Characterization of Unbound Materials for Mechanistic-Empirical Pavement Design Guide (MEPDG)
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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) already started the implementation of the MEPDG for the structural design of flexible and rigid pavements. The resilient modulus of the unbound materials remains an important parameter in pavement design. This parameter also used to characterize the unbound materials in the MEPDG. The MEPDG follows a hierarchical approach in defining the required engineering properties of the pavement structure. Three levels of input are specified in the AASHTOWare® Pavement ME design software. This includes direct measurement from the laboratory testing offering the highest level of accuracy (i.e., Level 1), estimated values using correlations with soil properties (i.e., Level 2), and typical values offering the lowest level of accuracy (i.e., Level 3). NDOT currently uses R-value to estimate the resilient modulus of unbound materials which is not originally developed for Nevada. The major objective of this study is to develop the resilient modulus model for new design and rehabilitation projects. The unbound materials were sampled from District I and various testing were conducted to determine numerous properties and characteristics including the classification of the evaluated material (i.e., soil classification), R-value, moisture density, unconfined compressive strength, and resilient modulus test. The resilient modulus test was conducted according to AASHTO 307 procedure. The stress dependent resilient modulus models were developed for the unbound materials. In summary, the stress dependent behavior of the resilient modulus for base material in Nevada District I was found to fit very well the theta model. Meanwhile, the stress dependent behavior of the resilient modulus for the subgrade materials fitted very well both the universal model and Uzan model. The MEPDG procedure was used to find the design resilient modulus for the new design projects. On the other hand, for the rehabilitation projects, a different approach was followed to determine the design resilient modulus in this study. For that, two different resilient modulus models were developed for new design and rehabilitation projects for the unbound materials. Additionally, it was concluded that the current available NDOT resilient model equation overestimates the resilient modulus.