Empirical relationship between overburden thickness and natural frequency based on borehole data in Beijing region
The thickness of the site overburden layer has a significant impact on the distribution of earthquake damage.Moreover,studies have shown that when calculating the surface ground motion parameters,the uncertainty of the overburden layer thickness will lead to obvious differ-ences for the calculation results.Therefore,giving the thickness of the site overburden layer in a scientific,reasonable and accurate manner can improve the accuracy of calculating the ground motion parameters and better estimate the earthquake damage.Traditionally,the overburden layer thickness is obtained by borehole drilling.However,borehole drilling is expensive,time-consuming and unfriendly to the ecological environment.In order to obtain the thickness of the site overburden layer economically and efficiently,many researchers have conducted in-depth studies on the typical site indicators(site natural frequency,overburden layer thickness)and es-tablished the empirical relationship between the overburden layer thickness and the site natural frequency in various research areas.However,so far,no researchers have given the empirical relationship between the site natural frequency and the overburden layer thickness in Beijing.In view of this situation,this paper conducts research on the borehole data obtained in the seismic safety evaluation work in Beijing and obtains the following results:1)According to the current Code for Seismic Design of Buildings in China,1142 bore-hole data were classified,and 512 borehole data of class Ⅱ sites and 630 borehole data of classⅢ sites were obtained.Then,calculations and analyses were carried out on these borehole data,and the distribution characteristics of vS20 and vS30 of class Ⅱ sites and class Ⅲ sites in the main distribution areas of the boreholes were obtained,that is,the values of vS20 and vS30 in the west and southwest are higher than those in the east and northeast.As the geographical location moves from the southwest to the northeast,vS20 and vS30 are gradually decreasing.2)Using the model y=a(1+x)b,the borehole data were fitted under the conditions of distinguishing the site categories or not,and the empirical relationship of the soil shear wave ve-locity changing with depth under different conditions were obtained.Without distinguishing the site categories:vS=124.6(0.6408+Z)0.3074;class Ⅱ 1 site category:vS=134.4(0.1176+Z)0.3085;class Ⅱ2 site category:vS=183.1(-0.3965+Z)0.2739;class Ⅲ site category:vS=124.6(0.6408+Z)0.4115.3)The empirical relationship of the soil shear wave velocity changing with depth obtained above were derived,and the empirical relationship between the overburden layer thickn-ess and the site natural frequency were obtained.Without distinguishing the site catego-ries:h=84.33 fr-1.4438;class Ⅱ1 site category:h=94.54 fr-1.4461;class Ⅱ2 site category:h=124.63 fr-1.3772;class Ⅲ site category:h=61.55 fr-1.6992.And the obtained empirical relation-ships were verified.The verification results show that the empirical relationship between the overburden layer thickness and the site natural frequency established in this paper can estimate the overburden layer thickness relatively accurately.
Beijingshear-wave velocityoverburden layer thicknessfit analysisnatural frequency
万方数据
维普
