| Comparing with the ballast bed in the conventional railways with lower operation speeds,the high-speed railway ballast bears very frequent and repeated high-speed train loads.This makes adverse effects to the accumulative deformation and the service condition of ballast.As a result of improper control,it will increase the amount of railway maintenance work greatly.In order to well know the evolution of dynamic performance and accumulative deformation of ballast under the high-speed operation states,it is necessary to study the mechanical behaviors and degradation mechanisms of railway ballast from both the macro and meso scale specific to the structure and transportation characteristics of the high-speed railway ballast.In view of this,laboratory tests and discrete element analyses are carried out in this dissertation to investigate the macro-meso mechanical behaviors of high-speed railway ballast.Firstly,in the aspect of building the discrete element models of ballast particles at the meso scale,a three dimensional(3D)laser scanner is used to capture and reconstruct the irregular geometry morphology of the ballast particles with a high resolution.After the particle geometry is well captured,the methods for building the discrete element models that can simulate the real morphology of ballast particles are proposed and demonstrated.A shape library and a database of the discrete element models of ballast particles are established,and a computer code is written for automatically discrete element modelling of high-speed railway ballast.In the aspect of macro-meso mechanical behaviors of high-speed railway ballast,both the crushing test of ballast stones under the static compaction load and the direct shear test of high-speed railway ballast are carried out.Discrete element analyses are also made to simulate those laboratory tests.By measuring the load-displacement responses of ballast stones under the static compaction load,the strength characteristics of ballast particles are statistically analyzed.The bonded particle models are adopted to simulate the statically crushing behaviors of ballast stones.According to the results,the evolution of contact force chains and the propagation of the broken bonds in ballast particles are analyzed,and the breakage mechanism of ballast stones under the static load is revealed.A home-made large-scale direct shear apparatus is adopted for carrying out the direct shear test of high-speed railway ballast,and the shear strength and deformation of ballast are measured in the test.A discrete element model is also established for simulating the direct shear behavior of ballast.According to the test and simulation results,the meso mechanical parameters between ballast particles are calibrated.The meso scale mechanical behaviors of the ballast during the direct shear process including the distribution of the contact force chains,the evolution of the contact force anisotropy and the movement trend of the ballast particles are also analyzed.Based on the discrete element models of ballast particles and the basic mechanical properties of the ballast,both 2D and 3D discrete element analyses are carried out to deeply investigate the dynamic behaviors of high-speed railway ballast bed from both the macro and meso scale.A 2D discrete element model for high-speed railway ballasted track is established to simulate the dynamic behaviors of high-speed railway ballast bed under the moving wheel-rail loads induced by the high-speed trains.The distribution characteristics of contact forces and vibration responses of ballast particles in the longitudinal plane are qualitatively analyzed.Specific to the structure of the high-speed railway ballast in China,a 3D discrete element model for high-speed railway ballast bed is also established using the discrete element models of ballast particles with the real shape.By applying the dynamic rail-sleeper force induced by the high-speed trains into the discrete element model,the macro and meso scale dynamic behaviors of high-speed railway ballast are simulated.The dynamic response characteristics of the ballast under the high-speed trains are analyzed,and the influence of the train running speed on the meso scale dynamic behavior of the ballast is ascertained.The meso scale mechanisms for the degradation and the stability reduction of the high-speed railway ballast is preliminarily revealed.Numerical results show that with the increase of the train running speed,the vertical vibration velocity and acceleration of ballast particles,and the horizontal vibration responses of the shoulder ballast also increase continuously.This will aggregate the attrition and abrasion of ballast particles,and therefore accelerates the accumulative deformation rate of the ballast.Under the high speed operation states,the severe ballast vibration also probably increase the possibility of ballast flight,loose the ballast,reduce the ballast stability,and even cause the instability of the ballast slope and the liquidation phenomenon.It is necessary to take some measures to strengthen and protect the high-speed railway ballast.To investigate the deformation and the degradation of railway ballast under the long term cyclic loads,a full-scale laboratory model test of high-speed railway ballast under the cyclic load is carried out,and the degradation of ballast particles during the test is studied.The accumulative deformation of the ballast is measured under a cyclic load simulating the high-speed train load up to 4 million load cycles.The test results demonstrated that the settlement of the rail and the sleeper are less than 1mm,and under the cyclic loads,the settlement of the high-speed ballasted track mainly comes from the accumulative deformation of the ballast.After the cyclic load test,82.5% ballast particles used for the degradation test are still intact and undamaged,namely,no clear degradation phenomenon is observed for these ballast samples.Four kinds of degradation phenomenon are observed for other ballast samples including the attrition of particle corners and edges,the surface polishing scratch,the surface crack and the produced fine particles with quite small size.Among these four kinds of degradation phenomenon,the attrition of corners and edges of ballast particles are the most popular one.In order to further investigate the influence of the transportation characteristics of high-speed railways on the deformation law of the ballast bed and reveal its meso scale mechanisms,discrete element simulations are carried out to study the permanent deformation behavior of the ballast under the cyclic loads and under the repeated high-speed train loads.Specific to the problem that the loading frequency in the existing permanent deformation tests of ballast is too low to reflect the frequency range of the high-speed train loads,the discrete element analyses in this dissertation ascertain the remarkable influence of the frequency of the high-speed train loads on the permanent deformation of ballast and its meso scale mechanism,and show the importance of considering the loading frequency and the rest period in the load when carrying out the permanent deformation tests of the ballast.Numerical results show that when the loading frequency exceeds 15 Hz,the influence of the loading frequency and the load amplitude are dependent on each other,especially,when the loading frequency exceeds 20 Hz,the permanent deformation of the ballast increase remarkably.In the permanent deformation tests of the ballast,if only the cyclic loads with the low frequency is applied to the ballast,the permanent deformation of the ballast will be much underestimated.It is suggested to consider the influence of the loading frequency and the rest period of the high-speed train loads.If the loading frequency is set to the passing frequency of the bogie distance of the train,and two cycles of cyclic loads are used to simulate one vehicle load,the produced permanent deformation under the cyclic load will be close to that under the repeated high-speed train loads. |
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