Stabilisation of aggregate layers over weak subgrade soils is frequently acomplished using geogrids for unpaved roads, working platforms, or foundations. The performance of various stabilised aggregate layers over a soft silty clay subgrade soil was evaluated at full scale under highly controlled loading conditions.  In order to provide the required reaction load, a moveable loading platform was constructed onto which 4 loaded gravel trucks could be driven.   The overall system consisted essentially of a moveable steel bridge which was placed over a trench within which multiple test pads were constructed with different combinations of gravel, gravel thickness and geogrid layers.

Load was applied to a 1m² plate area by a hydraulic piston capable of reaching well over 1000 kN, thus representing loading at essentially field scale.  All tests were taken past ultimate bearing capacity failure to a maximum stroke of 250 mm. A photogrammetric procedure was  developed to define in 3 dimensions the deformed surfaces of the gravel and soft clay subgrade below. The displacements of the surfaces can be evaluated by taking before and after images, and comparisons can be made between different tests combinations. The photogrammetry procedure yields three dimensional digital elevation models which clearly illustrate the nature of the displacement of the ground surface, the geogrid reinforcement and the aggregate – subgrade interface.  The variations in the precise pattern of deformation serve to highlight aspects of the aggregate – geogrid interaction and will be valuable in future numerical back-analyses or modelling of these full scale tests.