Amplification Of Earthquake Ground Motion By Metro Group Cavities In Layered Half-space

As an important ingredient of today’s urban railway transit system, subway playsa significant part in easing serious traffic congestion in large cities. Subway tunnelsare usually composed of one or more parallel cavities, according to engineering wavepropagation theory, the effect of scattering by cavities on seismic waves shallundoubtedly exert influence on seismic motion in the vicinity of tunnels, and makeaseismic capacity of the existing buildings nearby inadequate. China is an earthquakefrequent country, over half of its large cities are located at regions of seismic intensity7or higher degree, and consequently, study about the influence of tunnels on seismicmotion nearby will be of great significance.By combing indirect boundary element method (IBEM) in frequency domain andFourier time-frequency transform, and taking typical linear sites in Tianjin district asexamples, effects of single cavity and twin cavities in layered half space on surfaceground motion are studied, the sites include dry site and saturated poroelastic site, andthe vibration modes include anti-plane vibration and in-plane vibration. It is shownthat, existence of cavity remarkablely changes magnitude and spectrum of groundseismic moiton nearby. Under the excitation of Taft wave and El Centro wave, as fortwin cavities, the amplification of PGA can be up to45.2%. Besides, spectralcharacteristic of response spectrum will also be changed.A new algorithm coupling FEM and IBEM is established, and by introducingequivalent linear method into such algorithm, nonlinear characteristic of soil layer canbe taken into consideration, on this basis, by taking typical nonlinear site in Tianjindistrict as an example, effects of single cavity and twin cavities in layered half spaceon surface ground motion are further studied. It is shown that unlinear amplificationof surface ground motion by cavity is also obvious. Under the excitation of Taft waveand El Centro wave with PGAs of0.1g and0.2g, while as for twin cavities, theamplification of PGAcan be up to30.7%.Significant amplification of seismic motion by underground metro tuunels isrevealed in present study, such amplification should be better taken into account whendesigning subway tunnels.