Dynamic Response And Long-Term Accumulated Deformation Characteristics Of Subgrade For Heavy Haul Railway

Dynamic interaction between train and line is significantly enhanced with the increasing of vehicle axle load,and the resulting subgrade defect problems,such as rail abnormal abrasion,sleeper fracture,ballast powdering,subgrade settlement and embankment side slope collapse,have seriously threatened the running safety of trains.Main reason of that can be attributed to the change of the maximum pressure on sleeper,load sharing between sleepers and effective depth of subgrade dynamic stress due to the increase of axle load.For this reason,the research status of some problems related to subgrade dynamic response at home and abroad was summarized.Based on which,considering the application background of subgrade design and maintenance for heavy haul railway,the dissertation carried on some fundamental researches on dynamic response of heavy haul railway embankment subgrade,dynamic performance of coarse grained soil,and long-term deformation characteristics under cyclic loading by means of theoretical analysis,laboratory experiment,in-situ vibration test and numerical simulation.The main research work and results of this thesis are as follows:(1)To get the actual property as well as the attenuation rule of vibration source,typical embankment sections at Gan-Quan,Bao-Shen and Ta-Han Railway lines were selected,and the vibration responses of rail,sleeper and embankment surface induced by heavy haul trains were recorded.The measured non-stationary vibration signals were pre-processed with Empirical Mode Decomposition method(EMD),and the difference between vibration in vertical and horizontal directions was compared from time domain and frequency domain,respectively.Moreover,effects of vehicle type,velocity and embankment height on subgrade vibration responses were also analyzed.(2)According to the features of subgrade dynamic stress,laboratory dynamic triaxial tests were carried out to investigate the effects of confining pressure,compaction degree and consolidation ratio on dynamic parameters of the subgrade coarse grained soil.Relationships between dynamic stress and strain were described by equivalent linearization model,the backbone curves were fitted with classical Hardin Hyperbolic Model.On this basis,correction factors of compaction and consolidation ratio were introduced to refine the empirical expressions of the maximum shear modulus,maximum dynamic shear stress and maximum damping ratio.(3)Dynamic finite element analysis model for Gan-Quan railway embankment section was established,considering the interaction between rail,sleeper,ballast,subgrade and site.A three-dimensional viscous-spring artificial boundary was used to model the infinite domain condition of the subgrade.The train moving load was modeled with vertical uniformly distributed load in place of unmovable point load,and applied in real time with subroutine method.The non-linear constitutive relation of the subgrade structure layer was taken into account with equivalent linearization model.Using this model,the transmission and sharing function of wheel-rail force among sleepers,the spatial and temporal distribution characteristics of subgrade dynamic stress,as well as the stress path change rules were analyzed.Take the depth where dynamic stress equal to 0.2 times of subgrade gravity as effective action depth,with which effects of axle load,velocity,ballast thickness and subgrade bed thickness on subgrade dynamic stress were compared.On this foundation,the peak value of dynamic stress on subgrade surface was expressed as a function of axle load,velocity and ballast thickness,and the attenuation relation along the depth direction was depicted with exponential function.In-situ measured data showed the validity of the simplified computing formula.(4)Dynamic strength and long-term deformation characteristics of coarse grained soil were investigated with dynamic triaixal tests.Residual strength was introduced to normalize the dynamic strength obtained under different initial contions,and the relationship between dynamic strength ratio and failure cyclic number was established with hyperbolic model.According to the long-term deformation development mode of coarse grained soil,a permanent strain empirical model,which took into account the contribution of cyclic stress amplitude,initial mean stress and initial stress ratio,was established and validated.Combined with the calculated subgrade initial conditions and dynamic stress distribution,the variation of embankment subgrade permanent deformation with number of load cycles or freight volume was obtained.