Trackbed Deterioration And Controlling Measures Of Ballasted Track Under Long-Term Train Loads

The recent increase of train speeds and axle loads has produced significantly higher dynamic responses and vibrations in ballasted track,which could eventually cause excessive cumulative deformation and reduced long-term performance.Therefore,the study of the ballasted track behavior and ballast degradation under the long-term train loads of increased train speed and axle load is of great significance to provide better underst and ing for preventing track deterioration and improving long-term performance.To gain insights into the long-term trackbed behavior and the ballast degradation evolution,physical model testing and discrete element method(DEM)were adopted in this paper.Through the full-scale ballasted track model test,the ballast deterioration mechanism,the evolution of dynamic responses,the cumulative migration of trackbed particles and the accumulated trackbed deformation under the long-term train loads were analyzed.The geogrid reinforced ballast mechanism was carried out by triaxial test and discrete element method.The following main conclusions are drawn as follows:(1)A full-scale ballasted track with eight sleepers was designed and constructed based on a validated physical model test platform(ZJU-i HSRT)in the laboratory,the real train loads were determined and reproduced through a distributed sequential loading system coupling with a trainrail-substructure dynamic interaction model.The plastic deformation of the newly constructed trackbed was mainly caused by the compaction of the ballast layer.The movements of individual particles inside the ballast layer were also captured using “Smart Rock” wireless sensors.(2)Sieving analysis together with computer-aided ballast morphology analysis were adopted to quantify the ballast degradation in terms of both the ballast particle size and morphological evolution.In the test,the long-term train loads caused significant ballast degradation in both the particle size and the morphology properties,and the ballast particles in the middle zone produced more severe ballast breakage which could be due to the stronger confinement in the mid-span under the sleeper.The overall ballast shape class became more compact after the test,and ballast particles with larger diameter and specific shape classes(Platy,Bladed,Elongate)were more likely to degrade.Sleeper support stiffness was found almost doubled after the long-term train load of 500,000 trains,which was studied based on the sleeper displacement measured by a laser displacement sensor during the test.At the same time,the elasticity of the ballast bed increased and the plastic cumulative deformation hardly develops.(3)Vibration velocity sensors and “Smart Rock” wireless sensors were installed at specific locations in the ballast layer to compare the ballast dynamic responses before and after the implementation of long-term train traffic loads.It is found that the increase of train speed significantly improved the vibration response,particle movement and elastic deformation at different locations inside the trackbed.Ballast breakage and particle rearrangement made the small particles fill the ballast voids,which enhanced the trackbed densification,thereby reducing the dynamic trackbed responses.The long-term train loads decreased the amplification effect of train speed but improved the linearity between trackbed dynamic responses and train speed.(4)A particle migration analysis system was established to capture the ballast movement and with an accuracy of 0.1 mm.The cumulative migration of ballast particles in different areas of the trackbed surface under long-term train loads was revealed: the shoulder ballast with a slope had a strong lateral migration away from the sleeper as lacking lateral restraint;the crib ballast had obvious longitudinal migration along the train running direction as the strong contacts of the sleepers;while the shoulder ballast on the trackbed surface had limited accumulated migration.(5)The trackbed accumulated settlement,sleeper support static stiffness,the ballast particle breakage index(BBI)and the particle migration of trackbed surface under the long-term train loads showed a similar trend,and these were significantly intensified by the increase in train speed,then stabilized with the number of train passages,and increased sharply again due to the increase in axle load.Over 50% of the permanent accumulated settlement of the ballasted track resulted from the ballast bed deformation,and the increase in the train speed caused intense particle rearrangement in the trackbed and accelerated the development of the accumulated deformation,while the increase in the axle load caused serious particle breakage and contributed to a greater deformation.(6)The large-diameter static triaxial test was carried out to obtain the geogrid reinforced ballast behavior under various confining pressure and compaction degrees.It was found that the implementation of geogrid can effectively restrain the radial deformation of ballast specimen,thereby significantly increasing the deviator stress that the ballast specimen can withst and.In the test,the higher the confining pressure,the later the geogrid constrain effect,and the less significant the effect is.Under a condition of high confining pressure and compaction degree,almost all the ribs of the geogrid broke and the reinforcement effect failed.Therefore,the situation where the geogrid is directly arranged in the upper area of tracked should be avoided.(7)The characterization of the real ballast shape was reconstructed in DEM based on the multi-sphere element and the tuff morphological characteristics obtained through a computeraided ballast morphological analysis system.The three-dimensional geogrid and latex membrane based on parallel bond model were established and model the geogrid tensile properties and lateral confining pressure,respectively.The upper particles of the ballast sample produced the largest axial displacement,while the particles in the middle had obvious radial displacement.Increasing the confining pressure enhanced the ballast force chain and the integrity of the sample,making it difficult for the ballast particles to move.Increasing the initial compaction degree can reduce the porosity and fabric anisotropy of the ballast sample,so that an effective axial force chain can be formed earlier.Thus,the deviator stress that the ballast sample can withst and increased.The implementation of geogrid can significantly constrain the radial movement of ballast particles,thereby forming a stronger force chain and bearing capacity.The reinforcement effect of the geogrid on the ballast samples was more obvious under the condition of lower confining pressure and compaction degree,while under the condition of higher confining pressure and compaction degree would increase the tensile stress and make the geogrid fail prematurely.