10 Composite Breakwaters - Description

10 Composite/Vertical Wall Breakwaters, Description
Ref:
Shore Protection Manual, USACE, 1984
Basic Coastal Engineering, R.M. Sorensen, 1997
Coastal Engineering Handbook, J.B. Herbich, 1991
EM 1110-2-2904, Design of Breakwaters and Jetties, USACE, 1986
Breakwaters, Jetties, Bulkheads and Seawalls, Pile Buck, 1992
Coastal, Estuarial and Harbour Engineers' Reference Book, M.B. Abbot and W.A. Price,
1994, (Chapter 29)
Coastal Engineering, K. Horikawa, 1978
Topics
Description of Composite/Vertical Wall Breakwaters Design
--------------------------------------------------------------------------------------------------------------------Composite/Vertical Wall Breakwater Design
Description
Composite or vertical wall breakwaters are vertical structures built on rubble
mound foundations. The vertical wall portion usually consists of caisson structures. "A
caisson is a large, generally rectangular concrete box, usually not solid, but composed of
walls and internal diaphragms to give it strength, and filled with sand or gravel to give it
weight. It rests on the sea bottom, or on a layer or mound of stone, relying on its mass to
resist wave action. A string of such caissons can make a continuous breakwater.." (Goda,
1991)
A "vertical breakwater" is generally a free standing caisson (or string of caissons)
resting directly on the bottom, built on an improved foundation probably with a rubble
mat flush with the natural ground. The most common type is the "low mound
breakwater" where the vertical portion rests on a low (relative to water depth) rubble
mound which distributes the load of the caissons to a larger area and reduces the
settlement. The mound therefore allows construction on weaker soil. The foundation is
usually prepared by placing layers of rubble until adequate bearing pressure is obtained
for the complete structure. The combination of weak soil and deep depths may require the
mound size to increase, thus the structure becomes a "high mound breakwater."
The basic design procedures are similar to rubble mound breakwaters, except that
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the wave forces must be explicitly calculated and
the stability of the vertical structure is of concern.
In the rubble mound design, the wave forces are implicitly considered by using
the wave height to calculate the armor layer size and the structure stability is dependent
on the armor layer stability.
The main design concern for composite breakwaters is stability of the vertical
section. The stabilizing force is the net gravity force (i.e. structure weight reduced for
buoyancy) and the upsetting forces are due to wave reflection with additional wave
impact loading considered. The structural stability concerns are sliding and overturning
of the vertical section.