Mechanical Behavior And Deterioration Mechanism Research On Railway Ballast Bed

Ballast bed is one of the most important structural forms of railway track in China.Compared with slab track,ballast track has many irreplaceable advantage such as repairable,economical,good adaptation in extreme climate condition and adverse geological conditions.Ballast track consist of ballasts,under the cyclic loading,abrasion,breakage and dust accumulation will come up.All of these may cause mechanical performance degradation of ballast bed,indeed the uneven distribution of ballast bed stiffness and the track geometry instability.Once without well maintained,the track irregularity may come up and operation safety may be threatened.However,the shapes of ballasts are very different and both ballast shape and graduation of ballast have observably influence on mechanical property of ballast bed.These make the behavior under the cyclic loading of ballast bed very complex.Since the traditional research method can hardly study the complex shape of ballast,the research of interaction and deterioration mechanism was confined.Thus,a set of refined DEM numerical model was set up based on 3-D reconstruction result of real ballast particle.Both the numerical and laboratorial method were adopted to study the interaction and deterioration mechanism in different views,the main work of this paper summarized as follows:(1)3-D reconstruction of ballast shape and selecting of typical ballast particles based on similarity algorithm3-D digital reconstruction platform based on 3-D laser scanners and double-mirror intersection method was set up,and the complicated shape of ballast was captured.Shape characteristic quantification method and similarity algorithm based on normalized Fourier descriptor were proposed.Based on analysis results of different ballast particles,the typical ballast particles were selected for building numerical model of ballast bed.(2)Correction and construction of refined ballast DEM numerical modelThe construction method of clump element based on reconstructed result of real ballast shape was proposed for building the DEM numerical model of ballast.Correcting algorithm of inertia moment and overlapped mass was proposed for correcting the ballast numerical model.Based on these,the refined numerical model of ballast was set up.(3)Study of wear mechanism of ballastConstruction method of crushable ballast numerical model based on real ballast shape was proposed.Both the numerical and laboratorial method were adopted to study the influence of particle shape on wear mechanism of ballast,as well as the influence of particle abrasion on ballast bed.(4)Research of the interaction and degradation mechanism of ballastThe large-scale direct shear apparatus was researched and developed for studying the influence of normal and sand fouling on shear strength of ballast bed.And numerical model was set up for studying the influence of graded degradation on shear strength of ballast bed.Based on this,a comprehensive fouling evaluation index was proposed.(5)Research of settlement behavior of ballast bed and sleeper under cyclic loadingA biaxial ballast box cyclic loading numerical model was set up.And related laboratory test was adopted for verifying and calibration the mechanical parameters.Based on these,settlement behavior of ballast bed under cyclic loading of different conditions were studied and revealed.Based on the analysis results of existing settlement theory,an optimized settlement predicted method was proposed.(6)Research of loading transfer mechanism between ballast bed and subgrade based on coupled discrete element-finite difference methodThe discrete element method(DEM)and finite difference method(FDM)coupling algorithm was introduced in mechanical analysis of railway ballast bed structure in this paper.The ballast bed model which consists of crushed stone was well performed based on DEM simulation of realistic appearance of ballasts.Both sleeper and roadbed foundation of successive medium were simulated based on FDM.A constitutive model of coupled transition layer based on DEM,FEM and FDM was set up.Three different computing method were coupled by means of secondary development of different soft wares.Loading transfer mechanism between ballast bed and subgrade of high-speed and heavy-haul railway were studied deeply.And an empirical equation of dynamic stress attenuation with depth was proposed based on the analysis results of loading transfer laws of different railway condition.