我国z<30 m正常停孔V_(s30)线性外插公式可行性研究

The research on the feasibility of a linear extrapolation equation to predict V_(s30)for z<30 m borehole with normally stopping in China

我国现行规范(GB 18306-2015)场地类别划分定义中覆盖土层的规定,导致我国钻孔测速中在正常停孔条件下产生大量深度小于30 m的钻孔数据。此类钻孔无法直接计算得到V_(s30),阻碍了我国借鉴和使用国外与V_(s30)相关的研究成果。观察我国滇中引水工程深度小于30 m实测钻孔波速分布,得出z_(500)(即剪切波速第1次达到500 m/s土层位置)至终孔的平均剪切波速V_(sz)沿深度近似呈线性分布;进一步通过其与KiK-net台站中108组同类数据的线性相关性分析,建立了对数坐标下2个竖向深度处平均剪切波速V_(sz1)、V_(sz2)(z_(1)<z_(2)<30 m)线性外推V_(s30)的公式LEE。经验证,LEE公式z_(1)、z_(2)最小换算间距无严格要求,其为最小可能间距1 m时也有较好计算精度。由于我国正常停孔下,深度小于30 m钻孔和深度大于30 m的钻孔所对应的场地特征不同,因此以我国深度大于30 m的实测钻孔数据建立的V_(s30)预测公式会低估我国深度小于30 m钻孔的V_(s30),且计算深度越浅,低估现象越严重。由于KiK-net钻孔数据非我国直接代表性数据,且其记录方式相对粗糙,因此,文中的研究仅对我国未来V_(s30)线性外插公式的研究提供可行性支撑。

It is the specification of covering soil layer in definition of classifying sites in Chinese code(GB 18306-2015)that leads to a large number of normally stopping boreholes with depth less than 30 m in China.Under this situation,V_(s30)fail to be directly calculated by definition equation from code,and referencing the correlated research results which include the parameter of V_(s30)are prevented.By observing the velocity profiles of the boreholes from Water Diversion Project in Central Yunnan with the depth less than 30 m,from the depth of z_(500)(z_(500) is the depth at which the shear-wave velocity reach 500 m/s firstly)to the stopping depth,linear feature of the averaged shear-wave velocity profiles is observed.Moreover,based on the linear correlation analysis of the velocity profiles mentioned above and the 108 homogeneous data from boreholes of KiK-net stations,in logarithmic coordinate,a linear extrapolation equation LEE expressed by two averaged shear-wave velocity V_(sz1) and V_(sz2)(z_(1)<z_(2)<30 m)is proposed for estimating V_(s30).And they are verified that there is not special requirement of the minimum interval between z_(1) and z_(2) and there is good accuracy even if the minimum interval is 1 m.In China,under the premise of borehole normally stopping,because the boreholes with depth less than 30 m and the boreholes with depth more than 30 m come from the sites with different characteristics respectively,the predicting equations of V_(s30),which are based on the data of the boreholes with depths more than 30 m,underestimate the V_(s30)of the boreholes with depths less than 30 m.Moreover,the shallower the calculation depth is and the more serious the underestimation is.Because boreholes data from KiK-net stations cannot directly represent the boreholes data from China,and their record format is relatively rough,the researches in this paper can only provide feasibility support for future studies on the linear extrapolation equations of the V_(s30)in China.