Evaluation of Permanent Deformation Characteristics of Unbound Granular Materials by Means of Multistage Repeated-Load Triaxial Tests

Abstract

Rutting in flexible pavements is often associated with permanent deformation of the unbound granular layer. The current permanent deformation models are applicable only to a single stress path in repeated-load tri-axial (RLT) tests, in which the load pulses are of constant amplitude. In this paper, a general approach using the time-hardening concept was introduced to model the permanent deformation of unbound granular materials (UGMs) continuously in multistage (MS) RLT tests, in which load pulses of a range of different amplitudes are applied, to represent field conditions realistically. With this formulation, three existing permanent deformation models were reconstructed, and one of the models was slightly modified, to suit MS loading conditions better. The material parameters of these models were then optimized for three UGMs used in pavement construction with data from MS RLT tests and application of a least squares curve-fitting method to the test data. The goodness-of-fit statistics were computed to evaluate and compare the quality of fit achieved with these models. The shakedown ranges were also calculated for each stress path of the MS RLT tests to compare these models in simulating these ranges. Generally, the time-hardening approach, for which the quality of fit is dependent on the chosen model, was found to work successfully. In this study, the selected models performed quite well, with the modified model showing the closest agreement to the test data. Thus, this approach has the potential to be applied for better modeling and prediction of pavement performance.