Seismic Design Method Of Underground Structures In Solid-liquid Two-phase Medium

Earthquake resistance of underground structure has became one of the fundamental problems in the field of earthquake engineering.The main research contents include: soil constitutive model,wave propagation characteristics in soil media,artificial boundary conditions,seismic wave input method and seismic design method of underground structure.It is well known that most of the soil in nature is solid-liquid saturated two-phase media or solid-liquid-gas unsaturated three-phase media.And the existence of soil liquid phase in an earthquake will significantly increase the seismic response of a structure.However,at present,researches on seismic design method of underground structure are mostly based on the single-phase soil particle media,which is inconsistent with the actual project.Therefore,the contents of this thesis are as follows:1)An equivalent load input method was proposed on the basis of infinite element boundary,and the method is implemented in the ABAQUS software.Then,the analytical results of the proposed method were compared with those of the equivalent load input method of viscoelastic boundary,considering the soil as single-phase media and two-phase media respectively.The results show that the proposed method not only saves computational time,but also has higher precision,which provides a new idea for seismic design of underground structures.2)The reflection of SV wave at the free field of unsaturated soil was analyzed based on the wave equation of unsaturated three-phase media.Potential function of wave equation was used to solve the amplitude reflection ratios for reflected SV wave,reflected P1 wave,reflected P2 wave and reflected P3 wave at the free field of unsaturated soil and the displacement response of the free field while SV wave incident in two-dimensional problem.In the numerical analysis,the effect of saturation level on the amplitude reflectance ratios of each reflected wave and the seismic response of free field was investigated,especially in the case of high saturation.It was shown that:(1)There is small effect of saturation on the amplitude reflectivity of reflected SV wave.(2)There has a great effect of saturation on the amplitude reflectivity for reflected P1 wave and reflected P2 wave,especially in the case of high saturation,and the reflectivity increases obviously with the increase of wave incident angle;(3)There is rarely effect on the amplitude reflectivity for reflected P3 wave.And the amplitude reflectivity for reflected P3 wave is so small to be ignored.(4)Saturation level has little effect on the seismic response of unsaturated soil in free field,which can be neglected.3)Based on the seismic response displacement method of underground structure,the soil-structure interaction caused by seismic excitation was simulated by equivalent load though damping.A seismic design method,named response velocity method,was proposed.In order to verify the efficiency of the method,the reaction velocity method and finite element dynamic analysis are conducted to calculated the Dakai subway station.The numerical calculation results show that: There are less relevant parameters in the reaction velocity method and the damping coefficient is easy calculated.The calculation results of structure are relatively safe compared to the finite element method considering soil as single-phase soil particle media.While for the solid-liquid two-phase media,the calculation of damping coefficient considers the influence of the liquid phase,and the calculation results of the reaction velocity method is well consistent with FEM results.