四川、甘肃地区VS30经验估计研究

Empirical Estimation of VS30 in the Sichuan and Gansu Provinces

摘要:

目前我国建筑工程抗震设计规范中对于工程场地条件的判断依据主要是地表以下20 m深度范围内土层的等效剪切波速,简称V_S20。相比之下,国外应用较广的是地表以下30 m深度范围内的等效剪切波速,简称V_S30。这种差别导致国内科研工作者在应用国外的地震工程、工程抗震模型时经常遇到对场地条件描述不准确的困难。为了解决这个问题,本文根据147个四川、甘肃地区国家强震动台站20 m左右深度的钻孔剪切波速数据,利用延拓方法、场地分类统计方法以及基于地形特征的V_S30估计方法研究各台站V_S30与V_S20的经验关系,对比发现基于速度梯度延拓的结果最为可取。参考国际上通用的Geomatrix Classification场地分类标准,最终得到四川、甘肃地区各类场地的平均V_S30,此结果可以为缺乏钻孔数据的工程场地的V_S30估计提供参考。

Abstract:

At present, Chinese seismic design codes for engineering projects classify site conditions primarily by their equivalent shear wave velocity at a depth of 20 m below the surface, which is referred to as V_S20. However, the parameter widely used abroad in seismic design codes is the equivalent shear wave velocity at a depth of 30 m below the surface, V_S30. This non-standardized parameter has led to inaccurate site condition descriptions when foreign models for earthquake engineering or seismic resistance are applied to Chinese engineering projects. Several methods have been used in an attempt to estimate V_S30 from site profiles with 20 m-deep boreholes for strong motion stations located in the Sichuan and Gansu Provinces. These estimation methods include: extrapolation (constant and gradient), Geomatrix site classification correlation via shear-wave velocity, and remote sensing (terrain and topography). In this study, gradient extrapolation was the preferred estimation method for sites with shear-wave velocity profile data. When combined with Geomatrix site classification, the average V_S30 for each site was derived from 147 estimated V_S30. Thus, gradient extrapolation followed by Geomatrix site classification can be used to assign a V_S30 to a site without 30 m-deep borehole data, unifying global engineering design codes.