Study On Regional Differences Of Engineering Site Conditions Based On Borehole Data In China

The spatial distribution of ground motion is significantly influenced by local site conditions.And site effects are one of the important factors that must be considered in the fields of structural seismic design,ground motion parameters zonation,and ground motion prediction models.Differences in site conditions will lead to significant variations in site response,so it is crucial to research on the regional dependence of site conditions.Although there are many studies on the characteristics of parameters such as geodynamics,ground vibrations,and soil characteristics for local areas,no systematic study has been conducted on the regional dependence patterns of engineering site conditions nationwide.In this paper,we analyzed the characteristics and regularity of regional differences of site cover thickness,equivalent shear-wave velocity,average shear-wave velocity within 30m below the surface,site shear-wave velocity profile and site amplification response,using the borehole profile data collected from engineering sites in China.The main work contents and results obtained are as follows:1.The borehole profile data of more than 6000 engineering sites in Sichuan,Yunnan,Beijing,Tianjin,Hebei,Guangdong,Guangxi Provience and Xinjiang Uygur Autonomous Region in China were collected.Based on the spatial distribution of borehole locations,regional differences in engineering site conditions were studied for four regions.Through the analysis of typical boreholes in Xinjiang,it is found that there exists a kind of engineering site borehole with special distribution of shear wave velocity with depth in China,and the existing empirical extrapolation model is not applicable to the estimation of V_S30 value.To this end,this paper presents two new V_S30extrapolation models:the walking time gradient model and the constant extrapolation correction model,using the statistical analysis of the eligible borehole profile data in China.Finally,by comparing the prediction accuracy and ease of use of different extrapolation models,a recommended procedure for V_S30 calculation and estimation applicable to engineering sites in mainland China is given.2.For the four site categories defined in China’s seismic design codes,this paper verifies that there are regional differences in site conditions between different regions by comparing and analyzing the percentage of boreholes,the probability density distribution of VSe and H,and the median curve of shear wave velocity profiles for Class II sites in the four regions.The cumulative probability distribution curves of V_S30in the four regions were plotted and their differences were analyzed with respect to the V_S30 parameter defined by European and American codes.Based on their overlap,parallelism and crossover phenomena,the characteristics of site condition variability in the four regions were summarized.3.Soil seismic response calculations were carried out for the Class II borehole sites in Xinjiang and Beijing-Tianjin-Hebei regions,and the sensitivity of the input ground shaking PGA and seismic magnitude to the site response was analyzed in this study.From the perspective of the remarkable frequency and amplification of the site response,it is argued that there is regional variability in the average site response in the two regions;the site amplification adjustment factors given in the zoning map specifications are compared,and it is found that for the engineering site boreholes studied in this paper,the zoning map will underestimate the actual site amplification effect to some extent.