5. Rubble Mound Structures
169
This encompasses an approach trestle (an open jetty supported on piles),
a breakwater (may be of caisson type or rubble mound type). Any of the
foresaid OWC System can easily be integrated into breakwater or even
with the approach trestle. This however needs a careful and detailed investigation.
5.4 Design Principles of Rubble Mound Structures
The design of rubble mound breakwater involves establishing the unit
weight of armour stones which is primary requirement. The weight of
armour stones for the primary layer is based on the Hudson’s formula
given by
w=___ >xg3___
Kd x (Sr — l)3cota
(5-1)
7S : Unit weight of angular quarry stones or artificial armour blocks =
2.65 t/m3 and 2.76 t/m3 for quarry stones.
H: Design wave height at toe of breakwater
Kd- Stability coefficient — 3.5 for roughly angular randomly placed
quarry stones
= 6.0 for tetrapods.
7s
unit wt of material
2.65
0 T •
—
_
—
— . ô (
7y
umt wt of water
1.03
a: Slope of the breakwater (for a slope 1:2, Cot a — 2)
The dimensionless stability coefficient, Kd in Hudson formula accounts
for ail variables other than structure slope, wave height and spécifie gravity
of water at the site.
These variables include:
1. Shape of armour units
2. Number of units comprising the thickness of armour layer
3. Manner of placing armour units
4. Surface roughness and sharpness of edges of armour units (degree of
interlocking of armour units)
5. Type of wave attacking structure (breaking or non-breaking)
6. Part of structure (trunk or head)
7. Angle of incidence of wave attack
The weight of the individual armour unit is proportional to H3 which
warrants careful évaluation of H. For deeper waters wherein, the wave