Characterization of Unbound Materials for Mechanistic-Empirical Pavement Design for NDOT Districts 2 and 3

The American Association of State Highway and Transportation Officials (AASHTO) adopted the Mechanistic-Empirical Pavement Design Guide (MEPDG) as an interim pavement design standard in 2008. The Nevada Department of Transportation (NDOT) started the implementation of the MEPDG for the structural design of flexible and rigid pavements. The resilient modulus (Mr) for unbound materials is an important parameter in pavement design and is used to characterize unbound materials in the MEPDG. The MEPDG follows a hierarchical approach in defining the required engineering properties of the pavement structure, where three levels of input are specified in the AASHTOWare® Pavement ME design software. The levels include using direct measurements from laboratory testing offering the highest level of accuracy (i.e., Level 1), estimating input values through correlations with other soil properties (i.e., Level 2), and using typical values offering the lowest level of accuracy (i.e., Level 3). NDOT has been using the resistance value (R-value) to estimate the Mr of unbound materials through an equation which was not originally developed for Nevada materials. In 2017, NDOT took a major step towards updating the process for determining the unbound materials’ design Mr in District 1 by developing models correlating it to other physical properties. The major objective of this study is to conduct similar research to develop resilient modulus prediction equations for NDOT Districts 2 and 3, and ultimately combine all collected materials in the aim to develop statewide design Mr prediction models for Nevada materials. The unbound materials were collected from Districts 2 and 3 and tested for particle size distribution, Atterberg limits, specific gravity, moisture-density relationship, unconfined compressive strength, R-value, and Mr. The Mr test was conducted according to AASHTO T307, and the stress dependent constitutive models for the unbound materials were obtained. In summary, the stress-dependent behavior of the Mr in Districts 2 and 3 was found to fit well with the Theta model for base materials, and with the Uzan and Universal models for the subgrade materials. The developed Mr constitutive models were used along with the MEPDG procedure to obtain the design Mr for new projects. The design Mr for rehabilitation projects was obtained differently through a stepwise mechanistic approach. The design Mr values were used with the measured empirical and physical properties to develop prediction models for Districts 2 and 3 unbound materials for new and rehabilitation projects. The results from this study and the one conducted for District 1 materials were compiled and analysis was done to develop Mr prediction models for Nevada materials