Instability on the sandy ground under breakwater due to earthquake and tsunami

Abstract

In 2011, the Great East Japan Earthquake occurred and it generated a long-period earthquake motion and tsunami. Many coastal structures were damaged with instability of the bearing ground under structure due to earthquake and tsunami. However, the mechanism of the earthquake-tsunami disaster for the coastal structure has not yet been completely revealed. This paper focused on the destabilization of breakwater due to an earthquake and a tsunami. Additionally, destabilization of a breakwater due to earthquake motion and liquefaction were examined using a soil-water coupled finite element analysis based on the elasto-plasticity constitutive equation and tsunami simulation was calculated by using the particle method. In the case of a complex disaster caused by a huge earthquake followed by tsunami, when the earthquake includes long-period motion on a breakwater for a long time, the excessive pore water pressure was generated in the sandy ground and consequently liquefaction occurred. As the result, the breakwater was settled. Therefore, tsunami will overflow on the breakwater. Next, we investigated stability of breakwater that received tsunami force after the earthquake acted. The margin of bearing capacity on the breakwater was loss by the decreasing of strength in the sandy ground due to tsunami seepage when tsunami acted on it. Moreover, we discussed on the performance of the breakwater by use of numerical simulation result of tsunami flood depth in the land with damage level of the breakwater.