Macro-and-micromechanical responses of ballast under triaxial shearing using coupled DEM–FDM with flexible and rigid membranes: a comparative study

Railway ballast undergoes rearrangement, abrasion, and even breakage, when subjected to high-speed train loads. To reproduce the deformation and degradation behavior of ballast under realistic boundaries used in laboratory triaxial tests, bonded particle clusters and clumps sampled within flexible and rigid boundaries were established, using the discrete element method and finite difference method. The models were then calibrated and validated against a series of experimental results. It is found that boundary condition has a considerable effect on the contact force chains and coordination number. The flexible boundary induces more uniform stress distribution between particle contacts, and consequently higher strength, lower dilation, and impartial breakage. A unimodal frequency distribution of the coordination number is observed when using flexible boundary, while rigid boundary can result in multi-modal distribution in breakable specimens. The flexible boundary also induces more particle breakage with high fragmentation. The rigid boundary specimens exhibit a bimodal distribution of particle breakage along the specimen height after test, with fewer fragments existing in the middle part; however, a unimodal distribution of particle breakage is found in the flexible boundary ones, which agrees more with the laboratory observation.