Dynamic Responses Of Stratified Poroelastic Soft Soil Foundation Under High Speed Train Load

High-speed train has become the development tendency of world transportation at present.With the rapid expansion of the scale of high-speed rails,the running speed has been greatly improved either.Published researches show that the dynamic responses of the ground induced by the low operation speed have tiny differences with the analytical responses solved by static procedure.However,the enhanced vehicle-rail interaction and the vibration of substructure induced by the improved running speed of the trains will threaten the safety of train operation.In the southeastern coastal areas of China,saturated soft soil is widely distributed in the ground.Since the propagation velocity of Rayleigh wave in soft soil is low,the running speed of the high-speed trains can easily exceed the Rayleigh velocity and induce violent ground vibration.This phenomenon can not only affect the operation safety and passengers' comfort,but also bring enormous influences to the ambient environment.Therefore,it's necessary to study the dynamic responses of poroelastic soft soil foundation under high speed train load.Firstly,a model of a saturated poroelastic halfspace interacting with an infinite beam of finite width based on Biot's theory and Euler-Bernoulli beam theory is established in this paper.To guarantee that there is no separation between the beam and the halfspace when a vertical load is applied,an improved version of smooth contact condition is assumed for the interface of beam and halfspace.The values of equivalent stiffness under different cases are calculated after deriving the expression for the equivalent stiffness analytically using a contour integration approach and the Rayleigh pole and the dispersion branches associating with the three body waves(PI,P2 and S waves)can thus be explicitly taken into account.Trajectories of the Rayleigh pole and the branch points are presented for different soil permeability.Then the influences of the continuity conditions at the beam-halfspace interface and the soil permeability on the equivalent stiffness are investigated.Secondly,a model of 3D stratified saturated ground is established.By introducing the Fourier and orthogonal transformations,Biot's equations for the poroelastic soil are generated into equations governing Love and Rayleigh modes.The ground is divided into several thin layer elements along the vertical direction.By introducing linear interpolation function for the elemental displacement fields,the equations governing Love and Rayleigh modes for a single thin layer element are derived and then assembled to obtain global modal equations.The Green's functions for the 3D poroelastic multilayered half-space is then obtained for point loads by mode superposition method.In the calculating procedure,every two layers is considered as a periodic cell.The numerical results are presented to investigate the dispersion characteristics of Love and Rayleigh modes with respect to the increasing excitation frequency and varying soil parameters.The distribution of stop-bands and pass-bands of wave propagation affected by the number of periodical cells can also be obtained.Finally,a model of 3D railway viaduct is established.The bridge is built as a vertical interaction model of a slab track composed of components including the infinite rail,the rail fastener,the discontinuous slab with underlying cushion layer and the concrete base resting on infinite spans of elastically-supported girders.The Green function of the 3D saturated ground is established with the help of thin layer element method and the piles were discretized into 3D Euler-Bernoulli beam elements.Series of moving harmonic point forces represent the train loads.Impedance of pile group and spatial distribution regularities of ground vibration velocity and pore pressure are obtained through the numerical analysis.Then the variations of ground vibration velocity and pore pressure with respect to the increasing running speed and excitation frequency are studied.The above investigations can enrich the wave theory of poroelastic soils and also can provide theoretical analysis models and methods for forecast of ground vibration induced by railway viaduct.The study has great guiding significance of design and construction for surrounding buildings.