A reinforcement interlayer retards the development of cracks in the asphalt overlay by absorbing the stresses induced by both heavy traffic and underlying cracking in the old pavement. The experimental work in this study used laboratory model tests to developed an understanding of the behaviours of sand used as a subgrade under earthquake condition and the its effects of this on flexible pavement and base course layer. The sand subgrade layer has a thickness of 600 mm and the base course was set to 200 mm; the asphalt layer was prepared as a panel with dimensions of 300 x 300 x 50 mm, representing the surface layer. These layers were then tested under the influence of earthquake loading with different frequencies 0.5, 1 and 1.5 Hz. The tests were performed in two parts without adding a geogrid, and with geogrids in the centre of the base layer and between the base and sand layer. Stresses in all three layers are measured using stress sensors, and the displacements of the asphalt layer were measured using two laser displacement sensors. The results for the tests on pavement layers showed that, the stress recorded in models reinforced with geogrids was higher than that seen in unreinforced models, while the displacement was decreased by adding the geogrid in the layers at all three frequencies. When the geogrid is laid between the base and sand layer, the stress in the subgrade layer also lower than that in the base and asphalt layers.