Mechanistic Classification of Unbound Materials

The effectiveness of an approach, based on the use of default resilient modulus (M_r) values in providing input to the Mechanistic–Empirical Pavement Design Guide (MEPDG) developed under the NCHRP 1–37A project, was evaluated. The process is intended to assist users of the guide in estimating the M_r of an unbound material based on its AASHTO soil class. Assessment conducted at the National Research Council Canada (NRC) involved comparing values of the parameter determined in the laboratory using the AASHTO protocol with default values included in MEPDG. Comparison revealed that the proposed default values frequently overestimate the value of the M_r parameter, hence yielding inadequate layer design. Examination of the AASHTO M_r test revealed that the procedure does not account for permanent deformations that accumulate during the laboratory test. These findings prompted the NRC team to develop a different approach for estimating the M_r parameter. The new approach yielded a mechanistic soil classification system based on the material response captured in the laboratory. A practical procedure for implementing the new mechanistic classification system was facilitated by establishing correlations between physical and mechanical properties of the materials. These correlations were found to be more accurate in estimating the M_r parameter and were expected to produce better design when used as input for running the MEPDG model.