苏州第四纪深厚地层剪切波速空间变化特征及其应用

土体剪切波速是工程场地地震效应评价的重要参数之一。基于苏州城区的大量剪切波速实测资料和工程地质特征,探讨了第四纪深厚沉积层剪切波速结构空间变化特征,给出了不同工程地质分区的土层剪切波速随深度变化的经验公式;基于不同深度范围内土层平均剪切波速间存在的显著相关性,建立了基于浅层剪切波速外推深层剪切波速的逐步外推法,验证结果表明:(1)苏州城区等效剪切波速ν_(s20)和ν_(s30)的空间展布具有显著的分区特征,覆盖层厚度d80m的Ⅲ类和Ⅳ类场地的等效剪切波速分界值ν_(s30)可以取为170 m/s;(2)平原区土层剪切波速随深度的变化呈幂律函数关系;丘陵区细化为3个小的区域后,各分区内土层剪切波速随深度的变化呈二次多项式关系;(3)逐步外推法具有很好的适用性,但当外推深度处的土层剪切波速存在剧烈变化时,剪切波速的逐步外推法失效;(4)基于剪切波速逐步外推法,可给出剪切波速不小于500 m/s和700 m/s的任意假想基岩面埋深H_(rock)及相应的微震动场地卓越周期T_g,发现T_g值和H_(rock)值的空间展布基本一致;选取剪切波速不小于700 m/s的土层顶面为地震基岩面时,各工程地质分区的T_g值和H_(rock)值空间展布的差异性显著增大。

Spatial variation characteristics of shear wave velocity structure and its application to quaternary deep sediment layers in Suzhou region

The shear wave velocity of soils is one of the most important parameters to evaluate the seismic effects of engineering sites. Based on large numbers of borehole shear wave velocity measurements and the engineering geological characteristics in Suzhou region, the spatial variation characteristics of shear wave velocity structure for the quaternary deep sediment layers are analyzed, and the empirical equations of shear wave velocities with depth in different engineering geological zones are given respectively. Moreover, based on the significant correlation between the average shear wave velocities to different depths, the gradual extrapolation method for estimating the values of deep shear wave velocity from a shallow shear wave velocity profile is established. The validated results show that: (1) The spatial variations of equivalent shear wave velocities for ν_s20 and ν_s30 in Suzhou City exhibit obvious regional dependency. It will be more appropriate to use ν_s30=170 m/s as the threshold value for the site class Ⅲ and Ⅳ with overburden thickness d>80 m. (2) The variations of shear wave velocities with depth in the western and eastern plain zones can be fitted by power-law functions. After subdividing hill zones, the variations of shear wave velocities with depth in the three subdivided hill zones can be fitted by quadratic polynomial functions. (3) The gradual extrapolation method has a good applicability for deep shear wave velocity, but when the shear wave velocities of soils under extrapolation initial depth have dramatic changes, the extrapolation method is inapplicable. (4) The depths (H_rock) of seismic bedrock surfaces with shear velocity not less than 500 m/s or 700 m/s and the corresponding predominant periods (T_g) under microseism can be obtained by the gradual extrapolation method. It is found that the spatial variations for both T_g and H_rock are similar. By choosing the interface of soils with shear velocity not less than 700 m/s as the seismic bedrock surface, the differences of T_g and H_rock among different engineering geological zones significantly increase.