ケーソン式岸壁の地震応答に及ぼす地盤物性値と幾何学的非線形性の影響に関する感度解析

Abstract

地盤・構造物系の地震時被害を数値解析により推定する場合, 解析に必要な地盤物性は確定値として用いられることが多い. しかし, 地盤物性のばらつきを考慮すると, その設定には常に不確実性が伴うこととなる。本研究では, ケーソン式岸壁を対象に, 地震時挙動に及ぼす地盤物性値の影響を把握するため, 液状化層の地盤パラメータを変化させた感度解析を実施した. その結果, ケーソン下の置換砂における内部摩擦角の不確実性が, 地震後のケーソン天端の残留変形量のばらつきに最も寄与することがわかった. また, 微小変形解析では地盤物性のばらつきが地震時のケーソン変形量に及ぼす影響を過大に評価してしまう危険性があり, 大変形解析により幾何学的非線形性を適切に考慮することで, より合理的な耐震性能の評価が可能になることが明らかとなった.

Past experience has shown that port structures are often susceptible to severe damage during major earthquakes. From field observation of 1995 Hyogo-ken Nambu earthquake, it is observed that the seismic behavior of port structures such as caisson-type quay walls shows significant variability due to the variation in soil properties. In this study, seismic response analyses using a two-dimensional FE mesh, representing a caisson-type quay wall located in Rokko Island, are performed to carry out the sensitivity analysis the seismic performance of the caisson-type quay wall. To investigate the effect of uncertainties of soil properties on the dynamic behavior of the quay wall, a tornado diagram is used. It has been found that the uncertainties in the friction angle of reclaimed soil contribute most to the variability of the residual displacement and inclined angle of the quay wall. In addition, a Monte Carlo simulation considering the variation in soil properties has revealed that taking into account the effect of geometrical nonlinearity by using a large deformation analysis makes it possible to rationally assess the seismic performance of port structures while infinitesimal deformation analyses can overestimate the effect of the variation in soil properties on the deformation of port structures during earthquakes.