Investigation of hanging crosstie problem at bridge approaches: a train–track–bridge model coupled with discrete element method

Abstract Nonuniform track support and differential settlements are commonly observed in bridge approaches where the ballast layer can develop gaps at crosstie-ballast interfaces often referred to as a hanging crosstie condition. Hanging crossties usually yield unfavorable dynamic effects such as higher wheel loads, which negatively impact the serviceability and safety of railway operations. Hence, a better understanding of the mechanisms that cause hanging crossties and their effects on the ballast layer load-deformation characteristics is necessary. Since the ballast layer is a particulate medium, the discrete element method (DEM), which simulates ballast particle interactions individually, is ideal to explore the interparticle contact forces and ballast movements under dynamic wheel loading. Accurate representations of the dynamic loads from the train and track superstructure are needed for high-fidelity DEM modeling. This paper introduces an integrated modeling approach, which couples a single-crosstie DEM ballast model with a train–track–bridge (TTB) model using a proportional–integral–derivative control loop. The TTB – DEM model was validated with field measurements, and the coupled model calculates similar crosstie displacements as the TTB model. The TTB–DEM provided new insights into the ballast particle-scale behavior, which the TTB model alone cannot explore. The TTB–DEM coupling approach identified detrimental effects of hanging crossties on adjacent crossties, which were found to experience drastic vibrations and large ballast contact force concentrations.