Research Trends and Results

Research on appropriate public capital facilitymanagement levels considering change over time ofactions and performance—Towards building strategic maintenance methods
for breakwaters—
MIYATA Masafumi, Head (Dr. Eng.)TAKENOBU Masahiro, Senior Researcher
ASAI Shigeki, Researcher
Port Facilities Division, Port and Harbor Department(Key words) Breakwater, maintenance, life cycle cost

2. Outline of results of the research
The following are results of a trial calculation of
appropriate block settlement management of a caissontype breakwater covered with wave dissipating blocks.
As shown in Figure 1, wave dissipating blocks on awave dissipating block covered breakwater often
gradually settle over time because of scattering
produced by high waves during typhoons. If thissettlement is ignored, wave force acts directly on the
caisson body, and according to circumstances, thewaves may break so the impact wave force acts on thecaisson, resulting in the high probability of even more
severe damage, such as the caisson sliding off the
rubble mound. But generally, even if a wave
dissipating block settles, it is often ignored, because atnormal times, breakwaters remain adequately stable
under the normal waves, ensuring a degree ofcalmness behind the breakwater, so its repair is judged
to be of low urgency.
So we used a newly developed tool to perform a
trial calculation to determine what degree of
settlement of a wave dissipating block requires repair
work (settlement repair start criterion) in order to beable to minimize the life cycle cost. Figure 2 shows
the results of a trial calculation of an actual breakwatersection, with the lateral axis showing the settlement
repair start criterion of wave dissipating blocks, andthe vertical axis plotting the anticipated value andstandard deviation of the total value of parts [2] and[3] of the life cycle cost explained above.
The figure shows that repairing the block
immediately after the settlement of the block becomesequal to the height of about 1 wave dissipating, it will
be possible to minimize the anticipated value of thelife cycle cost, and at the same time, limit theFigure 1. Image of Progression of Damage to aCaisson Type Breakwater Covered with Wave
Dissipating Blocks
Settlement of wave
wave force
消波工の沈下 → Increased
波力の増加
dissipating blocks
Increased risk of→大規模被災のリスク増大
large-scale damage
Wave
dissipating
消波工blocks

ケーソン
CaissonRubblemound捨石マウンドFigure 2. Relationship of Settlement Repair Start
Criterion and Live Cycle Cost of a Caisson Type
Breakwater Covered with Wave Dissipating Blocks
3,000
Value of direct damage+ indirect damage(millions of yen)
直接+間接被害額（百万円）

1. Outline of the research
The purpose of the research is to develop a methodthat can calculate anticipated values and scattering oflife cycle costs during the period of use of breakwaters,
to study appropriate maintenance and managementlevels (safety level) of breakwaters. Life cycle cost has
been defined as the cumulative total during the periodof use of [1] initial cost of construction of the
breakwater, [2] cost of repair of the breakwater when
it is deformed and cost of restoration when it isseverely damaged (value of direct damage), and [3]logistics rerouting cost incurred in a port if abreakwater protecting the port is damaged (value ofindirect damage).

(10m)

2.0
2.0個

2,500
1.5
1.5個

2,0001.0
1.0個

1,500

0.5
0.5個

標準偏差
Standard deviation
期待値
Anticipated value

1,000
5000
0.0

2.0

4.0

6.0

8.0

Block
settlement repair start criterion (m)ブロック沈下補修開始基準（m）

10.0

possibility of severe damage when a disaster occurs.
These are limited study results, but they clearly showthat at a wave dissipating block covered breakwater,
diligently repairing settlement of blocks is a rational
choice from the perspective of reducing total cost.
3. Conclusion
In the future, we will verify this model and improve
it so it can used for practical work.
[Sources]1) M. Miyata, et al.: Research on maintenancemethods considering the anticipated repair cost ofwave dissipating block covered breakwaters, Journalof the Japan Society of Civil Engineering, B2 (CoastalEngineering) Vol. 65, No. 1, pp. 911-915, 2009.
2) T. Nagao et al.: Study of target wave stability levelof breakwaters, Journal of the Japan Society of CivilEngineering, B2 (Coastal Engineering) Vol. 67, No. 2,
pp. 781-785, 2011.