The effect of geogrids on unbound granular fill by the mechanism of mechanical interlock has been established with particle confinement identified as being an important component influencing the performance of the resulting mechanically stabilised layer. Evaluation of the influence of geogrids on particles through the thickness of the mechanically stabilised layer has been carried out using a large multi-level shear box. The results obtained from the programme of testing carried out at the University of Gyor will be discussed by reviewing measured shear resistance at several levels in the granular fill to characterise the influence of the geogrid on particle confinement through the granular layer thickness.