Characteristic of unbound granular materials and subgrades based on multi stage RLT testing

The unbound granular layers (UGM's) and the subgrade of a pavement structure provide a significant support for the structure as a whole. Hence the mechanical properties of these materials are important for the overall performance of the structure. These materials exhibit both non-linear stress dependency and their mechanical properties are highly affected by their moisture content. This paper describes two of the most important material properties of the unbound layers of the pavements, i.e. the resilient and the permanent deformation properties. The stress dependency of UGMs can be captured by the universal model or the enhanced model by Cary and Zapata (2011) for materials with high fine content. The Bishop's effective stress approach can be used for high fine content materials for estimating the effect of suction. UGMs and subgrade materials are further highly dependent on moisture content. Either the AASHTO sigmoidal model can be used to describe the moisture dependency or the simple exponential model described in this paper. The paper further discusses four models to characterize the accumulation of permanent deformation with the number of load repetitions in repeated load triaxial (RLT) testing. The models are all developed to fit single stage (SS) RLT test results but have been extended to fit multi-stage (MS) RLT test results with the aid of the time hardening scheme. The advantage of the MS RLT tests over the SS RLT tests is that it applies a range of stress paths on a single specimen, thus taking into account the effect of stress history and reducing the time and effort required to test a separate specimen for each stress path.