A test system for the serviceability of highway subgrade considering the principal stress rotation was developed to accurately simulate the evolution of the three-dimensional stress state and assess the serviceability of the subgrade under traffic loads. The subgrade serviceability test system is composed of the core loading system and the auxiliary modules such as full digital servo control system and data acquisition system. The loading system with four dynamic and three static actuators was designed to achieve the rotation of the principal stress axis in subgrade soil elements at various levels based on the coordination of static and dynamic loading. A theory for three-dimensional dynamic stress recurrence in highway subgrade was developed based on the theoretical model of the subgrade’s dynamic response under traffic load and the loading system’s mechanical and spatial characteristics, and furthermore a method of transforming the “stress spectrum” into the “load spectrum” was proposed. Using the test system, the three-dimensional stress state in the highway subgrade under vehicle load was examined. The results show that the actual output load deviation of each static or dynamic servo actuator is less than 1%, confirming that the developed subgrade serviceability test system can accurately simulate the three-dimensional stress state of the subgrade under traffic loads. The time history curves of each dynamic stress component and the attenuation pattern of vertical dynamic stress closely match the theoretical ones. The vertical dynamic stress in the subgrade gradually decreases with depth, and the stress path experienced by the soil element resembles a “heart shape.” The “seven-cylinder linkage and coordinated loading” method can replicate the dynamic response of soil elements at various depths in the subgrade working area during the “far-near-far” movement of traffic loads, despite the loading device’s position remaining unchanged. Therefore, the subgrade serviceability test system developed in this study introduces a new concept, approach, and technology for investigating the evolution of highway serviceability under actual traffic loads.

highway engineering  /  highway subgrade  /  principal stress rotation  /  model test  /  loading system  /  serviceability