報告書番号

U01056

タイトル（和文）

台湾・花蓮地点における大規模耐震実証試験研究 --鉛直アレ－地震観測記録に基づく地盤異方性と地盤非線形性の評価--

タイトル（英文）

The Hualien Large-Scale Seismic Test for Soil-Structure Interaction Research-Detecting Ground Anisotropy, Identifying Ground Properties along Principal Axes and Evaluating Soil Nonlinearity from Vertical Array Seismic Records -

概要 （図表や脚注は「報告書全文」に掲載しております）

良く締まった砂礫地盤から成る花蓮地点にてこれまでに得られた地震記録によれば、3つの鉛直アレ－に共通して伝達関数に異方性が認められる。地表水平最大加速度(PGA)が50gal弱程度の地震記録を用いて逆解析した結果、強軸方向は地盤調査モデルと良く一致しているが、弱軸方向のVsはGL-5～-52.6mの範囲で15-43%程度小さい。この異方性は地震動強度が強くなっても変わらず保持されていることも確認した。一方，同定されたせん断波速度(Vs)・減衰定数(h)をPGAで整理してみると，強軸・弱軸方向を問わず，また成層を問わず，PGAの増加に伴ってVsは減少し，ｈは増加するとの基本的な傾向が認められた。鉛直アレ－での加速度記録からせん断応力～ひずみ履歴を求め，せん断剛性比・減衰定数～ひずみ関係を算定したところ，規準ひずみを多少下回る0.01%程度の最大せん断ひずみを得，ばらつきは大きいものの，室内試験が示す傾向に，基本的には沿う結果を得た。

概要 （英文）

Examining the seismic records obtained at the vertical array in the Hualien site, which consists of well-compacted sand and gravel layers, the noticeable discrepancies between the NS and EW components of the transfer functions of soil layers are observed consistently in all of the three vertical arrays. From the periodical characteristics of the coherence function, the angle of the principal axes of the soil layers was estimated. The angle of the strong axis of the soil layers at this site was estimated to be about 10 degree east from the magnetic north.The system identification was carried out to estimate the soil properties along the principal axes. The identified values of shear wave velocities (Vs) of the free field soil layers for the strong axis, using the seismic records obtained at the site, of which PGAs (Peak Ground Acceleration) are a little smaller than 50gal, almost agreed with those in the ground model 'UMFVT2' obtained by the site soil investigation, such as seismic prospecting. On the contrary, the identified Vs for gravelly layers (GL-5.0m～GL-52.6m) for the weak axis were 15-43% slower than those of 'UMFVT2'. Thus, it was made clear that this site is with ground anisotropy also from the application of the system identification. It was also confirmed that such anisotropic property at this site is kept as well, even when the seismic intensity at the site increases. On the other hand, putting the identified Vs and h (damping constant) into order in terms of PGA of the seismic events used in the analyses, the following basic trends were recognized, that is, Vs decreases and h increases with the increase of PGA, irrespective of strong or weak axis, and also irrespective of soil layers, which is the result of the soil non-linearity.The transfer function of the structure top over the ground surface was also studied. According to the result of the one-dimensional equivalent linear analysis, the predominant frequency shift of the soil-structure system during relatively large earthquakes may be explained by the soil non-linearity to a certain extent.In order to evaluate the site soil non-linearity in further detail, the site shear stress-strain histories at the depths of the observation stations in the vertical arrays were obtained, using the vertical array seismic accelerographs recorded at the site, selecting relatively large earthquakes, and the relationships between shear rigidity ratio, damping and shear strain were estimated. As the result, the curves were basically along those obtained from the laboratory tests, that is, the shear rigidity ratio decreases and the damping increases with the increase of soil shear strain, though they vary widely. The obtained maximum shear strain up to now is 0.01%, which is compatible with 1-D equivalent linear analysis, and which is a little smaller than "the reference strain".

報告者

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