Sensitivity of Axle Load Spectra in the Mechanistic–Empirical Pavement Design Guide for Washington State

The Guide for Mechanistic–Empirical Design of New and Rehabilitated Pavement Structures [referred to as the Mechanistic–Empirical Pavement Design Guide (MEPDG)] is proposed as an advanced pavement design tool that integrates up-to-date pavement practices. The use of axle load spectra instead of the equivalent single-axle load is a dramatic change. However, the collection of an adequate amount of data over years is required for the accurate characterization of future traffic for design; this gives primary importance to pavement designers having a good understanding of the axle load spectra. This paper presents the typical truck load spectra that satisfy the MEPDG requirements for the Washington State Department of Transportation (WSDOT) and that were developed on the basis of the data collected from selected WSDOT weigh-in-motion stations. Sensitivity analysis was conducted with various typical design parameters of WSDOT flexible pavements. The significant findings are that (a) one type of axle load spectrum can present load characteristics for WSDOT in MEPDG, (b) MEPDG is moderately sensitive to the axle load spectra for typical WSDOT pavement designs, and (c) WSDOT needs to calibrate MEPDG before use. The results have been verified in the calibration of the flexible pavement distress models for WSDOT. It is recommended that agencies that lack data resources test and use the default MEPDG axle load spectral inputs and that the bias may be corrected through model calibration efforts.