Cyclic settlement of ballast layer due to train passages at high speed and its reduction by asphalt trackbed

Ballasted track has been widely adopted in railway lines, even in high-speed railways. As the train speed increases, ballast layer has to undertake excessive settlement or severe degradation. To preserve rail network safety and ride quality more optimally, it is vital to get further insight into the cyclic accumulative deformation of ballast layer. This study concentrates on investigating cyclic settlement of railway ballast with an emphasis on the inside particle-scale interactions in conventional ballasted trackbed, and further in asphalt trackbed, a promising structural form of the granular track. The entire transversal cross-section of granular trackbed is modeled based on discrete element method with realistic polygon elements. The numerical model is validated with the measurements from full-scale model tests and then used to study the deformation behavior of railway ballast. Subsequently, the trackbed model with insertion of asphalt layer is established to investigate the reduction of permanent deformation in asphalt trackbed. The granular trackbeds under different train loading are analyzed in the end. Results indicate that the inter-particle structure in the ballast is stabilizing with an increasing number of loading cycles, contributing to less particle migration, so the growth rate of permanent deformation appears a slowing trend. Besides, the asphalt layer optimizes the distribution of contact chains in the ballast and reduces the intensities of contact forces, leading to a remarkable decrease in permanent deformation. Moreover, the higher train speed results in more active ballast particles so that the trackbed becomes hard to stabilize especially when the shakedown threshold is reached. The adverse impact derived from the train speed can be weakened effectively by the asphalt layer. This paper discusses the cyclic settlement of railway ballast and its reduction induced by the asphalt layer. These conclusions will guide the potential methods to alleviate the permanent deformation in high-speed railways, such as employing the asphalt trackbed or controlling the train speed.