the area inside the box, allowing for construction on the seafloor. Braced cofferdams are primarily used in shallow water environments for projects such as pier construction.
Cofferdam construction project underway
Single/Double-walled Sheet Pile Cofferdam
Sheet piling is an earth retention and excavation support technique that retains soil, using steel sheet sections with interlocking edges. They can be single or double walls depending on the demands of the project. These sheet piles are then often securely braced to support the weight of the water the sheet pile will be retaining, therefore allowing dewatering and construction to commence.
Earth Embankment Cofferdam
These are the simplest type of cofferdam construction methods, well suited for depths of water up to 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 riprap to mitigate erosion. A successful cofferdam does not necessarily need to be completely watertight. From an economical sense, it is not possible to make it watertight and hence some seepage of water into the excavation is usually expected and tolerated to an extent. The water collected through seepage is pumped out of the excavation site to keep the construction area as dry as possible. The earth embankment cofferdams are often constructed with a minimum freeboard of about 1m to prevent waves or high tides from spilling over the embankment. Sand-bag cofferdams are also used, primarily in an emergency situation, but can be less efficient.
Rock Fill Cofferdam
Rockfill cofferdams made of rockfill are sometimes used to enclose the site to be dewatered. Rockfill can be described as an embankment of compacted, free-draining sand and rocks. Rockfill cofferdams are permeable and are usually provided with an impervious membrane of soil to reduce the rate of seepage that must be pumped out to keep the area dry.
Cellular cofferdams are typically constructed by driving sheet piles of particular shapes to form a series of cells. These cells are designed to be interconnected to form a watertight wall rigid enough to retain water. These cells are then filled with soil to help provide an additional stabilizing force against the lateral pressure of the water. Cellular cofferdams are often built as a circular type or diaphragm type, depending on the project specifications and location.
How EDDY Dredge Pumps Excel at Cofferdam Construction Projects
Due to the fact that in many cases, the proposed construction site of a cofferdam is over a body of water, this area must be dredged to ensure a flat foundation in which to build the cofferdam. Typically this is accomplished with submersible dredge pumps, which can be prone to clogging and wear and tear from the constant pumping of abrasive sand, mud, and silt.
The EDDY Pump makes a top choice for a cofferdam dredge pump solution due to our ultra-recessed rotor which makes it highly resistant to abrasion and able to pump a much higher amount of solids than traditional dredge pumps. This allows for faster construction and far less pump maintenance, saving companies ample amounts of time and money and faster project completion times.
Depending on the project specifications, we have several dredging solutions to fit the needs of a cofferdam construction project including our Excavator Dredge Pump Attachment with Cutterhead, Diver-operated submersible dredge, or a cable deployed dredge that can be used from a barge or crane. Additionally, all of these units are completely modular, meaning the same unit can be used for multiple jobs if the scope of the project changes.
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