Triaxial testing and discrete-element modelling of geogrid-stabilised rail ballast

Geogrids have been found to improve the performance of unbound aggregate layers in transportation applications by providing confinement and arresting movement through interlock between individual aggregate particles and their apertures. Geogrid reinforcement offers an effective remedial measure when railway structures are susceptible to track geometry defects resulting from excessive movement and particle reorientation within the ballast layer. This paper presents an ongoing research study at the University of Illinois aimed at quantifying the effects of geogrid reinforcement on the shear strength and permanent deformation behaviour of geogrid-stabilised railroad ballast. Geogrids with triangular, rectangular and square apertures were tested in the laboratory experiments. Cylindrical ballast specimens were prepared and tested with geogrids placed at different heights within the specimen using a large-scale triaxial apparatus. An imaging-based discrete-element-method approach was developed to model triaxial test results and investigate geogrid-reinforcement mechanisms. With the capability to create actual ballast aggregate particles as three-dimensional polyhedron elements having the same particle-size distributions and imaging quantified average shapes and angularities, the modelling was able to capture the ballast behaviour with and without geogrid reinforcement reasonably accurately.