Prediction of Subgrade CBR using FWD for Thin Bituminous Pavements

Deflection based models developed by Jameson, Roberts et al and Queensland Department of Transportation and Main Roads are commonly used for predicting the subgrade CBR of asphalt pavements. The models utilise Falling Weight Deflectometer (FWD) deflection D900 recorded at 900 mm from the centre of the loading plate. The principal aim of the study is to enhance the prediction of the in-situ subgrade CBR using FWD for granular pavements with thin bituminous layers. The scope of the study included the comparison of subgrade CBR predictions from the three deflection based models and the predictions were verified using the in-situ CBR values derived from Dynamic Cone Penetrometer (DCP) from eleven pavement test sites. The study shows that the three models over predict the subgrade CBR because the deflections recorded at sensor D900 are consistently small. It is observed the impact load has little influence on the deflection at D900 for pavements with asphalt layer less than 50 mm. The inherent non-linearity behaviour of the thin pavement structure is also the reason for the discrepancies in the predictions. Consequently, a new predictive model is developed utilising the FWD deflection D450 observed at 450 mm from the impact load.