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Last edited 26 Apr 2021
Foundations can often be required for the construction of harbour walls, sea walls, breakwaters, and so on. Underwater foundations, also known as subaqueous foundations, may be used in situations where the use of a cofferdam or caisson is prohibitively expensive or unfeasible.
Typically, dredgers or grabs working from pontoons will excavate the ground and load the spoil into barges. If rock needs to be removed, drill and blast methods can be used, or drilling and cutting by jackhammer. Softer rock formations can be dredged or broken up by air tools and then grabbed. Gravel and sands can be excavated using a suction dredger or ejector tube.
Divers can then place temporary steel formwork, using sandbags to anchor them in place and prevent concrete leakage from under the toe of the form. If the foundation is relatively short and in shallow water, bagged concrete or cement placed in layers may be used as formwork and left in position. Foundations in deeper water may incorporate permanent formwork in the form of precast concrete blocks which are tied in to the foundation.
The concrete mix must have a high degree of workability to maintain a good flow and prevent too much cement being washed out of the mix. This ensures that the leading edge of the concrete is kept moving forward under the pressure of additional material. Extra cement should be added to offset any loss through the water, although too much cement will result in excessive laitance forming on the concrete surface.
Concrete is generally placed by one of three methods:
A tremie is a steel tube suspended in the water from a crane. A hopper is fixed to the top end to receive the concrete. The tremie is positioned vertically over the area to be concreted with the lower end resting on the bottom. After the pipe has been filled with concrete it is raised off the bottom to allow it to flow. This should allow the flow to continue to feed the interior of the initial mass.
The flow rate of concrete is controlled by raising and lowering the tremie, but care must be taken not to lift the tremie out of the concrete mass, otherwise the seal will be broken and the concrete will be weakened by water as it rushes out of the pipe. This may result in a damaged surface to the mass of concrete which will need to be removed before work can recommence. One tremie will serve an area of approximately 30 sq. m, and where all the concrete cannot be placed from one position, more than one tremie can be used for simultaneous placing.
Bottom-opening skips can be used, which are fitted with a top cover held in position by water pressure. The skip is filled with concrete before being gently lowered into the water. On reaching the bottom, the skip is emptied.
If it is lowered too rapidly, the covers may be disturbed and the concrete inside damaged. This is a slower method than a tremie pipe but can be more practical for thin beds, whereas tremies are more appropriate for large pours.
Pipe lines typically have a diameter of 110-150 mm and are generally capable of pumping for a distance of 300 m horizontally and 30 m vertically. Pumps may be mounted on a pontoon with an extendable steel boom. The pump works hydraulically with a diesel-driven piston that forces concrete along the pipes.
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