8. Design of Coastal Structures
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positioned in front of it. This is commonly referred to as composite breakwaters, which is now called the horizontally composite breakwater. This type
of structures are conventional, since the critical damages in a vertical wall
breakwaters are often strengthened by concrète armour blocks or rubbles
by dissipating the wave energy. Modem horizontally composite breakwaters employ shape-designed concrète blocks such as tetrapods. The design
begins with the study of tension at the base of the structure as shown in
the below équation.
f = If + M
7
A
Z
(8.34)
For zéro tension f — 0.
W
W x e
where, e is the eccentricity of the load, b is the width of the caisson, W is
the dead load of the structure, M is the moment due to the external force
especially wave force and Z is the section modulus of the base.
Check for sliding and overturning
The most commonly addressed failure mode of monolithic structures is
sliding backwards against direct wave action. This primarily dépends on
the horizontal loads however influenced by the uplift force. The design
of a breakwater’s upright caisson section must be stable against sliding
and overturning (Fig. 8.17), and to accomplish this, safety factors against
sliding and overturning must be greater than 1.2. In most cases, sliding
is more severe than overturning, especially when the breakwater crown is
relatively low.
Fig. 8.17 Sliding and overturning of upright section.