DEM Study On The Interface Characters Of Geogrid Reinforced Granular Ballast

With the steady advancement of the transportation strategy and the continuous expansion of the railway network,the rapid increase in transportation pressure has put forward high-speed and heavy-duty demand for the development of railways.Ballasted tracks,as one of the most important structural forms of railways,are widely used on general-speed railways and highspeed intercity railways.However,the natural granular characteristics of ballast determine that it will inevitably suffer from deterioration such as particle abrasion and sharp corner breakage under high-speed and high-density train cyclic loading.Long-term settlement accumulation can also easily lead to a decrease in the shear strength of the track bed and even overall lateral instability.Geogrids have been widely used in engineering reinforcement of granular materials because of its unique grid structure and high tensile strength.The geogrids and the surrounding ballast particles interact with each other to resist external disturbances.The excellent tensile strength of the reinforcement and the compressive capacity of the ballast form an effective complement.Although the use of reinforcement effectively restricts the lateral displacement of ballast particles,due to the complexity of the interaction of multiple factors in the interaction process of the reinforcement-ballast interface,the microscopic mechanism behind the deterioration of the macroscopic shear resistance of the interface is still unclear.There is still a lack of corresponding unified evaluation indicators for the selection of reasonable aperture sizes of reinforcements with different geometric structures,and the understanding of the load transfer mechanism of the reinforcement-soil interface is not deep enough.Moreover,this also restricts the transformation of existing reinforced soil research results to a certain extent.In view of the technical difficulties in the analysis of the interaction mechanism of the reinforcement-soil interface of the reinforced track bed,the geogrid-reinforced track bed is regarded as the research target of this study.Moreover,biaxial geogrids and triaxial geogrids with balanced mechanical properties in all directions were used as reinforcement materials.Taking into account the geometric characteristics of the reinforcement and the interlocking effect between the grid and the ballast,the discrete element numerical simulation was used to conduct an in-depth study on the mechanical response of the reinforced ballast under the pullout load.In addition,the precise calibrated meso-parameters were selected to establish the pullout test model of the reinforced ballast.The influence of ballast particle size,geogrid aperture size and node height on the shear strength of the interface between reinforcement and soil was systematically analyzed,and the bearing resistance of the transverse rib was quantified with the macro pull-out strength.The energy dissipation mechanism of the particle system was analyzed to clarify the normalization index for the selection of reasonable mesh size;To further promote the popularization and transformation of the existing research results of reinforced granular materials,starting from the meso-action mechanism,this paper focuses on the analysis of the evolution law of the meso-scale bearing structure of the particle system and the load transfer mechanism of the reinforcement-soil interface.The interaction mechanism and interlocking effect between the reinforcement and the ballast are revealed,and the internal relationship between the reduction of the macroscopic strength of the interface and the evolution of the microstructure is clarified.The main research contents include:(1)The influence of geogrid geometric characteristics on the performance of reinforced track bed and the normalized evaluation indexNumerical pull-out models of geogrids with different aperture sizes and shapes have been established to analyze the influence of the geometric characteristics of the grid on the reinforcement performance.A new evaluation index was proposed to improve the normalized evaluation system under different aperture forms.Based on the systematic analysis,the selection criteria for the reasonable size of the mesh were proposed to promote the transformation of related optimization results.(2)The internal relationship between the protrusion of the geogrid node and the response mechanism of the reinforcement-ballast interfaceThe geogrid node shape was reproduced by creating secondary particles,considering the ballast particle size distribution,combined with the evolution of the microscopic characteristics of the particle system and the development of macro-deformation,to explore the influence of node protrusions on the interface characteristics of the reinforcement and the ballast.(3)Evolution law of the bearing structure of the granular system under pull-out loadBased on the well-calibrated model,the evolution law of the load-bearing structure of the particle system was analyzed from the perspective of energy evolution.Based on the changes in frictional energy consumption and energy storage of geogrid,combined with the displacement field of the particle system,the evolution law of the bearing structure of the reinforced ballast system accompanied by the development of macroscopic strength was revealed.(4)Precise positioning of reinforcement-soil interaction area and load transfer mechanism of reinforced ballastBased on the visualization of the internal force distribution of the reinforcement and the distribution of the force chain of the particle system,the distribution range of the reinforcementsoil strong interaction was accurately located,and the development of the strong force chains were determined.The detailed division of the area based on the strength of the force chains clarified the relationship between the internal force distribution of the reinforcement under the load and the load transfer mechanism of the particle system.