Empirical approach for the use of unsaturated soil mechanics in pavement design

Historically, most of the pavement design procedures used in worldwide practice are based upon a simplifying (but costly) assumption that all aggregates and subgrade materials would eventually become fully saturated in their performance life cycle. However, the majority of all pavement layers will not exist in a fully saturated state. Field evidence and numerous numerical modeling studies have shown that even though the pavement acts as a cover for the unbound material, its moisture content will change with time, in an attempt to reach an equilibrium condition. This fact it is greatly influenced by climate regime and soil properties. The development of the initial mechanistic empirical pavement design guide (MEPDG) by the American Association of State Highway and Transportation Officials (AASHTO) incorporated, for the first time, a design methodology for pavement systems that considered unsaturated material response and prediction techniques based upon the site specific environmental conditions. This paper presents an overview of the models used in the MEPDG for the prediction of unbound pavement layer material performance and discusses several revisions to various models that can greatly enhance the MEPDG design procedure.