The role of predictive models for resilient modulus of unbound materials in pavement FWD-deflection assessment

The unbound materials in pavements are stress dependent and any changes in the moisture content will change the stress level and, therefore, the material behaviour. The objectives of this paper are to 1) investigate the stress dependency coupled with moisture sensitivity of the unbound materials on the estimated in-situ Falling Weight Deflectometer (FWD) deflection response; and 2) determine the most appropriate resilient modulus predictive model to predict the stiffness of the unbound materials without conducting backcalculation analysis. In the proposed approach, one stress-based constitutive model and one approximate empirical predictive model were evaluated in conjunction with the predicted and measured soil moisture content profiles in eight different unbound material types. Measured FWD data at four Long-Term Pavement Performance-Seasonal Monitoring Programme (LTPP-SMP) locations in Maine, Minnesota, Texas and Montana were used to estimate the in situ measured deflection. Then, layer elastic analysis was conducted using the soil moisture profile, asphalt temperature, material physical properties, groundwater table and depth to bedrock to examine the sensitivity of each model in the pavement response. The findings show that for non-plastic soil materials, the presented empirical model is adequate to predict the in-situ FWD deflection. In contrast, both approximate and stress-dependent models overestimate the in-situ pavement surface deflection for the plastic soils evaluated in this study.