土层剪切波速不确定性对场地刚性判断的影响

Impact of uncertainty in in-situ shear-wave velocity on the judgement of site stiffness

岩土工程中的不确定性是工程风险的重要来源。在我国7个主要地区的20个工程场地上开展了国内外首次大范围现场剪切波速不确定性专项试验,共由47家单位用11种工程常用仪器采用单孔法完成,获取了600组土层剪切波速Vs实测数据。依据实测结果拟合了场地时间平均剪切波速Vs, z的变异系数COV与计算深度Z的对应公式,揭示了土层剪切波速不确定性对场地刚性判断的影响,进而分析了我国场地分类的可能潜在误判区域。结论如下:在目前我国现场剪切波速测试水平下,Vs, z的变异系数与深度负相关--在近地表处达到极大值15%,并随着深度增加明显减小;地表至10 m深度区间内,Vs, z的变异系数迅速减小;10~20 m深度区间内,Vs, z的变异系数减小趋势变缓;超过20 m深度后,Vs, z的变异系数小于5%且基本不变;我国Ⅲ、Ⅳ类场地分类采用的等效剪切波速Vse和欧美场地分类指标Vs, 30的变异系数基本相等;依据我国建筑抗震设计规范GB50011-2010,对Ⅰ类场地以及覆盖层厚度小于5 m或等效剪切波速接近500 m/s的Ⅱ类场地分类时需考虑剪切波速不确定性的潜在影响,其他类型的Ⅱ类场地、Ⅲ类和Ⅳ类场地在分类时基本可忽略其影响。

Uncertainty in geotechnical engineering greatly results in engineering risk. In this study, the first large-scale uncertainty experiment on the in-situ shear-wave velocity(Vs) was carried out on 20 typical sites in 7 major regions of China. A total of 11 kinds of common engineering instruments were completed by using single hole method in 47 units, and 600 groups of Vs were obtained finally. According to this in-situ database, the corresponding formula between Coefficient of variation(COV) of time-average shear-wave velocity(Vs, z) and the calculated depth(Z) was fitted. We revealed the impact of in-situ Vs uncertainty on the judgement of site stiffness and pointed out the possible potential misjudging area of site classification in China. At the current level of in-situ Vs test in China, COV of Vs, z is negatively correlated with depth, which indicates that it reaches the max value of 15% near the site surface and then decreases significantly with the increasing of the depth. In the depth range from 0 m to 10 m, the COV of Vs, zdecreases rapidly;in the depth range of 10-20 m, the COV of Vs, z decreases slowly;after the depth of 20 m, the COV of Vs, z is less than 5% and basically unchanged;the COV of equivalent shear wave velocity(Vse) used for site classification of Class Ⅲ and Ⅳ sites in China is basically the same as that of the field classification index Vs, 30 used in Europe and America. Based on China building code GB50011-2010, Class Ⅰ and Class Ⅱ sites with the soil thickness less than 5 m or Vse close to 500 m/s should consider the potential impact of Vs uncertainty, while other types of Class Ⅱ and Class Ⅲ and Class Ⅳ sites could ignore the uncertainty.