For a deepwater site or one with an extremerange between high and low tides, a rubble-moundbreakwater or jetty may by topped with a cap struc-ture to form the composite type shown in figure10-23. In this case, the cap structure consists of aseries of precast concrete boxes called caissons,each of which is floated over its final location andsunk into place by filling with rock. A single-piececoncrete cap is then cast in place on the top of eachcaisson. Breakwaters and jetties are sometimes builtentirely of caissons. A typical caisson breakwater/jetty is shown in figure 10-24. A jetty may also beconstructed to serve as a wharfage structure. If so,it is still called a jetty.ALONGSHOREAlongshore structures include seawalls, groins, andbulkheads. Their main purpose is to stabilize a shore-line.SeawallsSeawalls vary widely in details of design andmaterials, depending on the severity of the exposure,the value of the property to be protected, and otherconsiderations. Basically, though, they consist ofsome form of barrier designed to break up or reflectthe waves and a deep, tight cutoff wall to precludewashing out of the sand or soil behind and under thebarrier. The cutoff wall is generally constructed oftimber, steel, or concrete sheet piling. Figure 10-25Figure 10-25.-Riprap seawall.10-13
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