Daiki Tsujio

A Study on Performance Assessment Framework of Breakwaters against Wave and Tsunami Action

This study discusses the performance assessment framework of breakwaters against wave and tsunami action. As both of the performance assessment methods were established individually, relevance of the two methods are not necessarily clear. Authors proposed to regard performance assessment against wave action as serviceability assessment and that against tsunami action as safety assessment. Criteria for serviceability assessment should be determined by economical point of view and life cycle cost analysis should be used for that assessment. Authors also discussed on the performance assessment method against tsunami action whose magnitude is larger than that assumed in the design procedure. Some design examples are shown considering both wave and tsunami action.

Applicability of Numerical Simulations for Ductility Design of Breakwaters against Mega Tsunami

The design to strengthen coastal defence structures with their ductility is going to be employed. This study applies three different numerical models; CADMAS-SURF2D-OpenFOAM and MPS, to verify their accuracy compared with the hydraulic experiments of breakwaters under tsunami load and to clarify their applicability for ductility design. In case that tsunamis overflow crown heights of breakwaters extremely, it was suggested that tsunami velocity behind breakwaters could not be simulated accurately by CADMAS-SURF2D, while OpenFOAM and MPS had good agreement with experimental results. It was also found that numerical results may lead to underestimate necessary weights of armor blocks.

A Study on Simple Estimation Method of Target Safety Level of Breakwaters

近年,ライフサイクルコスト(以下,LCC)最小化の 観点から,新設,既設を問わず,土木構造物のLCCを考 慮した設計手法が検討されている.道路や橋梁等の土木 構造物あるいは桟橋等の係留施設に対しては,構造物の 劣化予測から維持補修費を計算し,LCCを求める手法が 提案されている.しかし,本研究で対象としている防波 堤に関しては,上記の構造物とは異なり,毎年のように 来襲する高波浪に伴う被害に対する補修の方が構造物の 劣化よりも卓越する場合が多い. こうした背景のもと,防波堤に関するLCC最小化の検 討が進められている(例えば,吉岡・長尾,2004).防 波堤のLCC算出法は,滑動量解析から滑動量に応じた被 害額を算出し,これに初期建設費を加えたLCCが最小と なる断面を検討するものである.また,高山ら(2009) は,防波堤や護岸の被覆材に着目し,設計供用期間中の ブロック補修費を考慮して,初期建設費からLCCを算出 する方法を提案している.さらに,宮田ら(2009)は, 既存の消波ブロック被覆堤を対象として,高山らのブロ ック補修費に加えて,過去の被災事例から大規模な被害 に対する復旧費をモデル化し,ブロック補修費を加えた 維持管理費が最小となるブロック補修基準を検討してい る.また,長尾ら(2010)は,宮田らの復旧費モデルの 改良に加え,防波堤の平面的な被災を考慮して,直接被 害額を計上し,維持管理費が最小となるブロック補修水 準を検討している.LCCの算出に関して,長尾ら(2011) は,直接被害額に大規模な被災に伴う間接被害費用を計 上し,異なる耐波安定性を有する防波堤断面に対して, LCCを最小にする耐波安定性水準を検討している. しかし,防波堤に関する既往のLCC検討事例は,限ら れた検討条件によるものであり,LCC最小となる耐力作 用比を一般化するまでの情報は得られていない.また, モンテカルロシミュレーション(以下,MCS)は計算負 荷が大きいため,LCCの評価は設計実務には取り入れら れていないのが現状である. そこで本研究では,まず混成堤と消波ブロック被覆堤 (以下消波堤)に対して,様々な波浪条件,被害額条件 を設定して防波堤のLCC最小の耐力作用比(以下,目標 耐力作用比)を評価した.次に目標耐力作用比と設計パ ラメータの相関を議論した上で,目標耐力作用比の簡易 推定法を検討し,詳細法と簡易推定法による目標耐力作 用比の差異を検討した.

A Study on the Target Safety Level of Breakwaters against Wave Force

This study discusses the target safety level of breakwaters against wave force in view of minimizing the life cycle cost. We considered both damage recovery cost and economic loss cost in the calculation of life cycle cost. We found that target safety level changes according to the condition such as extreme value distribution of waves and ratio of economic loss cost to damage recovery cost. Target safety levels of composite breakwaters were found to be different from those of breakwaters covered with wave dissipating blocks. In addition, target safety levels corresponding to the minimum life cycle cost were found to be different from the allowable safety levels by the technical standards.

A Study on the Maintenance Strategy of Breakwaters Armored with Wave Dissipating Blocks Considering Uneven Displacement

It is necessary to maintain wave dissipating blocks for the armor of breakwaters properly because settlement of blocks often causes large disaster. Therefore, clarification of correlation between block settlement and breakwater displacement is important for the establishment of effective maintenance strategy. Previous study discusses maintenance strategy of breakwaters armored with wave dissipating blocks on the assumption of uniform displacement of breakwaters, however, uneven displacement of breakwaters is observed in field. This study considers uneven displacement of breakwaters and discusses the maintenance criteria for the minimum repair cost during working life of breakwaters.

ESTIMATION OF EXPECTED SLIDING DISTANCE OF A CAISSON AFFECTED BY DAMAGE OF ARMOR CONCRETE BLOCKS

The damage of the concrete blocks causes their own settlement together with the sliding of the caisson. The settlement increases wave forces acting on the caisson. The bigger wave forces enlarge the sliding of the caisson. Thus the damage of the concrete blocks is closely related to the sliding of the caisson. The present paper proposes the estimation procedures of expected sliding distance of a caisson affected by the damage of the armored concrete blocks. It concludes that the insufficient weight of the armor blocks causes the damage of the blocks which affects the sliding of the caisson itself and that the necessary width of the caisson which satisfies the selected allowable value of the expected sliding distance becomes larger under the consideration of the armor block damage than under no consideration of the damage.

A Study on Maintenance Methods Considering Expected Maintenance Costs of Caisson Breakwaters Covered with Wave-dissipating Concrete Blocks

In this study, we propose a new method for caisson breakwaters covered with wave-dissipation concrete blocks, which enables to evaluate cumulative maintenance costs including both ordinary maintenance costs (to raise work level of the blocks) and disaster recovery costs due to an extreme wave load. The method is based on a statistical model (Monte Carlo Simulation) calculating sliding distances of a caisson during its working life. This method can be effectively utilized for planning maintenance strategies, considering how the ordinary maintenance criteria for the settlement of the blocks affects the cumulative maintenance costs including information on both expected value and the its variations.

Factors of Caisson Sliding Failures Affected by Damage of Armor Concrete Blocks

Recently, caisson sliding failures affected by large waves have occurred. The damage of the breakwaters in Susami port due to Typhoon 0423 is a typical sliding failure and the caissons were tilted down in the rear of rubble mound. The damage at Fushiki-Toyama port on February 2008 is also another typical sliding failure and concrete blocks were settled down. The present paper describes the factors of their caisson sliding failures affected by damage of armor concrete blocks. It concludes that the caisson sliding failure at Susami port was caused by insufficient weight of the caisson. Even if heavier concrete blocks are placed as armors, the caisson would be slid. The numerical simulation suggests that the caisson sliding failure at Fushiki-Toyama port was caused by the combination of three factors of large wave height, long wave period and duration time.