DEM investigation of geogrid location effects on the depth-dependent micromechanical behavior of loose railway ballast

Railway ballast can remain relatively loose after initial construction, renewal, or maintenance, resulting in increased particle mobility and susceptibility to deformation. Although geogrids can effectively improve ballast stability, how their vertical location governs stress transfer and particle-scale behavior remains insufficiently understood. This study employs three-dimensional discrete element method (DEM) simulations, validated against large-scale triaxial tests and particle-rotation measurements, to investigate the depth-dependent response of loose railway ballast stabilized with geogrids at different elevations. Eleven configurations are examined, comprising an unstabilized specimen and single- and dual-layer geogrid arrangements. The results show that stabilization effectiveness depends strongly on geogrid position relative to the active deformation region. Single-layer geogrids placed near or slightly above the specimen mid-height effectively alter the internal load-transfer path, increasing stress mobilization in the upper and middle zones while reducing stress transmission to the lower zone. For the centrally placed geogrid, particle displacement at 10% axial strain decreased from approximately 25 to 13 mm in the middle zone and from 14 to 7.5 mm in the lower zone relative to the unstabilized specimen. Effective geogrid placement also delays dilation, suppresses lateral deformation, promotes contact-network development, and restricts particle rotation. In contrast, geogrids positioned below or farther from the specimen center interact less effectively with the dominant deformation region and provide weaker restraint. Dual-layer configurations further enhance confinement and stress redistribution when positioned close to the specimen center, but their effectiveness diminishes as the layers move away from the active deformation region. Overall, the results establish a micromechanical link between geogrid location, geogrid mobilization, stress redistribution, and particle kinematics, demonstrating that stabilization efficiency is governed by the interaction between geogrid placement and the dominant deformation zone. These findings provide new micromechanical insight into the role of geogrid location and a mechanistic basis for future track-scale evaluation of geogrid placement in railway ballast.