Enviromental Vibrations Induced By Underground Railways In The Saturated Soil

The problem of ground-borne vibrations caused by underground moving trains has received considerable attention in the past decades due to the increasing public sensitivity to environmental impacts.The complaints from inhabitants near the underground railways are likely to occur due to the noise and vibration pollution caused by the public transportation.In the southeast coastal area of China,the underground water level is high and most of metro lines are estabilished in the soft saturated soil.However,few researchers focus on the stuty of dynamic responses of the saturated soil generated by underground moving loads.As the presence of the pore fluid in the saturated soil has significant influence on the wave propagation,based on the exsisting research framework and Biot elasticity theory this thesis investigates the ground-borne vibrations from the underground railways in the saturated soil,as follows:(1)Using the Fourier transform,the dynamic response of a tunnel buried in a two-dimensional poroelastic soil layer subjected to a moving point load is investigated theoretically.Results obtained by this model can be treated as an upper estimate of ground vibration level generated by a metro line which is of significance in the practical vibration control.The critical velocity of the considered structure is determined from the dispersion curves.The proposed model can identify the different performance of the elastic soil medium and the poroelastic soil medium subjected to underground moving loads.In addition,a parametric analysis including the shear modulus of the solid skeleton,the permeability of the saturated soil and the load moving velocity,on displacement and pore pressure responses is presented to analyze the soil-beam interation.(2)Based on Biot governing equations and the shell theory in the cylindrical coordinate system,dynamic response of a tunnel embedded in a three-dimensional poroelastic full-space generated by a unit harmonic load applied at the tunnel invert is studied.Analytical solutions are obtained based on Fourier decomposition into ring modes circumferentially and Fourier transform longitudinally.Spatial distribution for the displacement,pore pressure and stress is presented induced by the dynamic loadings in the tunnel.The effects of the soil permeability and the load frequency on the responses of the tunnel and soil are analyzed.The different dynamic characteristics of the equivalent single-phase medium and the saturated poroelastic medium subjected to the dynamic loadings in the tunnel are investigated.The proposed analytical model can also serve as the benchmark for other computations with numerical or asymptotic approach.(3)On the basis of the analytical tunnel-soil model,dynamic responses of floating slab tracks and underground railway tunnel embedded in a poroelastic full-space under moving point loads are studied.The track and tunnel-soil model are coupled by force and displacement compatibility conditions at the tunnel invert.The vibration isolation effects of different railway structures can be evaluated by this coupling model.The different dynamic characteristics of the track with continuous slab and with discontinuous slab are investigated.(4)By using the basic solution for the elastic governing equations in the rectangular and cylindrical coordinate system,a closed-form analytical solution for the vibrations due to a moving point load in a tunnel embedded in a half-space is given.It also serves as an efficient tool to predict ground vibrations induced by underground railway tunnel in early design phases.Numerical results for ground vibrations due to a unit moving point load applied at the tunnel invert are presented for different load velocities and frequencies.Furthermore,the influence of the tunnel depth and the tunnel thickness on the propagation of vibrations is investigated.(5)By using the basic solution for the Biot governing equations in the rectangular and cylindrical coordinate system,a closed-form analytical solution for the vibrations due to a point load in a tunnel embedded in a saturated poroelastic half-space is proposed.The closed-form analytical solution can predict the ground vibration from an underground railway tunnel of circular cross-section with a reasonable computational effort and can serve as a benchmark solution for other computational methods.(6)Finally,the 2.5 dimensional finite element methods with artificial boundaries are extended to the poroelasticity.The coupling model of underground railway is established and used to study the dynamic response of track system and saturated soil.The model is divided into four parts,the moving train,floating-slab tracks,tunnel and saturated ground.The tracks are solved by the analytical method and by adopting the method of 2.5D FEM,the elastic theory and the Biot's theory are used to simulate the tunnel and saturated porous medium,respectively.Computed results for the dynamic responses of track system and ground surface are presented in both frequency and time domain.A series of theoretical model is established to predicit the gound vibrations induced by the underground railways.The present work definitely enriches the study of wave propagation in the saturated soil and of ground-borne vibrations from underground moving loads.It also provides policy basis and theoretical guidance for the future development of vibration predition and isolation.