益城町の2016年熊本地震被害集中域の表層に分布する凝灰質土の物理特性・動的変形特性  [in Japanese] PHYSICAL AND DYNAMIC PROPERTIES OF THE VOLCANIC ASH SOIL IN THE HEAVILY DAMAGED SITE OF THE 2016 KUMAMOTO EARTHQUAKE, MASHIKI TOWN  [in Japanese]

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Abstract

2016年熊本地震で益城町の狭い範囲に被害が集中した一因として表層地盤の増幅特性が推察されている.本研究ではこの推察の検証材料として当該地域に分布する凝灰質土の地盤特性の実測値を把握することを目的に,被害集中域内にある本震観測点の近傍でトリプルサンプラーにより不攪乱試料を採取し,物理特性と動的変形特性を室内土質試験より求めた.以下に凝灰質土の試験結果をまとめる.(1) 粒径のばらつきは大きく,その土粒子の密度は浅部の粘土よりもAso-4の砂・砂礫の方が小さい.(2) 動的変形特性は拘束圧と土質にほとんど依存せず,ひずみ依存性はせん断剛性比が砂,履歴減衰率が粘性土の一般的な傾向に類似している.また同地点で密度検層などから密度の深度分布を調査し,各土質の密度が周辺地盤と同程度であることも確認した.

 Some studies indicated one possible factor; the spatial difference in site amplification effect of the surface ground should bring on the heavy damage to the limited area in Mashiki town during the 2016 Kumamoto earthquake. In order to investigate the actual soil property of the area for the experimental proof of the above mentioned assumption, we obtained undisturbed volcanic ash soils of the surface ground at the site close to the seismic station recording the M<sub>J</sub> 7.3 mainshock in the heavily damaged area by using the triple-tube sampler. The obtained features of physical and dynamic soil properties of the soil core sample based on the laboratory soil testing are summarized as follows; (1) The grain size of the volcanic ash soil varies widely. (2) The soil particle density of sand and gravel is lighter than that of clay. (3) The dynamic properties of the volcanic ash soil do not depend on the soil type and the confining stress, i.e., the depth. (4) The strain dependencies of shear modulus ratio and damping ratio of the volcanic ash soil are similar to the standard ones of sand and clay, respectively. In addition, we investigated the depth distribution of density at the same site by the loggings, and recognized the similarity in density of each soil type around the area.

Journal

  • Journal of Japan Society of Civil Engineers, Ser. A1 (Structural Engineering & Earthquake Engineering (SE/EE))

    Journal of Japan Society of Civil Engineers, Ser. A1 (Structural Engineering & Earthquake Engineering (SE/EE)) 73(3), 552-559, 2017

    Japan Society of Civil Engineers

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