To accurately simulate the three-dimensional stress state and service performance of subgrade under long-term traffic loads, a subgrade service performance test system was developed. The test system consists of the loading system, a fully digital servo control system, and a data acquisition system. Based on the time-history characteristics of total stress components (three normal stresses and three shear stresses) of subgrade soil elements under traffic loads, the loading system was designed with four dynamic actuators and three static actuators. The loading system can simulate the rotation of principal stress axis in any subgrade soil elements through coordinated dynamic and static loading. The calculation method of load system was established to achieve the threedimensional stress state of subgrade soil element under traffic loads. Furthermore, the model tests were conducted on the developed test system to verify the three-dimensional stress state of subgrade under the typical traffic loads, such as highways, railways and airports. Results shows that the actual output load deviation of each dynamic and static servo actuator is under 1%. The time-history curves of dynamic stress components and the attenuation of vertical dynamic stress are in good fit with the theoretical calculations. Besides, the vertical dynamic stress in the subgrade decreases progressively with depth, and the stress path of the soil element is approximately heart-shaped. The above validated results indicate that the test system accurately simulates the three-dimensional stress state of subgrade under different traffic loads. Therefore, the subgrade service performance test system developed in this study offers a new concept, method, and technology for investigating the evolution of subgrade service performance under long-term traffic loads.

Geomechanics tests and theories have confirmed that soil exhibits noncoaxial behavior under the rotation of principal stress. A series of hollow torsional shear tests were conducted in this study on fiber-reinforced soil using a hollow cylinder apparatus (GDS-SSHCA). Factors including deviatoric stress, q, the coefficient of intermediate principal stress, b, and fiber content, FC, potentially influencing the shear strain, volumetric strain, and noncoaxiality of fiber-reinforced aeolian soil were evaluated in the tests. The results revealed that both shear and volumetric strains of the fiber-reinforced aeolian soil samples increased as deviatoric stress and the coefficient of intermediate principal stress increased. However, the impact of fiber content initially decreased and then increased. Maximum shear strain and volume strain values were measured at 0.44% and 0.517%, respectively, with an optimum soil content of 3 parts per thousand. During pure principal stress axis rotation, the fiber-reinforced aeolian soil exhibited noncoaxial characteristics and a fluctuating noncoaxial angle. The average noncoaxial angle decreased to a minimum of 23.59 degrees as the deviatoric stress, the coefficient of intermediate principal stress, and the fiber content increased. Based on the range-analysis method, deviatoric stress was found to have the most pronounced effect on the average noncoaxial angle, followed by the coefficient of the intermediate principal stress and the fiber content. A shear strain prediction equation considering noncoaxiality under pure principal stress axis rotation was established and verified against previously published data. The equation's accuracy was further confirmed through comparison with monitoring data. These findings may serve as a valuable theoretical reference for preventing geological engineering disasters.