Study On Seismic Response Characteristices Of Sand Gravel Soil Site

Sand gravel soil has the characteristics of irregular appearance,large grading span,poor stability of framework force transfer mechanism between particles and sudden change of particle crushing morphology,which made its dynamic characteristics significantly different from other soils.Sand gravel soil is widely distributed in Chengdu basin,Sichuan Province.The overburden of the site in that area is mainly composed of silty clay,clay and sand gravel soil,with complex structure and variable thickness.In the Wenchuan Ms8.0Earthquake,the earthquake damage of the whole Chengdu basin is relatively slight,which has attracted many scholars' attention to the seismic response of sand gravel soil site.However,the seismic response characteristics of sand gravel soil site are not obvious at present.Therefore,it is of great engineering significance to study the seismic response characteristics of sand gravel soil site,and the research results will provide references for seismic fortification of sand gravel soil site engineering.In this paper,shaking table tests and one-dimensional equivalent linearization analysis are used to study the seismic response of sand gravel soil site and its influencing factors,and the seismic parameters of sand gravel soil site in Chengdu basin,Sichuan Province are calculated.The main research contents and results of this paper are as follows:(1)The shaking table test of a homogenization sand gravel soil model site with a soil layer thickness of 100 cm was designed and carried out.The seismic responses of sand gravel soil site under different intensities and spectrum characteristics of earthquake excitation were studied.The results show that the seismic response of sand gravel soil site has obvious nonlinear effect,and the amplification effect of surface soil layer on seismic wave is obviously larger than that of lower soil layer.The soil layer in the underground of sand gravel soil site has amplification effect on the components with lower frequency of seismic wave,filter the component with higher frequency,and the upper layer has amplification effect on each frequency component of seismic wave.With the increase of excitation intensity,the dynamic earth pressure response of sand gravel soil site increases obviously,which shows the law of increasing after a sudden decrease.And the dynamic shear stress-strain relationship of sand gravel soil presents obvious nonlinear characteristics,i.e.,the damping ratio of soil increases and the shear modulus decreases.The change mechanism of acceleration distribution of each soil layer of site,acceleration spectrum composition and soil shear modulus were revealed.(2)Based on the shaking table tests of 5 different model sites,the effects of soil structure and soil thickness on the seismic response of sand gravel soil site are studied.The results show that:(1)The nonlinear effect of seismic response in clay site is stronger than that in sand gravel soil site.As compared to the homogenization sand gravel soil site,the layered interface of layered clay-sand gravel soil site weakens the seismic wave,the acceleration response of sand gravel soil layer in the lower part is smaller,and the amplification effect of clay layer in the upper part is larger.The nonlinear effect of seismic response of layered clay-sand gravel soil site is stronger in comparison with the homogenization sand gravel soil site.And The frequency of amplification and filtering frequency of seismic wave in the lower layer of layered clay-sand gravel soil site are higher,and the amplification effect of seismic wave with less than 23 Hz component in upper soil layer is weaker.The amplitude of ground acceleration response spectrum in the platform section and the descending section of the layered clay-sand gravel soil site is smaller than the homogenization sand gravel soil site,and the greater the excitation intensity is,the more significant the difference is.(2)With the increase of soil thickness,the amplification factor of ground acceleration peak value of sand gravel soil site decreases gradually or first decreases and then increases.The smaller the thickness of soil layer is,the smaller the amplification effect of low frequency component of seismic wave is,and the larger the amplification effect of high frequency component is.With the increase of overburden thickness,there is a shock absorption phenomenon in the rising section of the surface acceleration response spectrum,and the larger the thickness of the soil layer is,the more obvious the shock absorption is.From the platform section,the amplitude of the response spectrum generally increases.This study reveals the influences of soil layer structure and thickness on the seismic response of sand gravel soil site,and further provides a scientific basis for the theoretical research of seismic response and seismic damage analysis of sand gravel soil site.(3)Based on the one-dimensional equivalent linear analysis method,90 calculation conditions are designed to study the influences of excitation intensity,soil compactness and soil thickness on the seismic response of sand gravel soil site;according to the drilling information of Chengdu Basin in Sichuan Province,1080 calculation conditions are designed,and the calculation values of seismic parameters of sand gravel soil site in Chengdu Basin,Sichuan Province are given.The results show that: the amplification coefficient of peak ground acceleration decreases linearly with the increase of seismic excitation intensity.With the increase of excitation intensity,the predominant period of site increases linearly,and the greater the thickness of site is,the faster the predominant period increases.The denser the soil is,the stronger the amplification of seismic wave is.The increase of soil layer thickness may amplify or weaken the seismic wave.This research can provide reference for the engineering construction of sand gravel soil site.The current research is of great significance to determine the design ground motion parameters more reasonably and scientifically in areas with sand gravel soil site characteristics.Also,the study has a positive guiding role in analyzing the causes of earthquake disasters and improving the seismic fortification capacity in such areas.