Amplification Of Ground Motion By Tunnel Group

As the large-scale development and utilization of underground space the in the recent years, metro has become a major urban transportation vehicle. The analysis of ground motion amplification by tunnnel group is important both in engineering and theory.Ground motion amplification by tunnnel group is the scattering of seismic waves by tunnnel group essentially. Result is present by finite element method in time domain using consistent transmission boundary. calculations have been done in four kinds of media including linear homogeneous site, layered linear site, nonlinear homogeneous site and layered unlinear site.As for the linear homogeneous site, both seismic responses of free field and site with two tunnels are given. Comparison between the pre and post tunnels- construction reveals the law of amplification by two tunnels. As for the linear layer site, both seismic responses of pre and post tunnels- construction sites are given too. There are obviously different features between the response of homogeneous site and layered site.It is necessary and reasonable to use layered modal. Research shows that amplification is more significant in the site with tunnel compared with the free field. Layer has obvious amplification of seismic waves. Ground motion is the superposition of free field response and tunnel amplification. Both layer and tunnel group effects the ground motion.When the amplitude of incident seismic wave is large, constitutive relation is nonlinear rather than linear. After seismic response analysis of the linear site, the seismic response analysis of the nonlinear site with tunnnel group using equivalent linear method. Research shows that amplification is lager in the site with tunnel group compared with the free field. The nonlinear feature embodies the decreasing of shear modulus and damp increasing, therefore, the decreasing of shear wave velocity and fundamental frequency. Predominant period of nonlinear layer is lager than that of linear layer. Predominant period becomes longer when amplitude of incident seismic wave increases, which is an important form of nonlinear property. Amplification by nonlinear layer site with two tunnels is present, which shows that both layer and nonlinear has great effect on ground motion.In the four kinds of site, how the distance of tunnels and concrete lining effect the ground motion are considered. Research shows that distance of tunnels is a key factor. Even if the distance is large, there is great interaction between tunnels. There is remarkable difference between the scattering characteristics of tunnels with lined and without lined.