soil, they often cannot prevent slides where failure occurs beneath the surface.
This limits their
effectiveness to sites where the backfill and structure are low.
Bulkheads protect only the land immediately behind them and offer no protection to adjacent areas
up and down the coast or to the fronting beach. In fact, because bulkheads normally have vertical faces,
wave reflections are maximized, wave heights and overtopping may increase, and scour in front of the
structure is more likely. In addition, if downdrift beaches were previously nourished by the erosion of
land now protected, they may erode even more quickly. If a beach is to be retained adjacent to a
bulkhead, additional structures such as groins or breakwaters may be required.
Since scour can be a serious problem, toe protection is necessary for stability. Typical toe
protection consists of quarrystone large enough to resist movement by wave forces, with an underlying
layer of granular material or filter cloth to prevent the soil from being washed through voids in the scour
apron. Flanking (erosion of the shore around the ends of the structure) can also be a problem. This can
be prevented by tying each end into existing shore protection devices or the bank.
Sheet Pile Bulkheads
Sheet pile bulkheads consist of interconnecting or very tightly spaced sheets of material driven
vertically into the ground with special pile-driving equipment. The sheeting can be made of steel,
aluminum, or timber (Figures 8, 9, and 10). Sheet pile structures may be either cantilevers or anchored.
A cantilever bulkhead is a sheet pile wall supported solely by ground penetration, making it
susceptible to failure from toe scour. The sheet piling must be driven deep enough to resist overturning,
which usually requires penetration to a depth two to three times the free standing height, including the
anticipated scour depth (usually about one wave height).
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