Study On The Response Mechanism Of Shallow Subway Station And Surrounding Sand Ground Under Strong Earthquakes

With the rapid development of urbanization,the subway engineering has entered a period of rapid development in soft soil area.Combined with the basic situation of frequent and serious earthquake disasters in China,this thesis studies the strong earthquake response mechanism of liquefiable site and its shallow buried subway station through indoor small sample test and numerical simulation.The research results provide theoretical basis for the seismic design of subway station in liquefiable site.The main contents of this thesis include the following three aspects.?1?Based on the static and dynamic triaxial tests,the mechanical properties of saturated sand under different confining pressures are studied,and the relationship between the mechanical parameters of saturated sand and the buried depth is obtained.Based on the static triaxial test data,the linear relationship between the tangent modulus of saturated sand and the buried depth is obtained by fitting;Based on the dynamic triaxial test data,the relationship between the maximum dynamic shear modulus of saturated sand and the buried depth is obtained by fitting;The shear modulus ratio decreases with the increase of confining pressure and the damping ratio decreases with the increase of confining pressure.Based on the fitting analysis of triaxial test data,it is verified that the physical and mechanical parameters of saturated sand vary with the burial depth.Thus,in the numerical simulation,the homogeneous sand site with the average shear modulus and the heterogenous sand site with the linear change of the shear modulus with the burial depth are established.?2?Based on FLAC^3D software,two kinds of three-dimensional numerical models of free field are established.Through multi amplitude seismic input,the seismic response mechanism of liquefiable free field is explored.The development law of excess pore water pressure and effective stress of two kinds of liquefiable sand sites is related to the wave form of the applied seismic wave.The excess pore pressure ratio of liquefiable sand increases with the decrease of burial depth,and the shallow part of the site is the first to liquefy,and the shallow part of the heterogenous site is more likely to liquefy.With the increase of the peak acceleration,the liquefaction area of the site gradually develops to the deep,and the liquefaction needs a certain amount of energy.Liquefiable site has the effect of"isolation"and the effect of"isolation"is closely related to the development of pore pressure.The maximum horizontal displacement and vertical displacement of the liquefiable site increase with the increase of the peak acceleration,and the displacement development law of the site is related to the pore pressure development law.?3?On the basis of two kinds of liquefiable sand free field,this thesis introduces subway station,and explores the seismic response mechanism of shallow buried subway station in liquefiable field through multi amplitude seismic input.The liquefaction development of the site far away from the station is the same as that of the free field.With the increase of the peak acceleration,the liquefaction area develops from the shallow part to the deep part of the site.The lower part of the station near the structure is the first liquefied area.The ground surface far away from the station will subside,while the ground surface near the station will rise.The uplift of subway station is caused by the circulation of liquefied soil on both sides of the side wall to the bottom of the station and the buoyancy generated by the development of pore pressure in the lower site of the station.The uplift displacement of the station structure in the heterogenous sand site is significantly greater than that in the homogeneous sand site.The stress response of subway station increases with the increase of acceleration peak.The typical unfavorable stress position is at the junction of center pillar,center pillar and top,bottom,side wall and top,bottom.The stress value of station structure in heterogenous sand site is slightly higher than that in homogeneous sand site.There are 85 figures,17 tables and 122 references in this thesis.