Effect of Fracture Configuration on Macro–Micro Mechanical Behaviour and Degradation of Ballast under Triaxial Simulation

Particle breakage governs the degradation of railway ballast under repeated loading; however, the influence of fracture configuration in the Bonded Particle Model (BPM) on macro–micro responses remain insufficiently understood. This study compares three BPM configurations—Uniform Bond Breakage Model (UBBM), Corner Bond Breakage Model (CBBM), and Diverse Bond Breakage Model (DBBM)—using coupled Finite Difference Method (FDM)–Discrete Element Method (DEM) triaxial simulations with a flexible membrane boundary. The results show that all breakable models exhibit similar responses at the lowest confining pressure of 60 kPa, whereas clear divergence emerges at higher confinement (90–240 kPa). Compared with the rigid model, the breakable specimen under a confining pressure of 60 kPa exhibits an approximately 50% reduction in peak deviatoric stress. Particle breakage also suppresses dilatancy, and this suppression becomes more pronounced under higher confining pressures. At the microscale, breakage promotes fragment redistribution and drives the transition of force chain structures from anisotropic to more isotropic patterns. Gradation analysis indicates that fragmentation is localized within the shear zone and intensifies with increasing confining pressure and axial strain. Overall, the results demonstrate that bond strength heterogeneity and fracture topology jointly govern the mechanical response and structural evolution of ballast. Among the examined configurations, the DBBM provides more consistent agreement with experimental observations, highlighting the importance of incorporating non-uniform bond structures in DEM simulations.