Cyclic Behavior of Rubber–Sand Mixture under Multilane Traffic Loads Considering Principal Stress Rotation

Rubber–sand mixture (RSM), an environmentally friendly material, has been used in the pavement subgrade. Previous studies investigate the cyclic response of RSM with various rubber contents by triaxial devices under traffic loads without principal stress rotation (PSR). However, single-lane and multilane traffic moving loads generate unidirectional and multidirectional PSR, respectively, which may lead to more permanent deformation of the subgrade than the loads without PSR. To further understand the load–deformation relationship of RSM under the traffic loads, a series of cyclic drained tests involving the uni- and multidirectional PSR were performed on RSM and pure sand. The rubber contents determined by volume varied from 10% to 30%, and the specimens were consolidated under various stress levels to obtain different densities. The results indicate that multilane traffic loads generally produce more permanent vertical strain than single-lane traffic loads. The permanent vertical strain reduces with increasing rubber content, while a higher rubber content results in a larger plastic shear strain. In addition, an energy-based approach was employed to investigate the relationship between shakedown behavior and dissipated energy. The dissipated energy within one cycle tends to decline with loading cycles. The difference in dissipated energy between two different loading cycles is below a certain value when shakedown occurs. The findings provide an understanding of the stress–strain response of RSM under various traffic conditions and recommend a rubber content for RSM-subgrade design.