Previous investigations have shown that an abrupt stiffness change in track support is often associated with accelerated rates of deterioration of track geometry, high maintenance demand and poor ride quality. However, at present, there is no detailed understanding of the mechanisms of the deterioration of track geometry at transition zones. This paper aims to use the discrete element method to investigate transition zones from a micromechanical perspective. A simple track transition model with dimensions 2.1 m × 0.3 m × 0.45 m was simulated by using PFC^3D. In order to identify and evaluate appropriate mitigation methods, two kinds of transition patterns, including a single step change and a multi step-by-step change for subgrade stiffness distribution were tested. The influence of train direction, speed and axle load on the transition was also investigated. In addition, geogrid was used in the ballast layer to examine the effects of geogrid reinforcement. This paper provides insight into the factors that can cause or accelerate track degradation at the transition zones, in order to identify and evaluate appropriate mitigation design.

This paper presents an evaluation of the behaviour of geogrid-reinforced railway ballast. Experimental large box pull-out tests were conducted to examine the key parameters influencing the interaction between ballast and the geogrid. The experimental results demonstrated that the triaxial geogrid outperforms the biaxial geogrid and the geogrid aperture size is more influential than rib profile and junction profile. The discrete element method (DEM) has then been used to model the interaction between ballast and geogrid by simulating large box pull-out tests and comparing with experimental results. The DEM simulation results have been shown to provide good predictions of the pull-out resistance and reveal the distribution of contact forces in the geogrid-reinforced ballast system. Therefore, the calibrated geogrid model and the use of clumps to model ballast particles hold much promise for investigating the interaction between geogrids and ballast and therefore optimising performance.