Understanding degradation mechanisms at railway transition zones using phenomenological models

Due to the current climate crisis, railway transport is receiving increased attention owing to its capability of running fully on electricity, which can be generated from renewable sources. High-speed railway networks and the new concepts, such as Hyperloop, are already competing with road and aviation transport. However, the increased demand on railway transport causes an acceleration in infrastructure degradation leading to an increased frequency of maintenance and repair operations. Consequently, what before was considered normal "wear and tear" of the infrastructure is quickly turning into serious challenges causing disruptions to the normal operation of traffic.

When it comes to track degradation, the so-called transition zones require significantly more frequent maintenance than the regular parts of the railway track. Transition zones in railway tracks are areas with substantial variation of track properties (e.g., foundation stiffness) encountered near rigid structures such as bridges, tunnels, culverts, or rail-crossings. The occurrence of differential settlements at transition zones has been known for a long time and a multitude of mitigation measures have been designed to cope with this problem. Nonetheless, the mitigation measures have had just limited success and in some cases have even exacerbated the problem. Although the failure of some mitigation measures stems from inadequate design and poor implementation, overall, the lack of efficiency of mitigation measures can be attributed to the lack of understanding of the main mechanism(s) that drive(s) the differential settlement. Therefore, to design efficient mitigation measures, one needs to advance the understanding of the physical processes leading to differential settlements at transition zones. This constitutes the first objective of this dissertation.

The settlement mechanisms are studied in this dissertation through models rather than in-situ measurements or lab experiments. The majority of previous studies have used models to (i) understand and (ii) predict the response of railway tracks at transition zones. Researchers aiming at (i) have usually used simplified phenomenological models in which system characteristics that are not of interest are excluded. More recently, the models' complexity has increased tremendously by incorporating many system characteristics, making these models ideal for (ii), but less ideal for (i) due to the many mechanisms simultaneously at play. This led to the second objective of this dissertation, which is to investigate the effect of specific characteristics of the railway system on the degradation at transition zones. In other words, the second objective entails improving the simplified models by incorporating additional characteristics and determining which of these characteristics is of importance and which can be neglected.

Naturally, this dissertation can only focus on a few of the many aspects involved in this complex problem, and the two main constraints are presented in the following. Improving the maintenance operations themselves by employing new technologies could lead to a reduction in the maintenance frequency. However, to develop a long-term solution, one should aim at eliminating the root cause. Therefore, this dissertation investigated the \emph{initiation} phase of the settlement, and not the accumulation phase. Furthermore, this dissertation focused on the differential settlement stemming solely from the amplification of stresses and strains that occur at transition zones, which is significant at relatively large train velocities. Consequently, this dissertation has not treated other sources of differential settlements, such as the different rates at which autonomous settlement develops in the open-track and at the man-made structure.

Using a simple phenomenological model representative of the railway track, Chapter 2 demonstrates that the response amplification at transition zones is caused by the interference between the steady-state field and the free field generated by the transition process. Consequently, the more pronounced the free field, the larger the resulting amplification. It also shows that the soft-to-stiff and stiff-to-soft transitions have significantly different behaviour, strongly suggesting the need of different mitigation measure designs for the two types of transition. Finally, the transition radiation energy is shown to be invariant between the soft-to-stiff and stiff-to-soft scenarios, finding which was unexpected considering the above-mentioned difference in behaviour.

Investigating the vehicle-structure interaction, Chapter 4 demonstrates that the amplification of the wheel-rail contact force caused purely by a change in foundation stiffness and damping (i.e., a track without initial imperfections) can be significant. Previous literature studies concluded the opposite; however, these studies considered only quasi-static velocities and small effective changes in foundations properties. The findings presented in this chapter, thus, supplement earlier findings to offer a more complete picture. Nonetheless, even though the vehicle-structure interaction leads to a stronger transition radiation, it leads to a reduction of the response amplification at the critical locations in transition zones where settlement is usually observed.

Chapter 5 identifies three response amplification mechanisms at transition zones in systems that have a periodic nature. The amplification is the product of a system with periodic nature and with a local inhomogeneity, and if one of these characteristics is omitted, the amplification does not occur. While these mechanisms can be influential for the railway over-head wires and for the emerging Hyperloop transportation system, they have a negligible influence in the conventional railway track. Consequently, for investigations focused on transition zones and response amplification at low frequencies, the periodicity of the railway track can be successfully approximated by the equivalent continuously supported one without neglecting influential amplification mechanisms.

Chapter 6 introduces the ballast settlement and investigates its influence on the transition process. It shows that the development of the initial settlement leads to a redistribution of the transition radiation energy during the transition not only between frequencies, but also between the soft and stiff media. This redistribution is mainly attributed to the separation between the beam and foundation at the settlement location. Consequently, if the developed settlement is not large enough to allow for this separation, the influence of the nonlinear foundation on transition radiation is negligible.

Chapter 8 investigates the influence of the foundation nonlocality on transition radiation. It shows that the nonlocality of the soil layer has an increasingly pronounced effect on the steady-state response with its decreasing shear stiffness. Consequently, modelling the nonlocality of the supporting structure can be important for railway tracks founded on soft soils. Furthermore, for ballasted tracks founded on soft soils, the response amplification at transition zones can be more pronounced in the soil layer than in the ballast layer depending on the transition type. This is caused by the vertical stiffness of the ballast layer can be significantly larger than the one of the soil. This finding suggests that soil settlement should be accounted for if the long-term behaviour is to be correctly represented.

The investigation of several mechanisms of response amplification at transition zones performed in this study has led to a deeper understanding of the mechanisms leading to differential settlement at transition zones in railway tracks. This knowledge can serve future researchers and engineering in designing more efficient mitigation measures.