Study On Macro-Meso Dyanmic Mechanical Behaviour Of Railway Ballasted Track And Infrastructures Under Vehicle Loads

Nowadays,ballastless tracks have been widely used in Chinese high-speed railways,but ballasted tracks still occupy the majority of the existing lines at the speed of less than 200 km/h and the heavy haul railway lines.Compared with the ballastless track,the ballasted track has the advantages of low construction cost,proper elasticity,good vibration and noise reduction and easy maintenance.However,the geometry of the ballasted track is difficult to retain and the maintenance work is enormous.With the increasing speed of passenger train and axle load of freight train,dynamic loads acted on the ballasted track and subgrade are getting larger and denser,which threats the reliability of the track components,accelerates the degradation rate of track geometry and increases the maintenance work.Therefore,it is necessary to explore dynamic mechanical behaviour of ballasted track and subgrade system,so as to provide theoretical guideline and technical support for the design and maintenance of ballasted tracks.Many researchers have studied the track-subgrade system using the vehicle-track coupled dynamics and finite element method,in which ballast layers were simplified as rigid mass blocks or finite solid elements.These studies presented the wheel-rail interaction mechanism and track vibration characteristics,but the meso-mechanical characteristics(e.g.the slip,roll movement and breakage behaviour of the ballast particle)of ballast layer can hardly be simulated using these traditional methods,so that the influence of ballast meso-mechanical behaviour on service performances of railway infrastructures cannot be revealed.In recent decades,many scholars investigated the meso-macro mechanical behaviour of railway ballast using the discrete element method(DEM).Nevertheless,most of these studies ignored the elastic support of the subgrade and foundation and dynamic interactions between the ballasted track and the vehicle.Thus,these researches cannot be used to guide a comprehensive evaluation and design of the railway infrastructure.In this thesis,the coupled discrete-continuum model of the ballasted track and the infrastructures(subgrade,transition construction and foundation)and the meso-macro dynamic model of railway vehicle-track-subgrade system is built based on the DEM,the finite different method(FDM)and multi-body system dynamics.Then,these model are used to explore meso-macro mechanical behaviour of the railway infrastructure system.In the terms of discrete-continuum modeling and numerical calculation,the DEM model of the ballasted track considers complex geometric shapes of ballast particles and the particle size grading requirements.The sleeper was built as rigid blocks using the Clumps of the PFC software,and the rail was modeled as a beam linked with the parallel bond.On the other hand,the subgrade,the foundation and the transition structure was modelled by the Fast Lagrangian Analysis of Continua(FLAC),then the coupled discrete-continuum model of the ballasted track and infrastructures was generated by using jointly the PFC and FLAC software.Furthermore,the vertical dynamic model of the freight locomotive was built and integrated to the coupled discrete-continuum model.Especially,a calculation method of contact forces between the wheel and the DEM rail is proposed,and the calculation codes was developed using the FISH programming language.Thus,the full-time coupled numerical simulation of railway vehicle and infrastructure system was implemented in the PFC and FLAC environment.Finally,the laboratory experimental and field test results were used to validate these models,including the large-scale direct shear tests of ballast aggregates,the field tests on Shuohuang heavy haul railway and Suining-Chongqing railway track transitions.In the terms of the meso-macro mechanical analysis of the ballasted track and infrastructure system,dynamic responses of the track transition zone were analyzed considering various material parameters of wedge-shape backfills,the slope of backfills and transition structure forms.Numerical results indicate that the inverted trapezoid track transition is better than the trapezoid track transition,and some suggestions on the design of track transition and the selection of backfill material were given.Afterwards,the influence of the contact stiffness of ballast particle,the ballast density and thickness on the dynamic responses of ballasted track and subgrade system were studied.The comparative analyses show that the increase of the contact stiffness of ballast particle and ballast density can reduce the vibration of track components,but increase dynamic stress of the ballast layer and the subgrade surface.The larger stress would enlarge the possibility of ballast breakage and accelerate ballast abrasion,and then deteriorate the service status of railway infrastructure.In addition,the increase of the thickness of ballast layer would be helpful to slow down ballast particle breakage and decrease the stress of subgrade surface.Using the coupled vehicle-ballasted track-subgrade model,the influences of unsupported sleeper and the ballast layer with tire-derived aggregate on dynamic behaviour of railway system are studied.Firstly,dynamic responses of the vehicle-ballasted track-subgrade system under different conditions including the normal ballasted track,the ballasted track with one piece of unsupported sleeper and 2?5 pieces of unsupported sleepers were analyzed comparatively.Numerical results show that under the travel speed of 80 km/h one unsupported sleeper has few influences on dynamic performances of the vehicle,track and subgrade system.However,when the number of unsupported sleepers is over three,dynamic responses of the track and subgrade increase significantly.In the region under the unsupported sleepers,dynamic responses of ballast layer and subgrade decrease.In the region adjacent to the unsupported sleepers,the rail seating force,the stress of ballast layer and subgrade surface,the number of broken bonds in the ballast particle increase remarkably,which would speed up infrastructure degradation.Especially,the biggest dynamic response occurs when the number of continuous suspended sleepers is there,it should be paid more attention during maintenance work of the ballasted tracks.For the tire-derived aggregate(TDA)applied in the ballast layer,meso-macro mechanical behaviour of vehicle-track-subgrade system are analyzed considering the ballast layer mixed 0,10% and 20% TDAs.Numerical results show that vibration response of vehicle has tiny increase when some TDAs are mixed in the ballast layer,but it slightly affects vehicle dynamic performance.The TDA can change considerably dynamic response of the ballast layer and subgrade.For example,the TDAs alleviates dynamic stress of the ballast layer and increase significantly friction energy dissipation of ballast particles,all of these are beneficial to slow down the ballast deterioration(ballast breakage and abrasion),and then reduce the ballast degradation and cumulative settlement.The TDAs also reduce dynamic stress on the subgrade surface,which is helpful for reducing long-term plastic deformation of subgrade.Comparative analyses show the 20%TDAs with small size and narrow gradation can improve preferably dynamic performances of the ballast layer and subgrade.