Seismic Response Analysis Method For Underwater Foundation Site And Its Application

Seismic response research of underwater foundation sites has become an important hotspot in free-field seismic response analysis.The dynamic response analysis can not only provide a basis for the development of seismic planning of marine sites,but also lay a foundation for seismic response analysis of underwater structures such as underwater pipelines and submarine tunnels,and provide some scientific evidences to ensure the safe construction and operation of underground structures.At present,the calculation of the dynamic response of the underwater foundation site is mainly to assume the underwater saturated soil layer as a single-phase medium,and it neglects the characteristics of inertia,viscosity and mechanical coupling between the solid and liquid phases of the saturated soil.Biot's wave theory of saturated porous medium is applied in many available papers,however,research method of saturated soils is mostly focused on the influence of soil own parameters on the overall dynamic response,without considering the effects of water depth and underwater soft soil layers,and the study of saturated soils is limited to the scope of linear elasticity.A lot of experiments and theories suggest that soil is a nonlinear material,and it exhibits strong nonlinearity under the force action of small earthquake.Therefore,it is necessary to consider the nonlinear effect of saturated soil.On the other hand,the dynamics response of underwater tunnels mostly uses computational fluid dynamics(CFD)methods and the influence of water layer is considered,based on the assumption of simplified effective stress of the saturated soil layer.This does not simulate the true coupling interaction of fluid and solid phases and coupling interaction of porous medium and water layer.In addition,underwater tunnels are a long-line structure and small longitudinal stiffness.They become more susceptible to the non-uniform movement of the ground space caused by the earthquake.They lead to the great variety of internal force and deformation in the longitudinal direction.So underwater tunnels are worthy of attention to the study of the longitudinal dynamic characteristics.Supported by the National Key Basic Research Program of China(973 Program),the dynamic response of underwater foundation sites and the longitudinal dynamic characteristics of underwater tunnels are studied and the following main research results are achieved in this paper:(1)The model of underwater single-layer saturated soil foundation site is established,and the analytical solution of the dynamic response of underwater single-layer saturated soil foundation under plane wave incidence is obtained.The underwater single-layer saturated soil foundation site consists of bedrock,saturated soil and water layer.They are assumed to be impermeable single-phase elastic medium,water-saturated porous medium and ideal compressible fluid,respectively.Based on the elastic wave theory of single-phase elastic medium,Biot's saturated porous medium and ideal compressible fluid,the amplitude equations due to the oblique incidence of plane P-wave and SV-wave are established.The analytical expressions of displacement and stress in frequency domain are deduced,and the results of dynamic pore water pressure are compared with the existing literature to verify the correctness of the equations.The effects of saturated soil parameters and incident frequency and angle on displacement and stress response in frequency domain with different water depths are analyzed by the analytical expressions.(2)On the basis of the underwater single-layer saturated soil foundation site,the model of the underwater layered saturated soil foundation site is established,and the analytical solutions of the dynamic response of the site under plane wave incidence are obtained.Based on the wave theory of Biot's saturated porous media and the boundary conditions of displacement and stress between different layers of saturated soil,the wave amplitude equations of layered soil are established by the expressions of displacement potential functions.Then,the wave amplitude coefficient matrix and unknown wave amplitude vector of layered soil are assembled into the wave amplitude coefficient matrix and unknown wave amplitude vector of single layer saturated soil,respectively.The integral wave amplitude coefficient equation and unknown wave amplitude coefficient matrix of the saturated soil foundation site are established.The displacement results are compared with the existing literature to verify the correctness of the method.The solutions are compared with the results of the existing literature to verify the rationality of the treatment method of the layered soil emphatically.The influences of the thickness and burial depth of the soft soil layer on the frequency-domain response of the displacement due to the plane P-wave and SV-wave oblique incidence are studied with the variation of water depth.(3)Based on the one-dimensional time-domain algorithm of saturated medium,the one-dimensional time-domain algorithm of underwater foundation site is established.Rayleigh damping is assembled into the algorithm and it is extended to the calculation of two-dimensional free wave field by using the law of wave propagation.The method is compared with the time-domain analytical solutions of the underwater saturated soil foundation site by FFT to verify the rationality.Then,Rayleigh damping is introduced into one-dimensional time-domain method for underwater ground sites to take into account the effect of damping.The introduced method of Rayleigh damping is verified by comparing with the corresponding theoretical solution in the time domain by FFT.The application of Rayleigh damping to the one-dimensional method of underwater foundation site is discussed.The method is extended to the calculation of two-dimensional free wave field by the law of wave propagation and compared with the analytical results of time domain.(4)Based on the equivalent linearization method,the non-linear effect of saturated soil under water is considered,and the non-linear analysis method of the underwater saturated soil foundation site due to plane wave oblique incidence is established.On the basis of the analytical solution of the underwater layered saturated soil foundation site,an equivalent linearization method is used to consider the non-linear effect of saturated soil.Then,the underwater layered soil site is degenerated into layered single-phase site to illustrate the acceleration time history curve of the soil surface based on the numerical example of EERA software explanation manual.These two similar results are compared to verify the validity of the equivalent linearization iteration method.The influences of incident seismic wave intensity,water depth and thickness of soft soil layer on the site's non-linear acceleration response are studied.(5)Based on the longitudinal integral response displacement method and the equivalent linearization method of underwater foundation sites,the analysis method of the longitudinal dynamic characteristics of the underwater tunnel is established.Firstly,the equivalent linearization method of the saturated ground under water is adopted to consider the non-linear effect of the surrounding rock of the underwater tunnel.The time history of the non-linear displacement response of the free ground under water is obtained.Then,the longitudinal integral response displacement method of the tunnel is used to determine the input displacement at the most disadvantageous time.The free field model of the single-phase soil layer is established by ABAQUS finite element software and the equivalent seismic load is obtained.The load is applied on the model of soil-tunnel interaction by ABAQUS finite element software.Finally,the influences of the water depth,the stiffness of the saturated soil,incident seismic wave intensity and the stiffness of the tunnel on the longitudinal dynamic characteristics of the underwater tunnel are analyzed.