Incorporation of Subgrade Modulus Spatial Variability in Performance Prediction of Flexible Pavements

Recent efforts under the auspices of the National Cooperative Highway Research Program (NCHRP) have resulted in the development of a Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures. The recently developed Mechanistic-Empirical Pavement Design Guide (MEPDG) employs an iterative procedure for accumulating damage over the entire design period. It follows by mechanistically computing the pavement response (i.e., stresses and strains) and empirically translating these responses into individual distresses and pavement roughness. In the current procedure, the subgrade is divided into several sublayers, and an average modulus is used to define the mechanical behavior of each sublayer, without any consideration of the spatial variability in the subgrade modulus. This paper aims to evaluate the effect of such variability on the overall performance of flexible pavements. To achieve this objective, actual field data were collected and analyzed to determine the frequency distribution of the subgrade modulus, which was incorporated in three-dimensional (3D) finite element models of typical flexible pavement structures. The response obtained from the 3D finite element analysis was used to predict the pavement performance using the MEPDG transfer functions. The results of this study showed that the higher the variability is in the subgrade resilient modulus, the higher the variability is in the pavement response. Furthermore, the effect of the spatial variability in subgrade modulus was more pronounced on the prediction of rutting than that of the fatigue cracking.