易液化深厚覆盖层地震动放大效应台阵观测与分析

基于美国加州Caltrans/CDMG的两个井下台阵加速度记录,从时域和频域分析了百米级易液化深厚覆盖层在不同地震下的地震动放大效应,揭示了地震动从基岩向覆盖层传播的4个重要特征:(1)加速度放大效应受土层深度影响,近地表20~30 m以内放大效应突出;(2)地震动三分量的放大效应具有方向性,水平向与竖向地震动放大效应差异显著;(3)基岩加速度呈现"小震放大、大震衰减"的规律;(4)加速度放大规律与频率相关,深厚覆盖层放大频带较宽。初步分析了造成上述放大效应的可能原因。在此基础上,基于平面波动假设提出了考虑层间波阻抗比放大和传播路径衰减的深厚覆盖层加速度放大效应简化函数,改进了1/4波长法的参数取值,并结合自由表面效应,对前述台阵记录的放大效应进行了估算,发现无论时域还是频域,预测结果与实际观测都较接近。本文研究成果可为深厚覆盖层液化判别和抗震设计的加速度选取提供理论依据和简化分析方法。

Observations and analyses of site amplification effects of deep liquefiable soil deposits by geotechnical downhole array

Based on the accelerations recorded from two downhole arrays instrumented by the Caltrans/CDMG project, the site amplification effects of deep liquefiable soil deposits are analyzed in both time and frequency domains, and several important features of the wave propagation from the bedrock to the overlying soil layers are observed: (1) The depth of soil deposit affects the site amplification considerably, and large part of the amplification occurs in the near-surface zone within the depth of 20~30 m; (2) The amplification differs from one to another direction, and the difference between the horizontal and vertical shakings is significant; (3) The amplification occurs when the input bedrock motion is small, while the de-amplification effects are observed when the bedrock input motion is large enough; (4) The amplification is frequency dependent, and the deep deposits amplify the bedrock ground motion in a wide frequency band. The underlying mechanisms are preliminarily analyzed. The simplified function accounting for the impedance contrast amplification and thickness attenuation effects is proposed based on the plane wave assumptions, and the parameters are improved for the quarter wave length method. The amplification ratios are predicted for all four events in conjunction with "free-surface effect", where the predictions are found in good agreement with the observations either in time or frequency domain. The present study provides a theoretical basis and simplified method for estimating the ground motions for liquefaction evaluation and seismic design of deep liquefiable soil deposits.