Effect of Allowable Thickness Variation on Backcalculated Moduli

The effect of small variations in pavement layer thicknesses on the accuracy of backcalculated moduli was investigated. Deflection data were generated using simulated pavement structures in which layer thicknesses were varied by small amounts. This variation in layer thicknesses was kept sufficiently small so that all layers would conform to construction specifications. A backcalculation was then performed on the generated deflection data. Only the average design thicknesses were supplied to the backcalculation software. Errors between backcalculated moduli and those moduli that were used to generate the deflection data were then studied. It was found that even small variations in layer thicknesses influenced the accuracy of the backcalculated moduli to a significant extent. Where the subgrade depth was considered to be variable, the backcalculation error due to small variations in layer thicknesses was at least 5 percent. For the pavement structures studied, the subbase layer proved to be most sensitive to layer thickness variations. The subgrade appeared to be least sensitive to small variations in layer thicknesses. Thin, stiff layers such as a stabilized subbase were found to be most sensitive of all to layer thickness fluctuations. Despite the large variations in backcalculated moduli at individual data points, the average backcalculated moduli for the entire data set of each pavement structure compared well with the correct modulus value (i.e, the modulus values used to generate the deflection data).