Seismic Resistant of Breakwater Foundation Reinforced By Steel Sheet Pile and Gabion - Evaluation Through Model Shaking Table Test

Abstract eng:
Many breakwaters were damaged during past gigantic earthquakes and tsunamis. It was reported that breakwaters were failed mainly due to failure of their foundations. Mounds were scoured by the tsunamis. Caissons were slid down from the mound, and sank in sea. Due to failure of the breakwater, the tsunami entered in coastal areas. The tsunami led to catastrophic losses for population and structures near coastlines. The extent of damage of breakwaters during past earthquakes and tsunamis exposed vulnerabilities of conventional breakwaters, and raised the importance of developing earthquake and tsunami resistant breakwater. Countermeasures against compound disasters brought by earthquake and tsunami are essential for construction of earthquake and tsunami resistant breakwaters, which can reduce damage in the future. In order to provide resiliency to breakwater against earthquake and tsunami induce forces, new reinforcing techniques have been developed for foundation of breakwater. The techniques involve use of steel sheet piles and gabions in the foundation of breakwater as reinforcing materials. This paper evaluates newly developed reinforcing techniques for breakwater foundation that can render resiliency to breakwaters against earthquake and tsunami related compound disasters. The gabions improve bearing capacity due to confining effect. It can also protect mound against scouring and toe erosion during tsunami. In this way, subsidence and lateral displacement of the breakwater can be reduced or prevented due to these properties. Sheet piles behave as boundaries, and can restrict lateral deformation of the foundation soils during earthquake due to their bending characteristics. Thus, deformation of the foundation soil can be prevented during liquefaction (high excess pore water pressure) caused by earthquake. Effectiveness of the proposed techniques in mitigating the earthquake induced damage is described through physical modelling. As a part of the physical modelling, a series of shaking table test were performed, and comparisons were made between conventional foundation and proposed reinforced foundation. Test results reveal that the reinforced foundation performs well in reducing the damage to breakwater brought by the earthquake. Settlement and horizontal displacement of the breakwater were reduced significantly by the reinforcing techniques. Installation of sheet piles around the mound could prevent breakwater tilting or overturning during earthquake. It was observed that in the case of earthquake with high intensity and longer duration, the proposed reinforcing techniques provide better mitigation effect. One of the major causes of breakwater settlement is the lateral flow of foundation soils. During earthquake, sheet piles could restrict the lateral flow, and thus could prevent any differential settlement.

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