液化层特征量对场地卓越频率影响的理论解答

Analytical solutions for changes in predominant frequency of a site based on characteristic parameters of liquefiable interlayer

液化场地对地震动影响问题以及依据地表地震记录识别场地液化问题虽分属地震波的正反演理论,但土层液化对振动特性的改变是二者共同的物理本质,应是这两方面研究的出发点。提出液化层存在下场地卓越频率变化的理论解答,并分析液化层特征量对场地卓越频率的影响规律。将实际土层视为水平成层状,以三质点体系代表具有液化夹层的场地,推导出场地卓越下降率解答,讨论土层及液化层特征量对场地卓越下降率的影响。结果表明：液化夹层的存在将使场地卓越频率下降,下降率主要与覆盖层与液化层厚度比λ1、液化层与下卧层厚度比λ2、液化层软化程度这3个参数相关;λ1对场地卓越频率下降起主要的控制作用,场地卓越频率下降率随λ1变化的可分为3种模式：指数关系降低、指数关系升高以及定值附近先增大后减小的波动,其中后者为主要表现;液化层与下卧层厚度比λ2对场地卓越频率下降率起次要作用,场地卓越频率下降率随λ2增大呈快速上升阶段和平稳增长两个阶段,二者转换点与λ1相关;场地卓越频率虽与液化层软化程度呈递增关系,但绝对增幅最大为0.15,影响较小。

Although the effect of soil liquefaction on seismic ground motion and the recognition of site liquefaction based on seismic records respectively represent forward and inverse derivation problems,the change in the self-vibration characteristics in the horizontal direction caused by soil liquefaction is the physical characteristic that is shared commonly by these two issues.Thus,the natural vibration characteristics of a site should be the basis for studying these two issues.From this perspective,the present work proposes an analytical approach for predicting the proportional decrease in the predominant frequency of a site after the occurrence of soil liquefaction(δ).The soil is treated as a layered and parallel structure,and a three-particle system including a liquefiable interlayer is employed to represent the sites susceptible to liquefaction.The approach is based on the characteristic parameters of the liquefiable interlayer.The effect of these parameters on δ is also discussed.The results show that the presence of a liquefied interlayer reduces the predominant frequency of the site.The value of δ is mainly related to the ratio of the overburden layer thickness to the thickness of the liquefiable layer λ1,the ratio of the liquefiable layer thickness to the thickness of the substratum layer λ2,and the degree of softening of the liquefiable layer.The ratio λ1 plays the dominant role in controlling δ according to three modes：an exponentially decreasing mode,an exponentially increasing mode,and a mode where δ first increases and then decreases,with the latter being the main representation.The ratio λ2 plays a secondary role in affecting δ ,where δ initially increases rapidly with the increasing λ2 up to a transition point,after which δ increases slowly.The results show that the value of the transition point is related to λ1.Although δ increases with an increasing degree of softening of the liquefied layer,the maximum absolute increment is only 0.15,which provides a negligible effect.