Study On The Micro-mechanical Behaviour Of Ballast And Particle Breakage Using DEM Simulations

Under the dynamic load of high-speed trains,the structure of railway granular track bed will produce high-frequency vibration.Under the action of vibration and impact,the particles of the track bed will undergo different degrees of rearrangement,abrasion and particle breakage which will cause mechanical performance degradation of ballast bed and require continuous maintenance.The research work on the deterioration of the ballast bed is of great significance.In the aspect of building the DEM specimens of ballast,breakable models with rigid boundary condition were constructed through PFC^3D,the flexible boundary condition models were established by coupling FDM software FLAC^3D for comparison.The static and dynamic large-scale triaxial tests simulation were carried out to systematically study the micro-mechanical behaviour and degradation mechanisms of railway ballast.The main characteristics of conclusions are summarized below:（1）Compared with the flexible boundary specimen,the particle displacement is smaller under the rigid boundary condition,the degree of dilatation of the specimens and the particle breakage is more serious,the distribution of particles and fragments inside the sample is more uniform as well.Under the flexible boundary conditions,regardless of the kind of breakable model,the particles in the 25～53mm particle group in the sample are broken to different degrees,but the simulation results of the rigid boundary condition sample will be affected by the type of breakable model.However,the particle group with smaller size has higher proportion of total broken particles,whatever the boundary condition and ballast model,the number of broken particles in the d₀～d₁₀ particle group accounted for about 50%of the total.（2）From the static and dynamic triaxial test simulation results of the Bonded Particle Method（BPM）,it can be seen that the higher the strength of the breakable specimen,the closer the simulation result is to the unbreakable specimen.Compared with the unbreakable specimen,the deviator stress,average contact force and dilatation degree of the breakable specimen are smaller.In the unbreakable model,the contact number profile is approximately peanut-like and gradually evolves into a more uniform olive-like shape as the model strength decreases.Among the three breakable BPM models,the Uniform Bonded Breakage Model（UBBM）is more fragile,which may lead to more serious particle breakage.However,the Corner Bonded Breakage Model（CBBM）and Diverse Bonded Breakage Model（DBBM）models can take into account the simulation efficiency and particle breakage accuracy,which can better simulate the degradation of ballast.（3）From the static triaxial test simulation results of the FRM model,it can be seen that a reasonable fragment replacement method is of great significance for accurate simulation.During the loading process,the breakage degree of the sample increases,the gradation gradually tends to the fractal distribution,and the fractal dimension gradually increases.However,the fragments replacement method which can only simulate particle splitting,but cannot simulate edge and corner crushing,may obtain smaller breakage index and fractal dimension.When the fragment replacement method contains enough small particles,the breakage index and fractal dimension of the sample are larger.（4）Comparing the simulation results of FRM and BPM,the BPM model can obtain results similar to those of the test by modifying the property of contacts in the specimen.Although BPM model is accurate,it is also more time-consuming.The FRM model adopts a simplified fragment replacement method and particle breaking criterion,which can obtain similar results to the BPM simulation to a certain extent,but greatly improves the calculation efficiency,and can obtain more accurate simulation results after further improvement.（5）When using irregular-shaped particles to simulate grading ballast,the grading automatically generated by the DEM software based on the equivalent volume method will be different from the preset grading.The effect of particle shape on gradation can be determined by simulating the vibration sieving test of particles of different shapes,and calculating the difference between the real stratified particle size and the aperture diameter of each sieving plate.Correction before generating grading particles can result in more accurate simulation results.