The effect of ballast moisture content and fouling index on railway track settlement

A primary function of railway ballast is to maintain vertical track geometry, however over time it becomes contaminated with fines which reduce its ability to free-drain. Then, in the presence of moisture, the individual particles have greater scope for rearrangement thus leading to increased track settlement. This behaviour has received limited studied from a differential settlement viewpoint despite track geometry being the dominant metric used for scheduling railway maintenance. Therefore this paper presents a novel numerical modelling approach capable of simulating the effect of ballast fouling and moisture content on the evolution of track geometry. First the model is presented, which uses a 2.5D finite element approach with perfectly matched layers to simulate static and dynamic stress fields from trains. Non-linear track-soil effects are considered and loading is applied using a multi-body vehicle with train-track coupling. Differential track settlement is calculated in an iterative manner, being updated after every train passage. It is used to perform four analyses, the first of which is to investigate the effect of ballast fouling considering dry conditions. Next the effect of moisture of fines is studied. Then, the effect of earthwork stiffness and train speed is analysed. It is shown that elevated levels of ballast fouling lead to faster deterioration in track geometry, particularly in the presence of moisture. When the ballast is heavily fouled and the moisture of fines is high, rapid deterioration occurs. In contrast, when the ballast is kept dry, the increase in degradation rate for fouled ballast is limited. Higher train speeds and lower earthwork stiffness's make this effect more pronounced.