Unbound granular materials (UGMs) are widely used as road-base and subbase materials for bearing traffic loading. Significant road settlements can be caused by cyclic traffic loading after several years of operation. To investigate the settlements of base and subbase induced by traffic loading, a series of large-scale cyclic triaxial tests were conducted under low-amplitude and high-cycle loading. Special attention was paid to the effects of the grain gradation on the strain accumulation of UGMs under cyclic loading. Test results showed that the gradation had a considerable effect on the accumulated deformation of UGMs under various cyclic loadings. Based on the experimental results, an improved high-cycle accumulation (HCA) model, incorporating a new function to consider the influence of grain gradation, was proposed. Good agreement was observed between the results predicted by the improved HCA model and the validation-test results.

The traffic-induced cyclic stresses on the road base and subbase courses are usually in three-dimensional stress state, while so far most laboratory studies have focused on the deformation behavior of base and subbase layers in axisymmetric stress state. This study investigates the three-dimensional resilient behavior of coarse granular base and subbase materials based on a true triaxial apparatus. The factors of effective confining pressure, [ ], amplitude of cyclic shear stress, q^ampl, coefficient of cyclic intermediate principal stress, b_cyc, and fines content are involved. Test results indicate that the increase of either [ ] or q^ampl leads to a nonlinear growth of resilient modulus with a decreasing growth rate. b_cyc, which is proposed to represent the coupling of cyclic major and intermediate principal stresses, is found to have a promotion effect on the resilient stiffness, and the promotion effect tends to be enhanced by the increase of q^ampl. A modified model is established to predict the resilient modulus of coarse granular materials in three-dimensional stress state. There appears to exist a critical value of fines content, at which the variation trend of resilient modulus is changed from increasing to decreasing, and the critical fines content is related to both [ ] and b_cyc.