Influence of ballast gradation on repose angle using large-scale hopper flow tests and DEM simulation

This study investigates the influence of ballast gradation on the repose angle through a series of large-scale hopper flow tests and discrete element method (DEM) simulations. Bulk-scale and micro analyses of the entire ballast pile formation process reveal three stages: platform-stacking, peak-stacking, and peak-shaving, with particle interlocking during these stages playing a crucial role in establishing the angle of repose. Results indicate that ballast gradation significantly affects the repose angle, the maximum repose angle is observed at d₅₀ = 40.0 mm, within the tested range of 36.8 mm to 45.0 mm. As the gradation of the ballast transitions from uniform to non-uniform, the repose angle initially increases and then decreases, peaking at d₅₀ = 40.0 mm and C_u = 1.46. The spatial distribution of particles within the ballast pile follows a distinct pattern, large particles within the ballast pile is relatively uniform, while small and medium particles are more concentrated in the core regions. A better particle gradation allows smaller particles to fill the voids between larger particles, thereby optimizing the load distribution between strong and weak force chains. This research underscores the importance of proper gradation for ensuring stability and effective load-bearing capacity in ballast systems.