Dynamic Analysis Of Railway Ballast Bed Based On Coupled Discrete-Finite Element Method And Experimental Verification

Railway is a main approach of communication and transportation in China.The enhancement of speed and carrying capacity of train take the more and more high request to the carrying capacity of railroad.The dynamic and stability study of railway is very important for transportation safety.Railway structure contains both continuous and discontinuous materials.Due to the diversification of railway structure,the simulation of railway encounters a lot of difficulty.The development of numerical method for railway is necessary as a consequence.Discrete element method(DEM)based on discrete particle assembly modeling shows distinct advantage in the simulation of dynamic behavior of granular materials.Discrete element method can effectively obtain the microscomic dynamic information of granular materials,such as contact force between particles,distribution of force chain,porosity and irregular shape of particles.However,due to the over consideration of microscomic information,the computational efficiency of discrete element method is low.The computational costs are usually unacceptable in large-scale engineering problems.Finite element method calculation based on the macroscopic continuum model has unparalleled advantages in computational efficiency as compared to discrete element method.At first,the number of governing equations to be solved in the finite element method is much smaller than that required in discrete element method.Secondly,the time step size used in the finite element method calculation can be much larger than the critical time step size required by discrete element method calculation.However,finite element method based on the macroscopic continuum model is unable to simulate microscopic uncontinuous failure phenomena and capture the micromechanical mechanism of granular materials.In order to take advantage of discrete and finite element method,the coupled discrete-finite element method emerged.Railway ballast bed is a very complex structural system which contains both granular material(ballast)and continuous materials(sleeper,foundation).The coupled discrete-finite element method can be applied in the analysis of railway structure.In order to study the dynamic character of ballast bed,we establish a discrete element method for railway ballast and a coupled discrete-finite element method which could deal with the interaction between ballast particles and other continuous structures.The numerical analysis and experimental verification about the settlement of ballast bed,the reinforcement effect of geogrid to ballast bed,dynamic response of ballast-ballastless transition zone are carried out.The main studies of this paper are as follows:(1)Establish the coupled discrete-finite element model for the dynamic analysis of ballast bedIn consideration of the coexistence of discrete ballast,continuous embankment and foundation in railway structure,the coupled discrete-finite element model for ballast bed is established.The contact force between the two materials is obtained according to principle of conservation of energy.The relationship of mechanical parameters between discrete element and finite element areas is determined.The discrete and finite element method are coupled by the shape function calculation and relative displacement on coupled surface in the contact search between the two kinds of elements.In order to verify the reliability and accuracy of coupled discrete-finite element method for ballast bed,a direct shear test of ballast material is designed.The shear strength of ballast material is tested under different normal stress in direct shear test.A numerical simulation about this direct shear test of ballast material is carried out.The reasonability of numerical model is verified by the comparison with test results.(2)The dynamic character of the whole ballasted bed and the mechanism of nonuniform settlement of ballast bedDiscrete element method and finite element method are used to simulate ballast particle and subgrade.The effective elasticity and settlement of ballast bed under cyclic loads is analyzed with coupled discrete-finite element method.The chief factor of sleeper settlement is determined.Clump is employed to construct the ballast particle.Three-dimensional elastic-plastic finite element model is used to establish the embankment and foundation model.The coupled discrete-finite element method for ballast,embankment and foundation is also established.The traffic load under high speed and heavy load conditions is determined by tests and references,and applied to the numerical model of ballast bed.The buffering effect of ballast to traffic load is discussed by analyzing the force chain distribution in ballast bed.Moreover,the stability of large deformation area in subgrade and the bearing capacity of ballast bed are determined by analyzing the effect of material parameters to foundation under high speed and heavy load conditions.(3)The numerical analysis of reinforced ballast bed and its experimental verificationDiscrete element method and finite element method are used to simulate ballast particles and geogrid.The dynamic character of ballast bed reinforced by geogrid is analyzed with coupled discrete-finite element method.The effect of geogrid to ballast can be studied by the simulation with coupled discrete-finite element method and experiment.The inner mechanism of geogrid reinforcement to the ballast bed stability is revealed by study the interaction between ballast particle and geogrid.The settlement,side direction displacement,verticle displacement of ballast bed and the change of effective stiffness are compared by the dynamic simulation of reinforced and unreinforced ballast bed.The reinforcement effect of geogrid is assessed.The accuracy of numerical model is verified by comparation with direct shear test results of reinforced and unreinforced ballast material.(4)Study the effect of ballast glue to the smooth transition of settlement in ballast-ballastless transition zoneThe difference between ballast and ballastless bed leads to the obvious stiffness change in ballast-ballastless transition zone.The dynamic behavior of ballast-ballastless transition zone is analyzed with coupled discrete-finite element method.Main attention is payed to the relationship and coupled algorithm of mechanical parameters on the coupled surface between the two kinds of railway beds.A bonded discrete element model is proposed according to the glue pattern in ballast-ballastless transition zone.The settlement changes nearby the interface of ballast and ballastless bed are investigated to reveal the inner mechanism of nonuniform settlement in transition zone.