Discrete element modelling of lateral displacement of a granular assembly under cyclic loading

Railway ballast is an unbound granular medium which undergoes non-uniform lateral spread when subjected to high-frequency cyclic loading. In order to simulate the lateral spread of ballast upon loading, a modified large-scale process simulation test (PST) apparatus with five independent movable plates of equal height forming one of the side walls has been built at University of Wollongong, Australia. However, the experiments alone cannot provide a comprehensive study of the lateral flow of ballast in a micro-mechanical perspective. The discrete element method (DEM) has therefore been used to complement the modified large-scale PST apparatus and to provide much needed micro-mechanical insight to the lateral flow of aggregate particles, such as the contact force distribution, particle movement and porosity change. The simulation results provided acceptable predictions of the trend of vertical and lateral displacements of two-ball clump assembly upon loading. In addition, these simulations evaluate the effect of confining pressures (10-30kPa) due to the crib and shoulder ballast, on vertical settlement and the lateral spread of ballast under high frequency cyclic load. The vertical stress decreases significantly with depth from an applied vertical stress of 460kPa at the sleeper-ballast interface to 210kPa at the base wall (ballast-subballast interface), which is in agreement with the experimental results. In addition, geogrid model using PFC3D was used in the ballast layer to examine the ability of geogrid on arrest lateral displacement of ballast and reduce settlement under cyclic loading.