Flexible pavement structures are widely used in road construction, especially in circumstances where very high traffic volumes are not expected. These structures comprise of multiple layers of granular material, generally having decreasing strength with depth. Failure of these systems is typically observed as rutting on the ground surface, but the failure instigates at depth. In this research, for the first time, observations can be directly made of the progressive failure of these deeper layers under repeated wheel loading due to the combination of a new test apparatus, the Cambridge Accelerated Pavement Tester (APT), and Digital Image Correlation (DIC) technology. The use of a window allowing observation of soil displacements at depth during repeated wheel loading cycles allows the progressive failure to be observed and the changes in soil displacements and strains with different layer thicknesses to be quantified. It was observed that the critical failure mechanisms for thin and thick surficial layers are different, resulting in changes in the rates of surface rutting. Understanding these deformation mechanisms potentially allows savings to be made in road or airfield construction by using correctly-sized structural layers.