Hydro- and Soil- mechanical Study on Coastal Structure Damage due to Wave Motion and its Prevention

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So many damages of coastal structures due to storm waves are reported every year. We considered that such failures of structure are mainly caused by the liquefaction of sea bed around the structure. In this research we treated such dynamical problem as the interaction problem of soil and water. The following results are obtained in this research.1. The processes of progressive liquefaction, densification and re-liquefaction of loosely deposited sand bed under oscillating water pressure were clarified experimentally and theoretically for one-dimensional model. In the theoretical treatment the elasto-plastic model was adopted.2. The centrifuge wave test with dynamical similarity was carried out. The liquefaction and densification process of sea bed were reproduced in the experiment. These processes were explained theoretically by two-dimensional elasto-plastic model. Further more, the dynamic behavior of sand and rock around the rubble mound breakwater were observed in this experiment.3. The lift up of liquefied sand and the movement in the water were simulated numerically by the coupling of manifold method and discontinuous deformation analysis.4. The effect of pore water pressure in the sand bed on the sediment transport was investigated. The increase of the sediment transport was explained by the numerical movable bed model.5. The flow of back filling sand behind vertical sea wall was treated experimentally. The qualitative characteristics of flow were related to the water pressure variation and properties of material.6. The validity of the counter measure to prevent the flow of back filling sand was shown by the field test.7. Three-dimensional numerical model to analyze the movement of block in the liquefied sand layer was developed.8. The local scour around bridge pier under the co-existence of wave and current is experimented. It is found that the amount of maximum scour depth becomes larger than the scour depth under the steady flow condition.