Development of a New Mechanistic Empirical Rutting Model for Unbound Granular Material

This paper proposes a new mechanistic-empirical rutting (MER) model to evaluate the permanent deformation (PD) behavior of unbound granular material (UGM). To characterize the stress dependence of rutting behavior in UGM, the MER model incorporated a softening stress term and a hardening stress term into the Tseng-Lytton model, which is based on the Drucker-Prager plastic yield criterion. Repeated load triaxial tests were performed on two types of UGMs in this study, employing seven stress states to calibrate the model coefficients, and two stress states to validate the accuracy of the model predictions. The correlations of the two incorporated stress terms with the accumulated permanent strains were established based on the triaxial test results. It was found that the correlations are fitted by power functions with 0.97–0.99 R2 values. The proposed MER model was compared with the existing UGM rutting models, including the MEPDG model, Korkiala-Tanttu model, and UIUC model in terms of differences between the laboratory-measured and model-predicted PDs. Compared to the existing UGM rutting models, the MER model is better able to characterize the stress-dependent rutting behavior of UGM at various stress states. In addition, the prediction accuracy of the MER model is significantly higher than the existing models. A sensitivity analysis was also performed to evaluate the effects of cohesion and friction angle on the PD behavior. This demonstrates the potential of the MER model to characterize the moisture sensitivity of rutting behavior for the UGM. Finally, the MER model was implemented in a nonlinear finite element program to predict the rut depth of a flexible pavement. Compared to the MEPDG model, the MER model always predicts higher rut depths of the base layer and is more sensitive than the MEPDG model to the variations of load magnitude and base modulus.