Transition zones in railway tracks are locations with considerable changes in the vertical stiffness of the track support, which can be found near bridges, culverts and tunnels. In such locations, the stiffness variation and the differential settlement of tracks (uneven settlement of the track on the embankment and of the engineering structure) result in amplification of the dynamic track forces. This amplification speeds up the degradation of ballast and subgrade, ultimately resulting in deterioration of the vertical track geometry, which can lead to deterioration of the passenger’s comfort, failure of the track components, and in extreme cases to train derailment. Therefore, the track maintenance in transition zones requires substantial effort, up to eight times more effort than on open tracks (i.e. ballast tracks without any special elements).
Although the poor performance of the track in transition zones is frequently reported, the transition zones have not been paid enough attention. First of all, there is no specific experimental method for assessment of the track condition in transition zones. Therefore, the transition zones are usually treated as open tracks during inspections. Secondly, the effect of the differential settlement (one of the factors causing the transition zone problem) on the track degradation has not been sufficiently studied as compared to the effects of the stiffness variation. Also, due to the insufficient knowledge on the track behaviour in transition zones, the track settlement in transition zones cannot be predicted precisely. As a result, the maintenance is performed in a reactive way. Finally, although many countermeasures have been proposed for transition zones, the tools for assessment of their performance (especially on a long term) are still lacking, which causes difficulties for track designers when selecting the countermeasures. Clearly, the knowledge on the measurement, dynamic behaviour, degradation, and assessment of the track in transition zones should be improved.
This study intended to give answers to the following questions: (1) How to assess the condition of the tracks in transition zones? using which tool? (2) Which factor contributes more to the track degradation in transition zones, the uneven settlement or the stiffness variation? (3) How to predict the track settlement in transition zones on a long term? (4) How to assess the performance of the countermeasures for transition zones?
In attempt to answer these questions, an integrated methodology combing an innovative experimental method and numerical model for analysis of the dynamic behaviour and degradation of railway tracks in transition zones has been developed. The methodology consists of the following three parts:
- An advanced measurement technique based on the DIC (Digital Image Correlation) method that is used to measure the absolute dynamic displacements of rails/sleepers due to the passing trains. The advantage of this technique is that the vertical track displacements are measured simultaneously at multiple points, allowing obtaining the dynamic profile of the track section. Also, no track possession is required during the measurement. The measurement technique provides a basis for assessment of the track condition in the transition zones.
- A novel model for analysis of the dynamic responses in transition zones that uses the explicit Finite Element (FE) method. The track model accounts for both the vertical stiffness variation and the differential track settlement in transition zones. The nonlinear contact elements are used to model the sleeper-ballast interface, which allows the sleeper-ballast interaction to more realistically be described as compared to the existing models.
- A novel procedure to predict the long-term track behaviour (settlement) in transition zones, which is based on the developed FE model of the transition zones and an empirical settlement model of ballast (developed by Y. Sato). Using this procedure, the track settlement in transition zones due to multiple passages of trains can be predicted, which can provide a basis for planning track maintenance in transition zones.
To demonstrate the developed methodology, it was used in a number of applications in this study such as:
- Assessment of the track condition in various transition zones,
- Numerical analysis of the track behaviour and of the factors influencing initiation and propagation of the track settlement in transition zones,
- Assessment of the performance of various countermeasures for transition zones.
Some additional studies on the effect of the moisture condition on track performance in transition zones and on the feasibility of using satellite radar for structural health monitoring of transition zones have been performed as well. The main conclusions of these studies can be summarised as follow:
o The numerical and experimental results confirmed the higher degradation of the track near engineering structures in transition zones as compared to the open track observed in situ.
o The track degradation and the length of the settlement affected zone in the Embankment-Bridge (EB) and the Bridge-Embankment (BE) transitions, which is defined by the train moving direction, are different. That was confirmed by the measurement and numerical results, and by field observations. This phenomenon was explained using the numerical model, namely that the initial location of the track settlement in the EB transition is primarily defined by the pitch motion of the bogies, while in the BE transition it is affected by the ‘gliding’ and ‘bouncing’ motion of the vehicle. The settlement affected zone in the BE transition is longer (depending on the velocity, approx. 2 times for 140 km/h) than the EB transition.
o The track condition in transition zones was successfully assessed using the measurement method. The condition assessment results have good correlation with maintenance history, and satellite data of the considered transition zones.
o The performance of various countermeasures for transition zones was successfully assessed using the developed methodology. The numerical results have shown that the sleepers with modified dimensions (preventive countermeasure) and the adjustable fasteners (corrective countermeasure) can significantly improve the track performance, 51% reduction in ballast stress and 93% reduction in the wheel-rail contact force respectively.
Using the integrated methodology, the research questions have been answered. The proposed methodology provides suitable tools for measurement, assessment, analysis and improvement of the tracks in transition zones. The methodology can be further applied to the design and optimisation of the track in transition zones.
Haoyu Wang was on born in Shenyang, China in 1987. He received a bachelor’s degree in civil engineering and a master’s degree in railway engineering at Beijing Jiaotong University (Beijing, China). He joined the railway section of Delft University of Technology (Delft, the Netherlands) for PhD research in 2011 and worked partly as a teaching assistant in 2015 and 2016. In 2017, he worked as a consultant/railway specialist in Roadscanners (Tampere, Finland). Since 2018, he has worked as a developer/railway specialist in Fugro (Utrecht, the Netherlands). He is specialised in researching and solving problems in the railway industry, focusing on the track structure analysis, condition monitoring, and quality evaluation.