The deformation under traffic of crushed rock sub-bases over clay subgrades has been investigated; the experiments included an investigation of the effectiveness of a reinforcing geogrid between the sub-base and the subgrade. Sections with and without reinforcement were trafficked simultaneously by a rigid two-axle lorry with its rear axle loaded to about 80 kN and the number of vehicle passes to cause 40 mm deformation at the sub-base surface was related to the sub-base thickness, subgrade strength and the use of geogrid. Transverse trenching revealed the accumulated deformation in the sub-base and subgrade. The results were compared with existing designs for foundation layers and the performance of the reinforcement was examined over a wide range of test conditions.

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