Research Of High-speed Railway Ballast Mechanics Based On Discrete Element Method

Ballasted track is an important railway structure. It is used widely with its significant advantages. However, China has not finished enough investigations on high-speed railway ballast mechanics, and a lot of key scientific problems have not solved until now. The conventional finite element method cannot analyze many problems of ballast mechanics, including ballast gradation, breakage, ballast resistance, etc. High-speed railway ballast mechanics have not understood clearly, which prevents the wider application of ballasted track in high-speed railway.Considering the shortage of research on high-speed railway ballast mechanics, and based on the analysis of previous research and application of high-speed railway ballasted track, three dimensional model of ballast bed using discrete element method was established in this thesis. High-speed railway ballast mechanics from theory research and field test were analyzed and estimated. The field test results of dynamic and statistics tests were both took into account. Some advice on relieving ballast degradation was given in this thesis. The major work and conclusion are as follows:1. The effects of ballast parameters on ballast bed mechanics were investigatedRailway ballast is a kind of granular materials, and its particle size, density, friction coefficient and gradation have a significant effect on ballast mechanics. Previous research has not made an overall and systematic investigation on ballast basic parameters. Ballast bed model using the sphere element in PFC3D was established in this thesis, and the effects of ballast bed density, ballast friction coefficient and gradation on ballast mechanics were further analyzed.2. Three dimensional ballast bed with crushable ballasts was established, and the breakage characteristics of ballast bed under cyclic loading were investigatedUnder repeat impact loading of high-speed train, ballast may break into fine particles, which affects ballast mechanics significantly. Existing research has not investigated the breakage characteristics of ballast well. The process of ballast breakage is not clearly as well. This thesis simulated crushable ballast with cluster through self-compiled program in PFC3D. Ballast bed model using crushable ballasts was established and ballast mechanics were analyzed, especially the position and number of breakage. The effects of load frequency, load attitude and the strength of bond on ballast mechanics were also analyzed in details.3. Elastic sleeper and ballast bed model were established, and the effects of elastic sleeper to relieve ballast degradation were investigatedProblems of insufficient ballast thickness and excessive track stiffness, especially in areas like bridges and tunnels, were common in high-speed railway ballasted track. Elastic sleeper is one opportunity to meet the challenge of increasing ballast bed elasticity. Previous research mainly applied finite element method to study the working principle of elastic sleeper, this thesis applied discrete element method to establish elastic sleeper and ballast bed model, and investigated the effect of elastic sleeper on ballast mechanics, especially the vibration of ballast bed and breakage characteristics of ballasts. Furthermore, the effects of stiffness, density and friction coefficient of under sleeper pad on ballast mechanics were analyzed, and advice on reasonable design parameters of elastic sleeper was provided.4. DEM model of geogrid was established, and the interaction between geogrid and ballasts was analyzedBallast settlement is an important factor that determines the maintenance period of ballasted track. Geogrid is a preferable method to enhance the interlocking force between granular materials and reduce ballast settlement. China researchers have not made enough investigations on geogrid application for ballast bed. This thesis used DEM to simulate geogrid early. Using geogrid unidirectional pull test, the contact parameters of geogrid were determined reasonably. Through geogrid pull out test, the effects of vertical load, geogrid size and friction coefficient on pull out resistance were studied. Furthermore, the ballast box model was established. The influences of geogrid on ballast mechanics were investigated, and the effects of geogrid location, geogrid size and friction coefficient on geogrid behavior were also investigated.