Micromechanical Particle Interactions in Railway Ballast through DEM Simulations of Direct Shear Tests

Particle shape properties can significantly affect the load response behavior and field performance of the uniformly graded ballast layer in railway track structures. Particle contact and the related micromechanical behavior of the unbound aggregate ballast layer assembly influenced by the particle shape properties have not yet been thoroughly investigated. In this study, an aggregate imaging and particle shape-analysis approach based on computer vision technology was introduced to calculate aggregate morphological indices and construct polyhedral discrete elements with shapes close to realistic ballast aggregate particles. A model of direct shear test on railway ballast based on the discrete-element method (DEM) was developed using generated nonbreakable discrete elements as individual ballast particles and validated by closely matching the predicted shear stress-strain behavior with laboratory test results. The DEM model simulation predictions were then used to investigate the relations between microscale interactions of individual ballast particles, particle size effects in relation to shear box test equipment dimensions, and macroscale behavior trends of the aggregate assemblies. Stronger particle interactions and higher coordination numbers were observed as the imaging-based angularity index (AI) and flat and elongated (F&E) ratio increased. Furthermore, limiting individual particle movement was shown to increase strength and provide greater resistance to failure and deformation.