Investigating the Settlement Behaviour of Long Turnout Sleepers Using DEM

Ballast void formation is a known issue in railway turnouts, yet the underlying mechanisms remain insufficiently understood. This study investigates the mechanical response of a long turnout sleeper lying on a ballast bed under loading using both full-scale laboratory experiments and Discrete Element Method (DEM) simulations to study the correlation between applied load, sleeper deformation, sleeper-ballast interface pressure and residual settlement. The DEM simulations employed a deformable sleeper model using the PFacet approach in the Yade framework and an elasto-plastic contact law accounting for edge breakage (Conical Damage Model) to reproduce ballast-ballast and sleeper-ballast contact behaviour. Results show that the DEM model can replicate key experimental trends, including asymmetric sleeper bending, uplift, and the evolution of ballast pressure distribution in the short term. Under extended cyclic loading, the simulation reproduces the progressive formation of stable bedding conditions and the emergence of ballast voids, aligning with experimental observations. A simplified approach to represent USPs via reduced contact stiffness yielded realistic deformation and pressure behaviour, although residual settlement differed. The results demonstrate that DEM can reproduce and explain sleeper-ballast interaction mechanisms, providing mechanistic insight into how uneven pressure distribution and ballast rearrangement contribute to void formation in turnouts.