Influence Of Underground Tunnel On Amplification Effect Of Ground Motion Site

Seismic damage experience has shown that large earthquakes can cause serious damage to urban underground engineering structures,changing the conventional understanding of the positive seismic performance in the past seismic engineering community.Therefore,underground engineering earthquake resistance has gradually become one of the influential topics in geotechnical earthquake engineering.In the current seismic safety evaluation of underground tunnel projects,the underground tunnel is not considered,and it is also ignored that the existing free field conditions will be changed after the completion of the underground tunnel.However,the design ground motion parameters given based on the existing free field are severely inconsistent with the actual situation,which may initiate potential seismic safety risks for underground projects and surface ancillary projects.In view of this,this paper combines dynamic centrifugal model testing and numerical simulation techniques to conduct a study on the differences in dynamic response characteristics of tunnel site and free field under earthquake.A traditional spectral ratio method based on the autoregressive Auto-regressive model(AR model)is proposed to determine the site effect.Finally,a project example is compared with conventional seismic safety evaluation methods.Verify the reliability of the seismic response analysis method considering the impact of underground tunnel in this paper.The research results in this paper can further deepen and promote the understanding and development of the seismic field of underground engineering,and have certain reference value and engineering significance for the future revision of the code for seismic safety evaluation of underground engineering.The main research results are as follows:(1)From the base to surface,the peak ground motion acceleration and its amplification factor exhibit a phenomenon of first attenuation and then increase with the depth direction.The peak acceleration amplification at the surface of the free field train is between 1.042 and 1.072,and the peak acceleration amplification at the surface of the tunnel train is between 1.009 and1.024.The ground motion acceleration response spectrum presents a bimodal state.The greater the intensity of the input seismic wave,the higher the peak value of the response spectrum,but the corresponding predominant period moves towards a short period direction.(2)It is found that the overall trend of the spatial coherence function calculated by the periodic graph method and the AR model is consistent,but the results calculated by the AR model are generally smoother.The coherence of two points in the space of the ground motion site gradually attenuates in the low frequency band as the frequency increases.The larger the amplitude of the input seismic motion,the stronger the attenuation of coherence in the entire frequency band.(3)The variation of ground motion amplification coefficient with depth presents a nonlinear trend from the base to the surface.According to statistics,the probability that the value of the ground motion amplification coefficient of the higher order vibration mode is greater than the first order vibration mode is 35.0%,and the higher the vibration mode,the greater the probability.(4)The one-dimensional equivalent linearization method LSSRLI-1 and the finite element software PLAXIS 2D were used to study the actual engineering of Beijing Metro Line 16.There are some differences in the results of calculating ground motion response parameters between the two methods.The calculation results of the former mainly show a linear increase along the depth,while the calculation results of the finite element software PLAXIS 2D show a nonlinear change,which better reflects the actual ground motion site response characteristics.