濟州外港에서 暴風波浪荷重에 依한 砂質土地盤에 施工된 防波堤의 沈下

Abstract

In this study, to prove the wave-induced settlement, the long-term measured settlement data at Jeju harbor caisson breakwater were analyzed. Additionally, a wave load acting on the structure and seabed was calculated using 3D-NIT (Three-Dimensional Numerical Irregular wave Tank). The wave load data was used as an input data in geotechnical analysis program. The following conclusions were drawn.

Generally, the structure built on sand ground predominantly depends on immediate settlement. However, based on the field measurement in Jeju harbor area in which the silicate and carbonate mixed sand exists, it shows that in addition to the immediate settlement a considerable additional compression has been occurred with time due to crushing, shattering, and rearrangement of sand particles, especially carbonate sand particles. This material related settlement has been already studied. However, the settlement data show extra unordinary settlement which is related to wave load due to storm. The study on the wave-induced settlement has not been conducted so far. In this study, we focused on the wave-induced settlement affected by Typhoon.

In Jeju harbor caisson, the settlement caused by wave load was identified. Settlement, excess pore pressure, and effective stress path histories were estimated. The ground settlement was a little different at each measured location but practically the same. Settlement at the leeside is little bigger than that at the seaside.

As seen settlement history, wave load induced settlement clearly occurred so that the effect of wave on settlement was verified. Comparing with wave distribution, the ground settlement was not occurred during regular wave condition, but the settlement increased with wave. The rapid and abruptly large settlement occurred at the maximum wave load. After the maximum wave load the settlement still continuously occurred but the magnitude is smaller than that occurred at the maximum wave load.

The change of excess pore pressure ratio showed a similar pattern to location and depth of ground. The excess pore pressure ratio reached up to 0.9 at the ground just below the toe of rubble mound. When the excess pore pressure ratio reaches up to 1 and we can define the soil becomes liquefied. The rest of area showed 0.5 of excess pore pressure ratio. This excess pore pressure ratio was not enough to induce soil liquefaction, but could decrease the effective stress and cause the ground settlement.

As seen the effective stress path in p'-q' diagram, the ground at the toe of rubble mound was the most vulnerable location. As increased the excess pore pressure, the effective stress path moved to left side. Comparing with Mohr-Coulomb failure line, the ground at the toe of rubble mound closely reached to the failure line. It indicated the decrease of ground strength.

From this study, we can clearly see the effect of wave load on settlement in coastal structure built on sandy soil so that this effect should be incorporated in design and maintenance. Additionally a study of guideline about the structure installation considering the storm wave load induced settlement should be conducted urgently.