"Large-scale direct shear tests were performed to explore the behavior of rail ballast-geogrid interfaces and establish the role of geogrid aperture size on the interface shear strength. Fresh latite ballast with an average particle size (D-50) of 35 mm, and seven geogrids with equivalent aperture sizes ranging from 20.8 to 88 mm were used for this current study. The laboratory experimental results confirm that the interface shear strength is influenced by the geogrid aperture size. The interface efficiency factor (alpha), defined as the ratio of the shear strength of the interface to the internal shear strength of the ballast varies from 0.8 to 1.16, and the ballast-geosynthctic interface friction angles vary from 52 to 67 degrees. Moreover, a criterion to determine the minimum and maximum aperture sizes required to generate the beneficial effects of the geogrid has been identified in this study."