Effects of basecourse saturation on flexible pavement performance.
It is recognised that saturation of an unbound granular basecourse may lead to the premature failure of flexible asphalt pavements. Basecourse saturation results in increased flexural strains in the asphaltic layer and increased rutting leading to cracking of the surface layer. To obtain a physical and quantitative understanding of the deformation characteristics of saturated granular basecourse materials and the associated failure mechanisms of the pavement structure, laboratory testing equipment was developed including an automated kneading compactor, dynamic triaxial apparatus and a constant head permeameter. Tests were carried out on 250 mm diameter compacted samples of a basaltic aggregate having a maximum particle size of 38 mm. Dynamic tests were performed on both unsaturated and saturated samples. Characterisation tests established variations in resilient moduli and Poisson’s ratio with deviator and confining stress for use in multi-layer elastic analyses. The results of a testing program where samples were subjected to stress states simulating traffic loading, indicated that saturation approximately doubles resilient deformations and increases creep rates by as much as a factor of 10. These changes are caused by loss of capillary stress on saturation and increases in dynamic pore water pressures as a result of applied lateral pressures. A fundamental relationship between the creep rate and the resilient strain amplitude was established, and provides a simple key for the comparative study of different aggregates and the assessment of rut depths in design applications. To assess the effects of dissipation on the magnitude of the dynamic pore water pressures in a pavement structure, a permeability study was carried out on compacted basecourse samples. The permeability characteristics measured were used in a computer analysis to establish the effects of dissipation. Graphs were established showing the effects of permeability and traffic speed on the pore water pressure response. The permeability tests also showed that fines were transported at hydraulic gradients in excess of l resulting in clogging of pore channels and large reductions in permeability.
Comparative analyses were carried out to assess the response of saturated and unsaturated pavement sections using the deformation characteristics established in conjunction with a multi-layered elastic computer program. Results of the analyses indicated that the increase in rut depth due to basecourse creep deformation may be 100 times larger for a saturated pavement compared to an unsaturated pavement.