Mechanistic-empirical models for better consideration of subgrade and unbound layers influence on pavement performance

Abstract

It has been reported that the pavement performance predicted by the current mechanistic-empirical pavement design shows low or no sensitivity to subgrade and unbound layers. This issue has raised wide attention. Targeting this problem, this paper summarizes the process used by the authors to find better models of the influence of subgrade and unbound base course layers on the performance of flexible and rigid pavements. A comprehensive literature review is first conducted and the findings are categorized. It is found that the resilient modulus, permanent deformation, shear strength, and erosion are key factors. In particular, the properties that provide greater sensitivity are 1) the moisture-dependency of the modulus, shear strength, and permanent deformation; 2) stress-dependency of the modulus and permanent deformation; and 3) cross-anisotropy of the modulus. A number of unbound layer/subgrade models have been located and categorized. Three criteria are developed to identify the candidate models in terms of the degree of susceptibility, degree of accuracy, and ease of development. The first two criteria are used to evaluate the collected unbound layer/subgrade models, while associated development and implementation issues are planned as subsequent work. Two models that the authors previously developed are selected as examples to illustrate the improvement of the performance prediction, including the moisture-sensitive, stress-dependent, and cross-anisotropic modulus model for unbound layers and stress-dependent mechanistic-empirical permanent deformation model for unbound base layers. These two models are verified through laboratory tests and numerical simulations. Moreover, they are compared to their counterparts in the AASHTOWare Pavement ME Design. The advantages of accuracy and sensitivity to the operational conditions (e.g. moisture, traffic stress, and load-induced/particle-induced anisotropy) are obvious. In addition to these two models, the development of the shear strength model and erosion model are sketched. The candidate models need further development and implementation, which address issues such as hierarchical inputs, calibration/validation, and implementation. These are the on-going and planned work on this topic to better incorporate the influence of subgrade and unbound layers so as to contribute to the improvement of pavement designs.