Zonal micromechanics of geogrid-ballast interaction from triaxial tests and discrete element modeling

This study examines the zonal micromechanics of geogrid-ballast interaction using integrated large-scale triaxial tests and Discrete Element Method (DEM) simulations. Tests were conducted on loose and compacted ballast with and without geogrid inclusion, incorporating SmartRock sensors to capture local contact forces and particle rotations. Corresponding DEM models with representative ballast particles, an explicit geogrid, and a flexible membrane boundary were developed and validated against experimental responses. Results reveal that geogrid inclusion alters particle-scale load-transfer mechanisms. Stabilization promotes the formation of a more connected contact network and redistributes loads toward regions near and above the geogrid, while suppressing force transmission and particle kinematics below it. The stabilization effect is more pronounced in loose ballast, whereas compacted ballast retains higher inherent strength due to pre-existing interparticle interlocking. Although overall volumetric change is only marginally affected, the geogrid redistributes deformation, shifting dilation and bulging away from the specimen mid-height. These findings provide direct mechanistic evidence of how geogrids regulate force transmission and deformation in ballast, offering a basis for optimizing geogrid design and placement in railway tracks.