Characterization of Unbound Granular Layers in Flexible Pavements

The mathematical characterization of unbound granular materials should ideally be based on the behavior of the individual constituent elements and their interaction. Until particulate mechanics are developed to a level where it can easily be applied to characterize unbound granular materials, a nonlinear and cross-anisotropic model must be used to characterize the behavior of granular materials in pavements. Existing pavement design and analysis methods have generally taken a very conservative view of the relative strength properties of granular materials used as base and subbase layers in conventional flexible pavements. The mechanical properties of unbound granular layers in flexible pavements are important to the overall structural integrity of the pavement structure. Linear elastic analysis can be used with reasonable confidence for pavements with full depth asphalt layers, but it is inappropriate for unsurfaced or thinly surfaced flexible pavements unless the nonlinear behavior of unbound granular materials are properly taken into account. Work done by several researchers suggest that incorporating a cross-anisotropic elastic model significantly improves isotropic models and drastically reduces the tensile stresses computed within granular layers. This is due to the fact that the behavior of granular materials depends on particle arrangement. The laboratory determination of cross-anisotropic properties of granular materials has been a difficult task for researchers. In this study, a new laboratory testing protocol has been developed based on the theories of elasticity to determine cross-anisotropic properties of granular materials. The testing protocol is efficient and precise. The test is also an excellent tool for comparative analysis of compacted materials. The behavior of four unbound granular materials was studied. The resilient responses of the materials obey the Uzan type nonlinear model. It was observed that under low stress levels accumulation of permanent strain could stabilize in granular layers. However, at high stress levels, permanent strain will continuously accumulate. A finite element program was modified to incorporate the cross-anisotropic material model. Pavement sections were analyzed with the finite element program. It was observed that cross-anisotropic modeling eliminates the presence of tension zones predicted by isotropic resilient models. Deflection bowls predicted by nonlinear resilient models agree with field deflection bowls.