Characterization and Performance Prediction of Unbound Granular Bases with and Without Geogrids in Flexible Pavements

Abstract

Unbound granular materials (UGMs) constitute the supporting layer of flexible pavements. The performance of unbound granular base layer has been widely recognized to be influenced by its resilient modulus and permanent deformation, as well as whether it is reinforced by geogrids. The UGMs are found to exhibit moisture-sensitive and stress-dependent nonlinear cross-anisotropic behaviors, but are not adequately characterized by existing models in pavement engineering. The primary objective of this study is to develop a comprehensive methodology to accurately characterize the constitutive behavior of UGM, and predicting the performance of UGM in flexible pavements. Furthermore, this study aims at quantifying the influence of geogrid on pavement performance to facilitate the incorporation of geogrid into Pavement ME Design Software.

A new resilient modulus model is first developed to characterize the moisture-sensitive and stress-dependent nonlinear cross-anisotropic behavior for UGM. The moisture dependence of UGM is characterized by the degree of saturation and the matric suction. This model is validated by the laboratory resilient modulus tests on the selected UGMs at different moisture contents. The finite element approach is then employed to predict the performance of flexible pavements by incorporating this constitutive model for UGM. To accurately predict the rutting depth of base course, a new mechanistic-empirical rutting model is also developed to forecast the rutting behavior of UGM at different stress levels.

In this study, the repeated load triaxial tests are performed on a variety of granular materials to determine the resilient modulus and permanent deformation properties. The measured resilient modulus and permanent deformation properties are statistically related to a wide variety of performance-related base course properties. These regression models can accurately and efficiently predict the resilient modulus and permanent deformation properties of UGM.

A laboratory methodology is developed to evaluate the impact of geogrid on cross-anisotropy and permanent deformation properties of UGM. This impact is successfully predicted by an analytical model. Finite element models are developed to simulate the geogrid-reinforced structures by considering the geogrid-reinforcement mechanisms. These numerical models are validated by the large-scale tank tests. The validated finite element models provide a sound basis for predicting the performance of geogrid-reinforced pavements.