Storm Hazard Mitigation Structures 30.2 Coastal Armoring Structures 665
Part C | 30.2
cient produces the sliding friction resistance force F S
that opposes the horizontal wave force F H . The wider
the caisson width B the greater the sliding friction resistance force to produce a stable structure against sliding
failure.
The uplift force F U is the result of an assumed
triangular uplift pressure distribution with p u maximum on the front edge. Note that p u is not equivalent
to p 3 . The uplift moment arm is 2=3B and the horizontal wave force moment arm is the distance z for
clockwise moments about the heel of the caisson at
location e. The gravity force (W B) times its moment arm, t produces the counter-clockwise moment
about the heel location e to prevent overturning of
the caisson. Here again, the wider the caisson width
B, the greater the resistance to the overturning moments to produce a stable structure against overturning
failure.
The third failure test normally considered is the
soil-bearing pressure at the heel of the caisson structure. Space limitations do not permit a discussion here.
See the CEM [30.1].
Global safety factors can be applied for the sliding, overturning, and bearing pressure formulations but
they are not recommended because they are not related
to the probability of failure of the structure. Modern,
probabilistic design methods now employ partial safety
factors. For sliding failure, conditions for deep and shallow water, and design with and without model tests
are presented for both the load partial safety factor H
for wave height and the resistance partial safety factor Z for the friction coefficient. See Table VI-6-24 in
the CEM [30.1]. Note that two levels of knowledge of
the wave height coefficient of variability
0 (0:05 for
good knowledge and 0:2 for relatively poor knowledge)
are also presented for use to estimate the appropriate
five levels (0:01, 0:05, 0:10, 0:20, 0:40) of probability
of failure P f . Similar summary tables of partial safety
factors for overturning (Table VI-6-25) and for bearing
pressure foundation failure for both sand subsoil (Table VI-6-22) and clay subsoil (Table VI-6-23) are found
in [30.1].
In all the tables of partial safety factors in [30.1], the
design wave height, H design , is taken as the mean (carrot
symbol) of the long term, statistical probability distribution for the significant wave height (subscript s) at
the recurrence interval of choice (superscript T) noted
as O
H
T
S . Goda [30.4] recommends using H design as 1.8 H s
if seaward of the surf zone and the highest of the breaking waves when within the surf zone. The use of the
1:8 factor corresponds to 0:15% exceedance value in the
Rayleigh distribution included in 1=250 total number of
waves. Use of H design D 1:8H s in the specification of the
design wave height is equivalent to using a higher recurrence interval wave height. For example, in Fig. 30.3, if
the design wave height was 4 m (T D 50 yrs), then 1:8
H s is about 7 m at a recurrence interval, T of 5000 yr.
Tables of partial safety factors [30.1, Tables VI-6-22,
23, 24, 25], all specify using O
H
T
S .
The original Goda method is normally for design
of relatively long structures where the structure length
in effect spreads the wave loads along the length of
the structure. This horizontal spreading of the wave
loads calculated per unit length of structure is an added
factor of safety in applying the original Goda formulations for the pressure p 1 at the water surface as
shown in Fig. 30.8 for both nonbreaking and breaking
waves [30.37]
However, under some wave-breaking conditions,
very high wave impact (aka, impulsive, slamming,
hammering) loads of short duration occur that produce greater wave loads than estimated by the original [30.34] formula. Takahashi [30.38] has modified
the pressure p 1 at the still water level (SWL) to take
this slamming force into account. [30.1] advises that
these relatively small-scale model results do not correctly include air pockets trapped between the wall and
the slamming wave. The use of the Minikin method as
found in [30.39] is recommended [30.1, p. VI-5-162]
even though it is well know that the Minikin method
gives very high results.
The need for improved knowledge for the wave impact loads, geotechnical aspects, and structural analysis
aspects of vertical breakwater design resulted in the
PRObabilistic Design Tools for VErtical BreakwaterS
(PROVERBS) project in the European Union (1994–
1998) as summarized below.
The results of the PROVERBS project are summarized by Oumeraci et al. [30.5]. Only the wave impacts
are presented here. The interested reader should consult this reference for geotechnical aspects, structural
aspects, and probabilistic design tools that are applicable for vertical, monolithic structure design.
Three major advances have resulted from the
PROVERBS project, namely:
1. A parametric decision map has been developed
(Fig. 30.10) to provide a calculation procedure to
identify the breaker-type in front of the vertical
structure.
2. The breaker-type depends on the structure (four
structure types are identified and studied) as
well as the wave parameters approaching the
structure.
3. A 12-step design procedure has been developed that
includes new formulas to calculate the wave impact
Part C | 30.2
cient produces the sliding friction resistance force F S
that opposes the horizontal wave force F H . The wider
the caisson width B the greater the sliding friction resistance force to produce a stable structure against sliding
failure.
The uplift force F U is the result of an assumed
triangular uplift pressure distribution with p u maximum on the front edge. Note that p u is not equivalent
to p 3 . The uplift moment arm is 2=3B and the horizontal wave force moment arm is the distance z for
clockwise moments about the heel of the caisson at
location e. The gravity force (W B) times its moment arm, t produces the counter-clockwise moment
about the heel location e to prevent overturning of
the caisson. Here again, the wider the caisson width
B, the greater the resistance to the overturning moments to produce a stable structure against overturning
failure.
The third failure test normally considered is the
soil-bearing pressure at the heel of the caisson structure. Space limitations do not permit a discussion here.
See the CEM [30.1].
Global safety factors can be applied for the sliding, overturning, and bearing pressure formulations but
they are not recommended because they are not related
to the probability of failure of the structure. Modern,
probabilistic design methods now employ partial safety
factors. For sliding failure, conditions for deep and shallow water, and design with and without model tests
are presented for both the load partial safety factor H
for wave height and the resistance partial safety factor Z for the friction coefficient. See Table VI-6-24 in
the CEM [30.1]. Note that two levels of knowledge of
the wave height coefficient of variability
0 (0:05 for
good knowledge and 0:2 for relatively poor knowledge)
are also presented for use to estimate the appropriate
five levels (0:01, 0:05, 0:10, 0:20, 0:40) of probability
of failure P f . Similar summary tables of partial safety
factors for overturning (Table VI-6-25) and for bearing
pressure foundation failure for both sand subsoil (Table VI-6-22) and clay subsoil (Table VI-6-23) are found
in [30.1].
In all the tables of partial safety factors in [30.1], the
design wave height, H design , is taken as the mean (carrot
symbol) of the long term, statistical probability distribution for the significant wave height (subscript s) at
the recurrence interval of choice (superscript T) noted
as O
H
T
S . Goda [30.4] recommends using H design as 1.8 H s
if seaward of the surf zone and the highest of the breaking waves when within the surf zone. The use of the
1:8 factor corresponds to 0:15% exceedance value in the
Rayleigh distribution included in 1=250 total number of
waves. Use of H design D 1:8H s in the specification of the
design wave height is equivalent to using a higher recurrence interval wave height. For example, in Fig. 30.3, if
the design wave height was 4 m (T D 50 yrs), then 1:8
H s is about 7 m at a recurrence interval, T of 5000 yr.
Tables of partial safety factors [30.1, Tables VI-6-22,
23, 24, 25], all specify using O
H
T
S .
The original Goda method is normally for design
of relatively long structures where the structure length
in effect spreads the wave loads along the length of
the structure. This horizontal spreading of the wave
loads calculated per unit length of structure is an added
factor of safety in applying the original Goda formulations for the pressure p 1 at the water surface as
shown in Fig. 30.8 for both nonbreaking and breaking
waves [30.37]
However, under some wave-breaking conditions,
very high wave impact (aka, impulsive, slamming,
hammering) loads of short duration occur that produce greater wave loads than estimated by the original [30.34] formula. Takahashi [30.38] has modified
the pressure p 1 at the still water level (SWL) to take
this slamming force into account. [30.1] advises that
these relatively small-scale model results do not correctly include air pockets trapped between the wall and
the slamming wave. The use of the Minikin method as
found in [30.39] is recommended [30.1, p. VI-5-162]
even though it is well know that the Minikin method
gives very high results.
The need for improved knowledge for the wave impact loads, geotechnical aspects, and structural analysis
aspects of vertical breakwater design resulted in the
PRObabilistic Design Tools for VErtical BreakwaterS
(PROVERBS) project in the European Union (1994–
1998) as summarized below.
The results of the PROVERBS project are summarized by Oumeraci et al. [30.5]. Only the wave impacts
are presented here. The interested reader should consult this reference for geotechnical aspects, structural
aspects, and probabilistic design tools that are applicable for vertical, monolithic structure design.
Three major advances have resulted from the
PROVERBS project, namely:
1. A parametric decision map has been developed
(Fig. 30.10) to provide a calculation procedure to
identify the breaker-type in front of the vertical
structure.
2. The breaker-type depends on the structure (four
structure types are identified and studied) as
well as the wave parameters approaching the
structure.
3. A 12-step design procedure has been developed that
includes new formulas to calculate the wave impact