Transition zones in railway tracks: An experimental and numerical study on the structural behaviour

Abstract

The railway track continually degrades over its life cycle. This has negative consequences for maintenance and availability of the lines and is particularly evident at locations with abrupt changes in the track’s support conditions, such as transition zones to bridges or other structures. To mitigate this problem, modern lines comprise specifically designed backfills with high-performance geomaterials to provide an adequate approach to the structures. However, the problem of track degradation at transition zones is far from being completely solved. The research presented in this Thesis aimed at providing insight into the structural behaviour of transition zones using a novel and integrated approach to investigate the problem, contributing to an efficient management of railway assets, in the context of the life-cycle cost reduction paradigm of the railway system, in particular, the railway track. In the scope of a research project involving LNEC, FEUP and REFER, a 30 km-long railway line is used as case study and its construction is followed up closely. Laboratorial characterisation of bound and unbound granular material used in transition zones is carried out, in particular using cyclic load triaxial tests on large specimens. Thorough in situ characterisation of the track substructure is performed, including specific portancemètre surveys at transition zones. The deformation of the backfill of a transition zone case study to a bridge is monitored using different techniques, including sub-horizontal inclinometers installed during construction. The results obtained regarding the substructure at transition zones indicate that the design of the backfills was successful in minimizing settlements and achieving a gradual stiffness increase at the approach to the structures. The analysis of the evolution of the geometric quality of the line evidenced higher degradation rates at some transition zones and other track singularities, mostly associated with abrupt changes in the track substructure. To fill the gap between experimental and numerical studies, extensive field measurements are presented and used to calibrate and validate FEM models that consider the relevant track components of the superstructure and substructure. The models also account for the dynamic train-track interaction and are very accurate in reproducing the measurements. The experimental works comprised dynamic track characterisation with receptance tests and a new approach using the light falling weight deflectometer. Innovative solutions for transition zones, such as under sleeper pads (USP), are also analysed in detail in specific case studies of transition zones to underpasses. The experimental results evidenced the increase in the vertical stiffness of the track at the approach to structures, which could suggest the amplification of the dynamic track-interaction. However, because the design of the backfills provides a smooth stiffness variation and the track evidenced a very good track quality, the simulations indicated limited dynamic amplifications of the train loads in the tested scenarios. As regards USP, simulations suggested a beneficial contribution to reduce track degradation rates, but a careful design is required to prevent excessive dynamic amplifications when trains enter or exit sections of the track with USP. Later, the effect of the differential settlements on the dynamic response of the train-track system at the approach to structures is investigated. To that aim, comprehensive parametric studies using numerical simulations are carried out. The results of the simulations are integrated and used to establish performance indicators that affect the train-track interaction, based on the length and amplitude of track defects. A novel approach is then presented and applied to the line under study, in an attempt to associate the results of the parametric studies with locations denoting rapid track degradation or other critical scenarios. Tools and actions are suggested to Railway Infrastructure Managers to monitor the performance of transition zones and for a more efficient management of the infrastructure.