Damage analysis of volcanic ash embankment collapse in The 2011 off the Pacific coast of Tohoku Earthquake

東北地方太平洋沖地震では福島県会津若松市にて大規模な盛土崩壊が発生した。崩壊した盛土では液状化の痕跡が確認され，盛土材は火山性堆積物であることから粒子破砕の影響が懸念された。火砕流堆積物は全国各地に幅広く分布しており，人工盛土への適用事例も多い。本研究では現地採取土を用いて，等方圧密やせん断による盛土材の粒度変化を調査するほか，静的および繰返し載荷試験により強度特性を把握した。三軸圧縮試験より，福島現地採取土は有効応力p’=100 kPaまでの等方圧密で粒子破砕率が急激に増加すること，粒子破砕率の増加に伴い，せん断抵抗角が大きく低下することを明らかにした。また，得られた力学特性を用いて盛土崩壊事例に対し地震時変形解析を実施した。その結果，解析値が実変位量を再現し，液状化強度曲線より算出した残留強度と粒子破砕によるせん断抵抗角の減少を考慮することで妥当な解析結果が得られることを示した。A large-scale slope failure of embankment, which consists of volcanic sediment, at Aizu-Wakamatsu City in Fukushima Prefecture occurred due to The 2011 off the Pacific coast of Tohoku Earthquake. Some signatures of liquefaction have been observed in the collapsed embankment. When investigating the embankment slope failure mechanism, it is necessary to study the effects of particle breakage. In this study, some static and repeated triaxial tests were conducted for the soil material sampled from the site in order to investigate the change of the particle size distribution with isotropic consolidation stress and shear stress and the effect on the shear strength characteristics due to the particle breakage. From the test results, it was revealed that the sampled soil, subjected to the effective isotropic consolidation stress of 100kPa or less, increases the particle breakage amount rapidly and the shear resistance angle is significantly reduced with the increase of the particle breakage amount. Subsequently, seismic deformation analysis was carried out for the slope failure of the embankment based on the mechanical properties obtained in the test. As a result, a reasonable analysis result was obtained by considering the calculated residual strength based on the liquefaction strength curve and the reduction characteristics of shear resistance angle due to particle breakage