Pier settlement induced unsupported sleepers and nonlinear contact between sleeper and ballast bed in rack railway system

Pier settlement is inevitable in high-gradient elevated lines, which leads to vertical deformation of track structure. The uncoordinated deformation causes longitudinal slip and vertical gap between sleeper and ballast bed. In view of the above problems, this paper considers the vertical and longitudinal nonlinear mechanical characteristics of ballast bed, explores the influence of pier settlement on the contact characteristics of sleeper and ballast bed in rack railway system. The paper pays attention to the nonlinear contact characteristics of pier settlement and vehicle running load on the sleeper and ballast bed mechanical properties and dynamic responses. The results show that the influence of pier settlement on system characteristics is mainly concentrated in vertical direction, which directly leads to vertical gap between sleeper and ballast bed (i.e., the appearance of unsupported sleeper). Compared with pier settlement position, structure of adjacent pier area is more sensitive to the change of pier settlement. The vertical damping effect of ballast bed is worse than that of longitudinal damping effect at pier settlement and pier adjacent to pier settlement. The sliding caused by large slope lines and pier settlement has no effect on the mechanical characteristics of ballast bed.