Effect of loading frequencies on permanent deformation of unbound granular materials

Unbound granular materials (UGMs) show both permanent and resilient deformations under repeated traffic loads. Permanent deformation (PD) is the irreversible part of the deformation, commonly used for rutting prediction. Several factors, both stress and material related, are found to have influences on PD but some of these factors have not been thoroughly investigated. For example, there is no consensus in existing pavement design standards, guides and literature about the effect of loading frequencies and consequently, they have suggested different frequencies for testing. In order to address this disparity, this study investigates the influence of loading frequencies on PD of UGMs. Three frequencies (0.2, 0.33, and 0.5 Hz) were applied on two types of crushed rocks (basalt and granite) using the repeated load triaxial test (RLTT). The experimental results show that there is negligible effect of these loading frequencies for the tested materials under specified testing conditions. This result has an important implication in that by increasing loading frequency, RLTT testing duration could be drastically reduced. The experimental data were further modelled using regression and artificial neural network (ANN) modelling. The sensitivity analysis from the ANN modelling showed that the loading frequency has only 3% importance on the PD of the tested UGMs among the other factors.