Enhanced Model for Resilient Response of Soils Resulting from Seasonal Changes as Implemented in Mechanistic–Empirical Pavement Design Guide

The present study deals with the revision of the current model in the Mechanistic–Empirical Pavement Design Guide (MEPDG) used to predict the environmental factor for unfrozen unbound materials (F_U), which is used to adjust the resilient response of soils resulting from seasonal changes. A large database with data from the existing literature and studies at Arizona State University was developed to evaluate the model. The results suggest that the environmental factor is underestimated for fine-grained materials with high plasticity under dry (arid) conditions. However, insufficient data were available to enhance the F_U models for wetter conditions. Three fundamental factors that may have impacts on the F_U values were evaluated in this study: stress state, compaction energy (soil density), and soil type. The stress state was found to have little to no impact on the predictions of F_U. But density changes and soil type were found to be important. The potential for soil index properties to be predictive variables was assessed. Models dependent on enhanced moisture content accounting for the effect of soil type are proposed for nonplastic and plastic materials. The range of predicted F_U values is in close agreement with the actual measured F_U values found from laboratory studies. It is recommended that the new models be adopted in the revision of the MEPDG model for the drier conditions described in this report and that research be conducted to enhance the F_U approach in the current MEPDG for wetter conditions brought on for a variety of reasons (e.g., groundwater table change, increased rainfall, and frost effects).