Seismic Response Analysis And Influencing Factors Of Soil Layers In Karst Areas

In recent years,the issues of engineering seismic safety and seismic site effects in karst areas have received widespread attention from the seismic engineering community.However,in the seismic safety evaluation of major engineering projects in karst areas,the influence of the presence of karst caves or soil caves on determining the design seismic parameters of the site cannot be considered.The one-dimensional equivalent linearization seismic response analysis method recommended in the specifications can only simplify the soil cave karst area site to a free field model,and the site effect will be significantly underestimated for soft soil sites,which will inevitably lead to serious deviations in the seismic design parameters of the project.In view of this,the model of karst site and soil cave karst site is established by using FLAC 3D finite difference software,and the influence of soil cave development conditions and input ground motion on site seismic response in karst area is discussed.The study considered the working conditions of different soil cave sizes,changes in the thickness of the overlying soil layer,and changes in the shear wave velocity of the overlying soil layer;At the same time,the working conditions of inputting horizontal and vertical seismic motions with different peak values,as well as seismic motions with different spectral characteristics,were also considered.These works contribute to further understanding the seismic response mechanism of karst areas and provide reference for engineering design in karst areas.The main work and research results are as follows.(1)Three commonly used soil layer seismic response analysis methods(finite difference method FLAC3 D,time domain DEEPSOIL and frequency domain LSSRLI-1)in the current seismic engineering field are introduced and reviewed.The results indicate that both DEEPSOIL and LSSRLI-1 significantly underestimate the peak ground acceleration,and the error between the FLAC3 D calculation results and the measured records is only 4.97%,indicating higher accuracy.(2)Considering the development conditions of soil caves in karst areas,including the size of soil caves,the burial depth of soil caves,and the stiffness of overlying soil,the finite difference method is used to study and analyze the impact of soil caves in karst areas on the peak acceleration of ground motion,horizontal peak displacement,and vertical displacement,and at the same time,the results are compared with those of free field(without soil caves)seismic response analysis.The results show that as the radius of the soil cave increases,the peak surface acceleration gradually decreases.When the radius exceeds 2.5m,the peak surface acceleration of the karst site exceeds the free field result;As the thickness of the overlying soil layer in karst areas gradually increases,the peak ground acceleration shows a trend of first decreasing and then increasing.When the thickness of the overlying soil layer is about 6m,the soil cave has the greatest impact on the seismic effect of karst sites;As the shear wave velocity of the overlying soil gradually increases,the surface peak acceleration of the free site gradually decreases,and the surface peak acceleration of the karst site gradually increases.(3)Explored the impact of input seismic peak and spectral characteristics on the seismic response analysis results of karst areas.Using the horizontal SV wave as the base input,as the input peak increases,the soil cave in the karst area will lead to an increasing decrease in surface peak acceleration,with a maximum of 32.2%;Using vertical P-waves as the base input,as the input peak increases,the amplitude of surface peak acceleration decreases more and more,with a maximum of 19.3%.The impact of input seismic frequency spectrum is reflected in the enhancement of seismic effects on karst areas when approaching the model’s natural vibration period.