Study On Accumulative Settlement Of Ballast And Its Control In Ballasted High-speed Railway Track

The ballast layer,consisting of crushed aggregate particles with diverse sizes and shapes,is an essential component in ballasted track.The dynamic stresses acting on ballast surface are significantly increased in terms of frequency and intensity at very high train speeds.Owing to significant particle movement and rearrangement,the settlement of ballast layer is accelerated,which poses detrimental effects on the operational safety and ride comfort,and leads to increasing maintenance work.Hence,a comprehensive evaluation of the railway ballast ballast layer settlement behavior requires a better basic understanding and a careful insight into micromechanical particle interactions and movements,which is essential for effectively controlling ballast settlement.By means of carrying out laboratory tests and interpretating particle-scale behaviors from correspondingly established discrete element model,this dissertation focues on the meso-scopic mechanism of accumulative settlement of ballast in ballasted high-speed railway track and its contol.The main conclusions are listed as follows:(1)Particle imaging method had abilities of accurately capturing ballast particle shape parameters,and establishing a particle model library that can represent the real shape of ballast.By introducing the generated particle model into a polyhedral discrete element program,a discrete element method(DEM)was developed to simulate the realistic geometries of ballast.(2)The particle imaging-based DEM approach was validated by matching the predictions from the ballast direct shear test DEM model with the measured shear stress-strain relationships in laboratory tests.DEM simulations of direct shear tests on ballast with different shapes were carried out.When the ballast had larger angularity index and flat&elongation ratio,a stronger skeleton structure was formed in ballast by higher particle interactions.Furthermore,limiting individual particle movements were shown to increase strength and provide greater resistance to deformation.(3)A ballast triaxial test DEM model was built to investigate ballast strength and permanent deformation properties.The development of sample-scale plastic deformation under cyclic loading arose from the accumulation of particle-scale movements.By changing confining pressure and sample compactness,particles behaved differently in terms of movements,thus various plastic deformations of sample arose(4)A full-scale physical model test on ballasted track-subgrade system was carried out When train speed was lower,the accumulative settlement of ballast had a smaller growing rate and soon reached a stable state.When train speed increased to 300km/h.especially under the axle load of 22 tons,the amplitude of ballast vibration velocity was up to 37.5mm/s,such high vibration intensity was more likely to bring the ballast into an unstable state.Meanwhile,the dynamic stress level acting on the ballast increased significantly.Thus the accumulative settlement of ballast grew rapidly,and a stable state could not be reached even after 100,000 moving train loads,which would shorten the maintenance period(5)A ballasted track DEM model was built to analyse the effects of two different loading scenarios on the accumulative settlement of ballast.When the accumulative settlement of ballast tended to be stable after the same loading cycles,the permanent deformation of ballast was increased by 60%under moving wheel loading.The moving wheel loading induced many more particles to experience larger principal stress axis rotations,thereby mobilising greater particle movements,increasing ballast particle rearrangement.Finally,the ballast laver accumulated much larger permanent deformation(6)A geogrid-reinforced ballast layer DEM model was built to investigate the mechanism of geogrid reinforcement and its reinforced zone.Geogrid laying enlarged the distribution range of passive shear stress and lateral constraint stress as well as their magnitude in ballast,thus significantly reduced the particle lateral movement,meanwhile the particle rotation angle were lowered.As a result,development of the ballast accumulative settlement was restricted.The optimum reinforced zone was 67mm above the geogrid,where the lateral displacement of particles had the most reduction.