Effect of Longitudinal Displacement at Beam Ends on the Mechanical Behavior of the Ballast Bed Using a 3D Discrete-Element Method

Under the overall temperature loading, bridges undergo expansion and contraction deformations due to the thermal expansion and contraction effect. The resulting significant longitudinal displacement poses substantial challenges to track safety and operational stability. To investigate the influence of longitudinal displacement on the mechanical performance and quality state of the ballasted track at the beam end, a three-dimensional (3D) coupled model of the ballasted track, including a ballast guard board, was established. This model can effectively simulate the transmission of forces between the rail, sleepers, ballast bed, and bridge deck. On this basis, a detailed analysis was conducted to examine the influence of the longitudinal displacement of the beam end on the microcontact characteristics of the sleeper during lateral movement, the evolution of lateral resistance of the ballast bed, and the compactness of the ballast bed area. The results indicate that for the same lateral displacement of the sleeper, elongation of the beam end reduces the maximum contact force between the ballast, while contraction increases it. The evolution of lateral resistance with increasing sleeper displacement transitions from a linear growth phase to a strengthening phase. Elongation of the beam end will reduce the lateral stability of the track bed, while contraction will increase the lateral stability of the track bed. During the longitudinal displacement process of the beam end, complex force transmission characteristics exist between the bridge, ballast bed, and sleeper, resulting in varying degrees of density changes in different areas of the ballast bed. In addition, excessive longitudinal displacement at the beam end increases the likelihood of sleeper tilting, lifting, and potential damage, thereby compromising the operational safety of the track structure.