Study On The Seismic Deformation Characteristics Of Artificial Frozen Thawed Clay Around Subway Station

Artificial freezing method is widely used in subway station and tunnel construction,but after freezing and thawing,the strength of soft soil is significantly reduced,and the settlement deformation is more significant.Many historical earthquake disasters show that the engineering construction in the areas with wide distribution of soft clay has brought great security risks to the seismic fortification work.In this paper,the equivalent linear seismic response of undisturbed soil and frozen-thawed soil in frequency domain is analyzed by DEEPSOIL software.The dynamic characteristics of saturated soft clay before and after freezing-thawing under earthquake loading are analyzed by two-way dynamic triaxial test.The seismic dynamic interaction between the two sites and subway station are simulated by ABAQUS finite element method.The research results mainly include the following six aspects.(1)Through time history inversion of ground motion in frequency domain,it is found that the acceleration time history of bedrock ground motion is quite different from that of ground motion.The results of DEEPSOIL equivalent linear analysis show that the ground motion acceleration of soft soil site has obvious amplification effect,and that of frozen-thawed soil site is more obvious.(2)The maximum dynamic shear stress increases monotonously with the increase of depth.At the same depth,the seismic shear stress of frozen-thawed soil site is larger than that of undisturbed soil site.With the increase of soil depth,the shear stress reduction factor decreases gradually.The reduction coefficient curves of undisturbed soil site and frozen-thawed soil site are close.The shear stress reduction coefficient of clay site will be overestimated by Seed liquefaction resistance shear stress method.(3)In the static triaxial test,soil samples soften during shear process,and freeze-thaw soil samples are more likely to be destroyed.In the step loading test,the the dynamic shear modulus increases with the confining pressure.The dynamic shear modulus of the undisturbed soil is larger than that of the frozen-thawed soil,and the freeze-thaw effect will lead to the decrease of the shear strength of the soil.(4)Under the same confining pressure,the saturated consolidation deformation of frozen-thawed soil is larger than that of undisturbed soil.With the increase of loading times,the axial strain of soft clay tends to slow first and then fast.There is a limit minimum cyclic strength between the undisturbed soil and the frozen-thawed soil.The axial strain of the frozen-thawed soil is less than that of the undisturbed soil when the axial strain reaches the failure.Under the same seismic shear stress,the excess pore water pressure of frozen-thawed soil develops faster than that of undisturbed soil.The pore pressure produced by the soil with loading frequency of 1Hz is the largest.The pore water pressure of soil samples with depth of 12 m develops fastest and is more prone to failure.(5)Based on ABAQUS finite element analysis,when the input earthquake is small,the surface settlement is mainly concentrated in the soil layer in the middle and upper part of the roof span of the station.When the local vibration is large,the soil layer above the middle column of the station is deformed greatly,and the ground surface is more likely to deform.The whole structure of the station collapsed in the form of "M" under the ground vibration of 0.4 g in the undisturbed soil site,and the compression deformation of the middle column was serious.While the station in the frozen-thawed soil site has been damaged greatly when the seismic motion is 0.3 g.(6)Under the same seismic motion,the displacement angle between stations in the frozen-thawed soil site is more likely to reach the limit value and damage than the undisturbed soil site.During the earthquake,the middle column is damaged first because of its bearing large axial pressure.Because of the low strength of the frozen-thawed soil,the damage of the columns is more serious under the earthquake.Therefore,the key to seismic design of subway stations in soft soil area is to improve the artificial frozen-thawed soil around the subway station and improve the deformation capacity of the columns in the station.There are 90 figures,15 tables and 144 references in this paper.