苏州第四纪深厚沉积层一维等效线性和非线性地震反应对比分析

Comparison of 1D equivalent-linear and nonlinear seismic site responses for quaternary deep sediment layers in Suzhou region

合理描述土体动力本构关系对场地设计地震动参数取值的准确性有显著影响。以苏州城区200 m深的钻孔剖面为研究对象,对比分析了一维等效线性波传分析法(ELA法)和基于修正Matasovic本构模型的多自由度集中质量非线性分析法(NLA法)给出的深厚场地地震反应,研究了基岩输入地震动特性和地震基岩面的选取对深厚场地地震反应的影响。结果表明:(1)随基岩地震动强度(PGA)的增大,ELA法给出的地表PGA呈单调递增的特征,而NLA法给出的地表PGA呈先快速增大后缓慢减小或几乎不变的趋势;(2)ELA法和NLA法给出的地表加速度反应谱(Sa谱)在短周期范围内存在明显差异,ELA法对基岩高频地震动具有显著的滤波效应,而NLA法对基岩高频地震动的影响表现为随基岩PGA的增大先放大后减小的特征;(3)随地震基岩面深度的增大,地表Sa谱的谱值呈现出稍许增大的趋势,但对周期T<0.1 s部分,NLA法给出的地表Sa谱的谱值则呈现出稍许减小的现象;(4)中、大震作用下,地表地震动持时不仅与基岩地震动特性有关,还与地震反应分析方法和地震基岩面的选取密切相关,NLA法能更合理地反映基岩地震动强度和上覆土层厚度对地表地震动持时的影响。

A reasonable description of soil hysteretic constitutive relation has a significant impact on the evaluation of design ground motion parameters. Based on the 200-meter deep borehole profile in Suzhou region, the 1-D seismic site response analyses were performed by equivalent linear wave propagation analysis（ELA） and nonlinear analysis（NLA） using a multi-degree-of-freedom, lumped mass model in term of the modified Matasovic constitutive model. The effects of bedrock motion characteristics and bedrock interface depth on site response in deep sediment layers were analyzed. The results show that： 1） Surface peak ground acceleration（PGA） calculated by ELA increases monotonically as the PGA of bedrock motion increases. However, surface PGA calculated by NLA first rapidly increases and then gradually decreases or keeps constant. 2） Surface acceleration response spectra（Sa） for short periods computed using ELA and NLA show significant differences. The high-frequency contents of bedrock motions exhibit obvious filtering in ELA, while the high-frequency contents in NLA first magnify and then attenuate as the PGA of bedrock motion increases. 3） Surface Sa values become slightly larger with increasing bedrock surface depth. However, surface Sa values for the periods less than 0.1 s computed by NLA become slightly smaller. 4） For the medium or strong bedrock motions, the surface ground motion duration is closely related to bedrock motion characteristics, site response analysis methods and seismic bedrock surface depth. The influence of bedrock motion intensity and the overburden thickness on the duration prolongation of surface ground motion can be reasonably evaluated by NLA.