Design Deadman Anchors
Cofferdam
A cofferdam is a structure that is used to divide water from land, generally for the purpose of constructing a structure in a dry environment. Cofferdams are often built using a sheet piling structure that can be installed and then later removed once the structure to be built is complete. In many instances, structures such as bridges, bridge abutments, boat launches and foundation piles utilize cofferdams so that the construction project can essentially work in the “dry” instead of employing divers to build the structure under water. These sheet pilings are often designed using a cantilever design. In a cantilever design, generally 2/3 of the sheet piling is driven below the mud line. The existing 1/3 of the sheet piling is exposed which is what is holding the water out of the constriction area. Often times the interlocks of the sheet pilings have to be sealed so that the transmissivity between the interlocks can be brought down to close to 0 flow rate of water. This prevents seepage of water from entering through the interlocks which could saturate the project to a point that would make construction difficult. The sealant generally works similar to a bead of caulk being applied, but once the sealant comes into contact with a water borne agent it, will expand and create a water-tight barrier.
In recent years cofferdams are increasingly being built out of synthetic sheet piles. Synthetic sheet piles are generally made out of a composite or FRP (fiber reinforced plastic) or a vinyl sheet piling. These synthetic sheet piles are extremely light weight and therefore relatively easy to work with compared to a much heavier steel sheet piling. Additionally the synthetic sheet pilings can be reused on multiple projects or can simply be left in place to act as the new face of the new existing structure.
A cofferdam can also be supported by a system of wales helping to support and hold the sheets back. If a cantilever design is suggested where much of the sheet piling is driven below the surface of the sea bed, it may not practical from a drivability standpoint or cost of sheet piling standpoint to complete in a traditional manner. In this case a support structure of wales can be used to support the wall. It may be necessary in the cofferdam to run tie backs through the wales to a supporting deadman to help support the wall. The dead men will often consist of a driven in round pile, a temporary earth anchor, or in some cased another driven steel sheet piling to hold the wall in place.
Inside a cofferdam, often a series of water pumps will be running to make sure that all possible water is removed from the jobsite location. The pumps will run all day and all night to continue to pump out any water that may have seeped through the interlocks or worked its way through the cofferdam barrier in anyway.
Synthetic sheet piling can be extremely cost effective compared to the traditional sheet piling structure. In recent years, steel prices have escalated at an astounding rate, driving construction costs to a point where they were unable to be awarded due to price constraints. Of the times a composite sheet piling can be substituted for steel sheet piling in a cofferdam structure at a price savings of 25 – 30%. When you consider that shipping steel sheet piling compared to vinyl sheet piling can often triple the costs of freight. Several steel contractors have begun to consider the use of composite materials in cofferdams to lower their overall bid numbers and increase their probability of winning awarded jobs.
In one case recently a project in Florida called for the use of vinyl sheet piling to be used as a cofferdam in the construction of a bridge. The vinyl sheet piling was driven in the area where one the support structures of the bridge was to be. Once the sheet piles were connected in a square shape (picture a peninsula extending from the bank of the river) the construction site was de-watered. Following the dewatering of the site, the foundation was poured and the concrete footer for the bridge was given a chance to set up and cure. In this particular case, the vinyl sheet piling was designed to stay in place following the completion of the footer being cured. Thus the walls of the cofferdam became a permanent part of the completed structure acting as a seawall.
Cofferdams that were once constructed out of steel sheet piling are now being constructed out of synthetic sheet pilings. The benefits range from the saving of money, time of installation, freight charges, and reusability of product. Non-traditional materials are becoming ever popular due to these features and benefits. As composite sheet piling and vinyl sheet piling continue to become stronger and closer to the strength of steel, more and more synthetic sheet pilings will be used in cofferdams. Visit our website for more information and national services Cofferdam.
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Aesop Rock- One Brick