Geogrid belongs to geosynthetic family of construction site products. These were initially developed in the 1950’s and are now commonly used to stabilise ballasted trackeds, pavement and foundation bases. Geogrid can improve the construction’s design life, reduce the building materials usage and reduce carbon emission. The conventional method to study geogrid stabilisation is via laboratory tests, which can replicate the different field and loading conditions. However, test results are empirical and can hardly capture the geogrid-soil interaction, which is important to understand the stabilisation effect. In recent years, the discrete element method (DEM) has been used to develop geogrid and soil model. DEM simulation provides an approach to study the geogrid stabilisation at the particle level. In this study, a geogrid-soil unit model has been developed using DEM method. This includes a manageable number of soil particles, the geogrid model and a lateral deformable stress-control boundary. The lateral boundary comprises 12 rigid planes, each maintaining a constant force during vertical loading, hence a nominally constant confining pressure can be achieved. The geogrid model is composed of a series of bonded particles using the Edinburgh Beam Bond Model (EBBM). The geogrid-soil model allows an axial loading to be applied under different confining pressures assigned through the lateral boundary. A detailed study the soil-geogrid interaction is carried out using the unit model. The result indicates that the contact friction of the soil-soil and soil-geogrid particles affect the loading stiffness and the resulting strength at large deformation. At the local scale, the tensile stress distribution in the geogrid ribs is far from uniform, with some higher stressed regions concentrating at the geogrid node-rib connecting areas and the mid-rib area. This research sheds light on the geogrid-soil interaction and geogrid loading behaviour during vertical loading, which is helpful to understand the geogrid stabilisation effect.