Effect of Load Spectra on Mechanistic-Empirical Flexible Pavement Design

The transition from equivalent single-axle loads to load spectra represents a significant change in traffic characterization as many agencies begin the migration from empirically based pavement design methods to mechanistic-empirical (M-E) approaches. A key issue lies in determining appropriate representative load spectra that can be used for design. Previous studies have examined differences between load spectra and have largely found them statistically different and site-specific. This study proposes an approach to evaluating different load spectra in terms of practical effects on resulting flexible pavement thickness design. Load spectra from 12 sites in Alabama were evaluated against a statewide load distribution. Customized M-E design software featuring Monte Carlo simulation and layered elastic analysis generated pavement response distributions for a range of three-layer pavement structures. The responses were transformed through calibrated transfer functions into predictions of pavement performance from which required thicknesses were derived. For the 36 design scenarios considered (3 soil types and 12 load spectra), approximately 86% of the design thicknesses were within 0.5 in. of the thickness determined from the statewide load spectra. It was concluded that, in practical terms, statewide load spectra are warranted for use and will not adversely affect most pavement designs. However, local knowledge and experience should be used to determine when site-specific data should be collected for pavement design.