New England Verification of National Cooperative Highway Research Program (NCHRP) 1-37A Mechanistic-Empirical Pavement Design Guide

Abstract

In 1996, the National Cooperative Highway Research Program (NCHRP) launched Project 1-37A to develop a new design guide for pavement structures. The design guide recommended by the project team in 2004 is based on mechanistic-empirical (M-E) principles and is accompanied by software that handles the execution of the design and performance prediction. The mechanistic empirical pavement design guide (MEPDG) software has gone through various version upgrades and improvements to the incorporated models and user interface, the latest being DarwinME. The design inputs needed for the MEPDG software are classified according to a hierarchy system where the designer can select the level of data accuracy and sophistication based on the economic impact of the project. The selection is also a function of the state-of-knowledge and availability of the data. The levels vary from Level 1, for which design inputs are generally site specific and are determined from material testing and/or in-situ measurement to Level 2 and 3, where default or user-selected values obtained from national and regional experiences such as LTTP sites are used. The performance prediction models incorporated in the MEPDG were validated and calibrated using field performance of selected pavement sections throughout the United States. Coefficients incorporated in the models can thus be regarded as national averages derived from the performance measured from the sites selected for the calibration. While the State Highway Agencies (SHAs) can use those models with the “default” coefficients, a higher level of reliability can be achieved in predicting the distresses if the agencies adjust the coefficients to better suit the conditions prevalent in their states. It is widely recognized that local calibration of the models should thus be performed to take full advantage of the MEPDG. The main goal of this research was to offer the New England and New York state highway agencies guidelines for the implementation of the MEPDG for designing flexible pavements and AC overlays. This report documents the current design practices of the six New England States and New York as well as progress of MEPDG implementation initiatives undertaken by other states. A comprehensive sensitivity analysis of the MEPDG Level 2 and 3 inputs for each of the seven states involved in this study was conducted. The extensive software runs conducted allow for an evaluation of the MEPDG functionality and accuracy for the level of inputs used by comparing predicted distresses with field-measured distresses, and provide individual states with an idea on adequacy of their input database and accuracy that the embedded distress models with nationally calibrated coefficients provide. The findings can be used by the state agencies in their decision on whether to start implementing the MEPDG with current models and coefficients and for what level of analysis, and in prioritizing implementation activities.