Research On Seismic Dynamic Response Characteristics Of Shield Tunnel In Xi’an Area

Many cities are developing and constructing urban rail transit to relieve the pressure of urban traffic,effectively solve problem of traffic congestion,and improve the comprehensive strength of the city.The shield method is widely used in subway tunnel project with its safety,high efficiency,low cost,small surrounding rock disturbance and other advantages.Xi’an is located in high intensity seismic fortification area,with complex geological structure and special geology of the loess site,which greatly increases the difficulty of subway construction.In particular,a series of problems arise from metro shield tunnels pass through ground fissures.Therefore,it is necessary to make a profound study on the seismic dynamic response of shield tunnels in Xi’an area.Takes the Xi’an Metro 8th Ring Line crossing 14 ground fissures as main research project,a three-dimensional finite element model of shield tunnel-surrounding rock with discontinuous stiffness under the conditions of loess site and ground fissure site are established The seismic dynamic response regular of shield tunnels on loess sites and the effects of different ground fissures and tunnel intersection angles,different seismic intensities,and different seismic waveforms on the seismic dynamic responses of shield tunnels on ground fissure sites are studied.the main results of this thesis includes:(1)The effect of soil plastic deformation and absorbing wave energy increases with the increase of earthquake intensity,and the acceleration response of surrounding rock and tunnel structure decreases accordingly,moreover,the deformation of the soil controls the seismic response of the flexible shield tunnel structure,and the dynamic response of the top of the arch is stronger than that of the bottom of the arch.The greater the intensity of the earthquake,the greater the additional lateral earth pressure at the bottom of the arch.The surrounding rock produced obvious residual convergent deformation after seismic loading.During the earthquake,the axial stress of the arch waist of the lining is greater;the maximum peak of the hoop shear stress response is near the arch shoulder and the arch foot,and the maximum peak of the bending moment is at the arch waist.(2)Under the action of an earthquake,the acceleration response of the tunnel structure on the hanging wall in the ground fissure site is significantly greater than that of the footwall tunnel structure because the seismic response of the shield tunnel is controlled by the surrounding rock.The incoordination of the displacement of the upper and lower walls of the ground fissure strengthens the dynamic interaction between the surrounding rock and the lining structure in the area near the ground fissure,resulting in a sharp increase in the dynamic earth pressure of the lining structure across the ground fissure section,and the soil also enters a plastic state more quickly.The stress concentration of the segments near the hanging wall is more intense,of which the circumferential shear stress of the right arch foot is the largest,and the bending moment at the right arch waist increases significantly.(3)The comparison of several influencing factors on the seismic response of shield tunnels across ground fissures shows that: the seismic waveform has the greatest impact on the acceleration response of surrounding rocks and lining structures near the ground fissure,followed by the seismic intensity,and the intersection angle between the tunnel and the ground fissure has the least effect.As the intersection angle between the ground fissure and the tunnel decreases,the influence range of the ground fissure site hanging wall effect increases slightly.The acceleration response of the shield tunnel above the arch waist increases with the increase of the intersection angle,and the acceleration response of the shield tunnel below the arch waist decreases with the increase of the intersection angle.The peak and range of the principal stress response of the tunnel vault and the inner and outer sides of the ground fissure hanging wall increase with the decrease of the intersection angle.The intersection angle has a greater influence on the shear stress of the tunnel arch bottom and the bending moment at the arch waist.Compared with the artificial wave and Taft wave input conditions,the hanging wall effect of the ground fissure site under the EL-Centro wave input is more obvious,and the dynamic earth pressure between the lining and the surrounding rock and the lining stress response are more significant.