Dynamic Response And Settlement Induced By Rail Transit In Soft Soil Foundation

The mainly dynamic behaviors of track foundation under the train load are vibration and settlement.With the construction of rail transit facilities in the saturated soft soil areas and the increase of train running speed,the dynamic response of track foundation under traffic load has become a subject of great concern in the field of transportation geotechnical.There are many research achievements using single-phase elastic model,however,the theoretical and experimental research on saturated subgrade is still limited.In this paper,the dynamic response in high-speed railway and urban subway are studied based on Naiver and Biot elastic wave theory by combining 2.5 dimensional finite element method(2.5 D FEM)with field test and full-scale model testing.The main work and conclusions are list as follows:1.Based on Naiver elastic wave theory,the 2.5 dimensional format of vehicel-rail-foundation coupling equations based single-phase elastic wave theory are deduced.The 2.5 D FEM analysis models of ballast track and ballastless track are established and verified.The analysis results of the dynamic responses of these two different types of track show that the dynamic stress at the subgrade surface in ballast track is much higher than that of the ballastless track,if they are both built according to the design standard.The amplitude of the dynamic stress of ballast track with good condition is about 6 times of that in ballastless track.By built the ballast and ballastless track structure on the same track foundation-ground model,the role of the track structure played can revealed.It comes that the vibration amplitude in ballast track is about 1.1 times of that in ballastless track,and 1.3 times for the dynamic stress.Compared to the ballast track,the dynamic stress distribution on the subgrade surface of the ballastless track is more uniform and the amplitude is smaller,which indicates the ballastless track is an optimized track structure form that can effectively control the vibration and settlement.2.It's found the smooth track model can be used to predict the dynamic stress when the train speed is lower than 150km/h.However,the influence of track irregularity must be considered when the train speed gets higher.The new railway line built according to the standard can meet the current control limitation of dynamic stress.However,as time goes on,even for low-speed line,the dynamic stress on the subgrade surface will exceed the limitation value.3.The critical speed of the system of ballast and ballastless track that built on the same track foundation-ground model is consistent.Thus,the critical speed of the track system has nothing to do with the track form.The existence of subgrade greatly increases the critical speed of the system in soft soil area,and the increase amplitude decreases with the increase of ground stiffness.With a subgrade about 1.2m thickness can well improve the critical speed of the track system,but with the decrease of the subgrade thickness,the subgrade vibration amplitude will increase significantly.4.Based on the Biot theory,the 2.5 D governing equation of saturated soil is derived and applied to obtain the dynamic response in saturated soil.A parameter c/kD is defined to describe the combined effect of load moving speed and soil permeability on the generation of excess pore pressure under traffic load.It is found that c/kD determines the extent to which excess pore pressures build up during passage of the load,at any given depth.For a single moving load,there is a critical value of c/kD,above which the maximum pore pressure reaches a constant value;this critical value depends on the depth.Below the critical value,the pore pressure accumulated during the passage depends on the value of c/kD and is otherwise independent of the load speed.For the normal speed range of high-speed railway(200km/h-360km/h),when the permeability coefficient of saturated soil is greater than 10-3 m/s,the train operation will not cause the response of excess pore pressure in saturated soil,however,if the permeability coefficient of saturated soil is less than 2 × 10-6 m/s,any velocity within the velocity range will cause significant and equal excess pore pressure.For subway load with lower running speed,the corresponding permeability coefficient is 10-7 m/s.The pore pressure accumulated during the passage of a bogie pair is greatest for intermediate values of c/kD.5.A comprehensive set of measurements of wheel load,tunnel structure vibration and pore pressure is performed in Ningbo urban subway to study the dynamic response and its propagation in the surrounding soil.The moving effect of traffic load would bring dynamic amplification effect,which making the wheel-rail contact force greater than the static axle weight.The frequency characteristics of wheel-rail force response are determined by the train speed and the distance between adjacent bogies.The vibration on the subway track is uniformly distributed.The vibration amplitude and intensity depend on the speed of the subway train.The main frequency components of the track vibration are all below 85Hz.Traffic load will cause residual pore pressure in saturated soil and with the reciprocating action of traffic load,residual pore pressure in saturated soil will gradually accumulate.6.A 2.5 D FEM analysis model of Ningbo urban subway is established to study the difference of dynamic response in single phase elastic model and saturated one.It is found that the vibration transmitted to the surface through single elastic soil is larger than that through saturated soil.The increase of tunnel depth can effectively reduce the environmental vibration at the ground surface.The excess pore pressure caused by subway load are less than 1 kPa.Therefore,there is little difference between the vertical dynamic stress(effeective stress)and shear stress in single elastic ground and saturated ground.7.An analysis model of 2.5 D FEM cooperates HDQM one dimensional consolidation theory is established.Based on this model,it is found that the residual pore pressure in saturated soil will accumulate first and then dissipate under the long-term cyclic traffic load.The accumulation value of residual pore pressure increases with the increase of depth,and the time needed for the completion of residual pore pressure accumulation increases with the increase of depth.For the test section in Ningbo subway,it takes one month to arrive maximum residual pore pressure,about 8.19kPa.It takes 33 months to reach the stable consolidation degree.The settlement developes rapidly in the first year after the Ningbo urban subway is putted into operation.