Earthquake Ground Response Analysis And Nonlinear Critical Index Research

Scholars in the field of earthquake engineering have been keen to study the nonlinear response characteristics of sites under earthquake.At present,they study the site effect of ground motion mainly by adopting numerical simulation and empirical methods.At present,most scholars put their researches focus on the amplification effect of ground motion on the surface or topography,however,few of them study on the variation law of underground ground motion amplification with depth.There are several of them conduct the research on depth effect by using geotechnical array records,but they lack of inspection of strong earthquake records,and is not capable of probing into its nonlinear seismic effect mechanism.A better data foundation has been laid for further exploring the analysis of ground motion site response and site nonlinear response characteristics,as the construction level of seismic observation array keeps enhancing and the number of strong motion records keeps increasing.The author obtains the site response results of Garner Valley Downhole Array and two sets of centrifugal models after analyzing the ground motion records of Garner Valley Downhole Array and the acceleration time history records obtained from the centrifuge model test.This paper systematically studies and analyzes some key problems,such as the mode response and amplification law of site soil layer under earthquake,the depth effect of site amplification,the degree and index of site nonlinear response,etc,and the main work and research results are as follows:1.Many vibration modes will be excited in soil site during the process of seismic wave propagating from bedrock to surface.Among all these modes,the first mode is the most significant one as well as easy to identify in site amplification,but it is not the maximum amplification factor of all modes,as it may be smaller than the amplification factor of other modes under special circumstances.2.The variation of site amplification has a certain law as its depth increases.i.e.Its vibration mode characteristics changes regularly as its depth increases.The amplification factor of both Garner Valley Downhole Array and anhydrous centrifuge model increases nonlinearly and gradually from bedrock to surface in the first mode shape,and it is the maximum at the surface;The amplification factors of Garner Valley Downhole Array and two groups of centrifuge models are quite different with depth in the second-order mode and high-order mode.3.According to the research results of the degree and index of nonlinear response of the site,Garner Valley Downhole Array reached the threshold of nonlinear response when the peak acceleration of the surface was 64.03 gal,indicating the occurrence of the nonlinear response.Besides,the critical frequencies f _NL of the four strong earthquakes are all located in the middle and low frequency stage.The degree of nonlinear response increases gradually as the intensity of local vibration increases.Under earthquakes with peak acceleration greater than 100gal,the DNL value of nonlinear response degree index is greater than 8.As is shown in the centrifuge model test,it reaches the threshold of nonlinear site response while the substrate input is set as 0.025 g,indicating the occurrence of nonlinear site response.The basic frequencies move to low frequencies as the nonlinear site response occurs.The critical frequency f _NL value is located in the low frequency stage in both anhydrous or water model.From a general view,the critical frequency f _NL value shows a nonlinear decreasing trend,and the nonlinear degree index DNL value gradually increases as the base input is increased.When the base input is increased to a certain value,the growth trend of DNL value will slow down and is eventually stable at a fixed value.In this paper,the author makes a preliminary analysis on the research on a tough problem of depth effect of ground motion site response caused by storm leakage in the seismic resistance of underground engineering and the dynamic test of rock and soil mass,and further probes into the research on the site effect of ground motion in sea area.The research in this paper is conducive to promote and guide the research and development of seismic resistance of urban underground engineering,and has certain reference value and engineering significance for seismic fortification and safety design of engineering construction in China.