Behavior of fresh and fouled railway ballast subjected to direct shear testing: discrete element simulation

This paper presents the three-dimensional discrete element method (DEM) that was used to study

the shear behaviour of fresh and coal fouled ballast in direct shear testing. The volumetric

changes and stress-strain behaviour of fresh and fouled ballast were simulated and compared

with the experimental results. ‘Clump logic’ in Particle Flow Code (PFC3D) incorporated in a

MATLAB Code was used to simulate irregular shaped particles in which groups of ten to twenty

spherical balls were clumped together in appropriate sizes to simulate ballast particles. Fouled

ballast with various Void Contaminant Index (VCI), ranging from 20%VCI to 70%VCI, were

modelled by injecting a specified number of miniature spherical particles into the voids of fresh

ballast. The DEM simulation captures the behaviour of fresh and fouled ballast as observed in

the laboratory showing that the peak shear stress of the ballast assembly decreases and the

dilation of fouled ballast increases with an increasing of VCI. Furthermore, the DEM also

provides insight to the distribution of contact force chains and particle displacement vectors,

which cannot be determined experimentally. These micromechanical observations clearly justify

the formation of a shear band and the evolution of volumetric changes during shearing. The

reduced maximum contact force associated with increased particle contact area due to fouling

explains the decreased breakage of fouled ballast. An acceptable agreement was found between

the DEM model predictions and laboratory data.