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

每年都有许多海岸建筑因风暴潮而遭到破坏的报道。我们认为这种结构的破坏主要是由结构周围海床的液化引起的。在本研究中，我们将这类动力学问题视为水土相互作用问题。本研究得到以下结果：在一维模型下，从实验和理论两方面阐明了振荡水压作用下松散沉积砂床的渐进液化、致密化和再液化过程。理论处理采用弹塑性模型。进行了具有动力相似性的离心波试验。实验再现了海床的液化和致密化过程。用二维弹塑性模型对这些过程进行了理论解释。此外，还对碎石丘防波堤周围的砂石动力特性进行了观察。采用流形法和不连续变形分析相结合的方法，对液化砂的抬升和水中运动进行了数值模拟。研究了砂床孔隙水压力对泥沙输运的影响。输沙量的增加可以用数值动床模型来解释。对垂直海堤后充填砂流进行了实验处理。流动的定性特征与水压变化和材料性能有关。通过现场试验，验证了防治回填砂流动对策的有效性。建立了液化砂层中块体运动的三维数值模型。对波浪和水流共同作用下桥墩周围局部冲刷进行了试验研究。发现在稳定流动条件下，最大冲刷深度的量大于冲刷深度。

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.