Field Validation of the Cross-Anisotropic Behavior of Unbound Aggregate Bases

The ICAR Research Project 502 has focused on determining structural considerations of unbound aggregate pavement layers for a proper representation in the new AASHTO Pavement Design Guide 2002. The research team developed models for the resilient and permanent deformation behavior from the results of triaxial tests conducted at the Texas Transportation Institute (TTI) and at the University of Illinois. The studies have mainly indicated that the unbound aggregate base (UAB) material should be modeled as nonlinear and cross-anisotropic to account for stress sensitivity and the significant differences between vertical and horizontal moduli and Poisson's ratios. UABs were constructed at the TTI Riverside research facility and tested for response and performance using the one-third scale model of the Texas Mobile Loading Simulator. The resilient responses of the test sections were modeled. The nonlinear cross-anisotropic material models used in the base layer predicted vertical deflections that are close to field deflections in the analyzed TTI pavements. Field validation data were also collected from a full-scale pavement test study conducted at Georgia Tech. The test sections had extensive instrumentation and the pavement response variables, such as stresses, strains, and deformations, were measured in all pavement layers including the UABs. The validation of the anisotropic modeling approach was accomplished by analyzing these test sections using GT-PAVE finite element program, predicting UAB responses, and comparing them to the measured ones. Laboratory testing of the aggregate samples was conducted at the University of Illinois and the characterization models were developed for the stress sensitive, cross-anisotropic aggregate behavior. With nonlinear anisotropic modeling of the UAB, the resilient behavior of pavement test sections was successfully predicted at the same time for a number of response variables. In addition, the stress sensitive, cross-anisotropic representation of the base was shown to greatly reduce the horizontal tension computed in the granular base when compared to a linear isotropic representation.