The subgrade strain criterion for asphalt pavements was investigatedby instrumenting pavement layers and underlying subgrades. Verticalcompressive strains under wheel loads were recorded under acceleratedand normal rates of loading. Four test pavements in the CanterburyAccelerated Pavement Testing Indoor Facility (CAPTIF), Christchurch,New Zealand, and one in-service pavement were included in theresearch. The asphalt surfacings were 2.5 mm to 85 mm thick, thesubsurface granular layers were 135 mm to 300 mm thick, and thesubgrade CBR ranged from 4%to28%.

Vertical compressive strains measured in the unbound granularlayers and subgrade of flexible pavements are substantially greaterthan predicted values. Vertical compressive strains in unboundgranular layers under thin asphalt surface layers can be equalin magnitude to the subgrade strains. A new subgrade strain modelis developed and presented in this paper, which substantiallyreduces the required thicknesses of overlays.