The role of particle breakage on the permanent deformation of ballast

Ballast is one of the most significant components in rail track structure. It supports the rail and sleeper by transmitting the traffic load to the subgrade. Due to increasing traffic congestion and the price of fuel, the demand for high speed trains has been increasing on a daily basis. This contributes to a more permanent deformation and degradation of the ballast layer. Cyclic loads from heavy haul trains degrade and foul the ballast, directly contributing to track settlement. An understanding of how ballast reacts during cyclic loading plays a key role in reducing the maintenance costs of railway tracks, while optimising passenger comfort. Currently, numerous research studies have been carried out to understand the behaviour of ballast during cyclic loading. However, laboratory investigations and field assessments alone cannot provide a full insight into complex ballast breakage mechanisms and associated deformation when the discrete and heterogeneous nature of granular materials is considered. On the other hand, computer simulations using the Discrete Element Method (DEM) provides enough information from the particle scale level to help understand the deformation and breakage mechanisms that occur under complex cyclic loads. In this research DEM simulations using PFC2D were conducted to examine the degradation and deformation of ballast during cyclic loading, and a case study involving the Bulli track north of Wollongong city is also included. A cyclic biaxial simulation was conducted to supplement the DEM model for track direction of the major principal stress under cyclic loading. Those particles actually under the sleeper sustain more uniform CF chains and breakage compared to particles near the lateral boundary. Particle breakage and re-arrangement leads to more uniform CF chains in the direction of major principle stress, but as the cyclic loading continues, more particles are broken and rearranged, which causes the ballast to become denser and generate increased lateral displacement.