Inter-particle lubrication model for ballast bed response at medium moisture contents

For railway lines in rainy regions, ballast beds may remain at medium moisture contents for prolonged periods because of their good drainage, and surface water on ballast particles can lubricate inter-particle contacts. Existing research mainly focuses on the dynamic responses of the ballast bed under field capacity (i.e. low moisture content) or saturation conditions. This study proposes an inter-particle lubrication contact model for ballast at medium moisture contents, in which lubrication force, friction coefficient and capillary force vary with the thickness of the surface water film. A minimum contact distance, defined from ballast surface asperity and water-film thickness using previous laboratory results, is introduced into the model. The model is implemented in the EDEM software program through a user subroutine and used to reproduce a full-scale half-sleeper test under simulated rainfall. The discrete-element model results show that, macroscopically, sub-ballast stress fluctuates with moisture content whereas dynamic stiffness remains nearly stable. Microscopically, moisture content changes inter-particle contact behaviour and significantly affects particle angular acceleration, with clear spatial differences across the ballast bed. The simulated macro- and microscopic trends agree well with previous laboratory observations, confirming the applicability of the proposed model.