Abstract
Geogrids are well known for improving the performance of unbound aggregate layers in transportation applications by providing confinement and restraining movement through interlock between individual aggregate particles and geogrid apertures. Geogrid reinforcement offers an effective remedial measure when railroad track structures are susceptible to track geometry defects resulting from excessive movement and particle reorientation within the ballast layer. This paper presents findings from an ongoing research study at the University of Illinois aimed at quantifying the effects of geogrid reinforcement on the shear strength behavior of railroad ballast. The effects of two geogrid types on ballast shear strength were evaluated through laboratory testing and numerical modeling. An imaging-based discrete element method (DEM) modeling approach was used to identify the optimal position for geogrid reinforcement to achieve the maximum shear strength gain in cylindrical triaxial specimens. Geogrids were installed at five depths within the cylindrical specimen and tested for shear strength properties with a large-scale triaxial test setup to evaluate the effectiveness of both geogrid aperture shape and reinforcement depth. Placing two layers of geogrids in the middle of the specimen was found to result in the maximum shear strength gain. Such placement of the geogrid ensured the intersection of the shear failure plane with the reinforcement layer, ultimately leading to significant shear strength gains. The DEM simulations were observed to capture accurately the ballast shear strength behavior with and without geogrid reinforcement.