The Seismic Response Analysis Of Rectangular Utility Tunnels Across Metro Tunnels In The Fine Sand And Liquefiable Site

The rectangular utility tunnel is a new type of infrastructure that collects various pipelines including urban water supply,drainage,electricity and gas into the same space,and is called the "main artery" of a smart city.Tongzhou District is the sub-center of Beijing city.The utilization rate of underground space development is high.A large number of utility tunnel projects are under construction.The area is a moderately strong seismic activity area with widely distributed silt and fine sand layers.The rectangular pipe gallery is in the construction process.It will inevitably cross the liquefiable stratum,and the liquefaction of the soil will cause serious damage to the site and the underground structure inside.The seismic response of underground pipelines under complex crossing conditions has always been a hot and difficult research in the field of geotechnical engineering.Based on the project of crossing Metro Line 6 on the rectangular pipe corridor of Changhe West Road,Tongzhou District,Beijing,this paper uses nonlinear dynamic analysis to obtain the liquefaction parameters of the saturated silt sand layer,and establish the three-dimensional geological and numerical model under complex crossing conditions.Analyze the seismic response of the rectangular tube gallery over the subway tunnel at a close distance,and propose disaster prevention countermeasures for the typical earthquake damage that may occur.The main research results of the thesis are as follows:（1）Considering the seismic response characteristics of the saturated silt fine sand layer,using nonlinear dynamic analysis methods to obtain site liquefaction parameters,performing integral inversion and low-pass filtering of the seismic acceleration time history,and correcting the residual velocity and displacement generated by the integral inversion;Comprehensive consideration of the size effect,damping type,dynamic boundary conditions and other factors,the study obtained the effective stress,excess pore water pressure,and displacement of the liquefiable silt sand layer.The pressure ratio is used as an indicator to carry out the analysis of the liquefaction response of the silt fine sand formation and verify it in conjunction with the engineering specifications.The research shows that the dynamic pore water pressure time history curve shows an "S"-shaped upward trend,and the excess pore pressure ratio decreases with the increase of the buried depth,and the excess pore pressure ratio reaches the critical value at the depths of 3.3m,6.5m,and 8.5m.Value,the vertical effective stress of the soil is lost and lateral sliding deformation occurs.（2）Relying on actual engineering to establish a pipe gallery and tunnel combination model under complex crossing conditions,starting from the three aspects of structural displacement,stress,and acceleration response,quantitatively analyze the seismic response of the rectangular pipe gallery.Research shows that when there are liquefiable strata at the bottom of the subterranean structure,the upward flotation is the main cause.The excess pore water pressure uses buoyancy to provide power for the rise of the pipe gallery.The rise of the pipe gallery lags behind the change in excess pore pressure.Existence will accelerate the attenuation in the propagation of peak acceleration.（3）Based on the principle of floating and unloading of underground structures,the influence of the short span of the rectangular pipe gallery in the liquefiable stratum on the underlying subway tunnel is analyzed.The study shows that the middle of the underlying tunnel produces upward bending deformation,the maximum value is 2cm,and at the same time The phenomenon of stress concentration occurs and spreads along the length of the tunnel.The maximum principal stress appears as a local tensile stress,which may lead to tensile failure of the vault.Corresponding measures are proposed to reduce the damage caused by seismic liquefaction.The research results reveal the floating response law of the rectangular pipe gallery in the liquefiable stratum,and provide a valuable theoretical basis for the seismic anti-floating construction of underground structures under complex crossing conditions.