This paper presents a study on the interface behavior of biaxial geogrids and sub-ballast using a direct shear box and discrete element modelling. Large-scale direct shear tests are conducted on sub-ballast (capping layer) with and without geogrid inclusions. The laboratory test data shows that the interface shear strength is governed by applied normal stresses and types of geosynthetics tested. Three-dimensional discrete element method (DEM) is used to study the interface shear behavior of the sub-ballast subjected to direct shear loads. Irregular-shaped particles, in which groups of many spherical balls are clumped together in appropriate sizes and positions, are used to simulate sub-ballast particles. Biaxial geogrids are modelled by bonding small spheres together to form the desired grid geometry and apertures. The numerical results agree well with the laboratory test data, indicating that the DEM model can capture the interface behavior of sub-ballast reinforced by the geogrids. Of the three types of geogrid used, the triaxial geogrid presents the highest interface shear strength compared to the biaxial geogrids; and this can be associated with multi-directional load distribution of the triaxial geogrid. Evolutions of contact forces of unreinforced/reinforced sub-ballast specimens and contour strain distributions during shear tests are also investigated.