Determination of an Optimum Backcalculation Cross Section for Unconventional Pavement Profiles

Abstract

The validity of backcalculation results partly depends on the pavement cross section used in the procedure. However, the development of a backcalculation cross section is challenging, particularly for an unconventional pavement (subgrade stiffer than base). The unconventional stiffness profile poses challenges to backcalculation, including compensating layer effects. This study presents a systematic investigation to determine an optimum backcalculation cross section for seven unconventional pavements at the National Center for Asphalt Technology pavement test track. Apart from the unconventional stiffness profile, the granular materials had different stress sensitivities: the subgrade was stress independent, whereas the aggregate base material exhibited some stress sensitivity. Falling weight deflectometer testing provided 2,268 deflections for backcalculation, which was executed with Evercalc 5.0, and 126 deflection tests on embedded gauges for forward calculation, which was accomplished with WESLEA 3.0. Nine candidate cross sections were evaluated. The study converged on a three-layer backcalculation cross section that provided an overall good deflection basin fit, a balanced match between measured and predicted stresses, and reasonable moduli. A key lesson is that the as-built cross section of an unconventional pavement may not necessarily yield valid backcalculated moduli, although there may be a good deflection basin fit. A working knowledge of the pavement materials is useful for developing an optimum backcalculation cross section, which should be validated through field instrumentation when available.