Performance of unbound aggregate bases and implications for inverted base pavements
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The current economic situation has severely affected the US road infrastructure and funding has become inadequate for either maintenance or future growth. The inverted base pavement structure is a promising alternative to achieve high quality roads at considerably lower cost than conventional pavements. The proximity of the unbound granular base layer to the tire load makes the response of the granular base critical to the performance of the pavement structure. Therefore extensive material characterization is conducted on the granular materials that make the base. In particular, a true triaxial chamber is developed to study the mechanical response and the stress-dependent stiffness of granular bases compacted at different water contents. A novel method is developed to assess the as-built stress-dependent anisotropic stiffness of granular bases in-situ using both crosshole and uphole test configurations. The two inverted base pavements built in Georgia at the Morgan County quarry haul road and the Lagrange south Loop are tested as part of this study. A nonlinear orthotropic constitutive model is selected to capture the deformational behavior of compacted granular bases. The response of the pavement is analyzed by implementing this constitutive behavior in a three-dimensional finite-element model. Different pavement structures are simulated. It is shown that thin asphalt concrete layers resting directly on granular bases deform as membranes. Finally, numerical simulations are extended to compare inverted base pavements to conventional pavements used in practice. Results highlight the inadequacy of ASSHTO’s structural layer coefficient for the analysis of inverted base pavement structures as well as the potential economic advantages of inverted base pavements.