SUBGRADE STRAIN CRITERION FOR LIMITING RUTTING IN ASPHALT PAVEMENTS

The subgrade strain criterion for asphalt pavements was investigated by instrumenting pavement layers and underlying subgrades. Vertical compressive strains under wheel loads were recorded under accelerated and normal rates of loading. Four test pavements in the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF), Christchurch, New Zealand, and one in-service pavement were included in the research. The asphalt surfacings were 25 mm to 85 mm thick, the subsurface granular layers were 135 mm to 300 mm thick, and the subgrade CBR ranged from 4% to 28%. Vertical compressive strains measured in the unbound granular layers and subgrade of flexible pavements are substantially greater than predicted values. Vertical compressive strains in unbound granular layers under thin asphalt surface layers can be equal in magnitude to the subgrade strains. A new subgrade strain model is developed and presented in this paper, which substantially reduces the required thicknesses of overlays.