Mesoscale Mechanism of Asphalt Track Bed in Reducing Cyclic Settlement of Ballast Layer under High-Speed Train Traffic Loads

Asphalt track bed, in which an asphalt mixture is inserted underneath the ballast in a conventional granular track bed, has been experimentally proven to have better safety, stability, and economic efficiency. However, most existing theoretical studies were based on continuous mechanics theory, mainly focusing on the impact of the asphalt layer on the macroscale response of the track bed or subgrade, and rarely on the particle-scale mechanical behavior of ballast due to the limitations of the method. Hence, aiming at the improved performance of asphalt track bed and its mesoscale mechanisms, the discrete element method (DEM) model of asphalt track bed is established in this study and then validated with the measurements obtained from on-site tests. Subsequently, the validated model is used to investigate the effect of asphalt layer on the dynamic response and cyclic settlement of ballasted track bed under high-speed train moving loading. Results reveal that the asphalt layer has an ability to reduce vibration of the track bed, which contributes to less active ballast and more stable interparticle structure, thereby decreasing the accumulative settlement of the ballast layer. Besides, the asphalt layer can improve the uniformity of the force chain network inside the ballast layer. The well distributed and less intense force chain network in the ballast results in a slower degradation rate. A series of parameter analyses is performed in the end, concluding that the required thickness of the ballast layer in the asphalt track bed could be reduced by approximately one-third without changing the bearing capacity of the subgrade, and the asphalt track bed structure is a more economical and effective track form for high-speed or heavy-haul railway lines. This study discusses the improvement of ballasted railway track by insertion of asphalt layer and its mesoscale working mechanisms, and the results provide guidance for the design of asphalt track bed in high-speed railways or heavy-haul railways.