a b s t r a c tRailway tracks degrade faster at transition zones to railway bridges. In modern lines, backfills with bound and unbound granular geomaterials have been used to minimize this problem. To provide insight into the behaviour of the train-track system and to fill the gap between numerical and experimental studies, the authors carried out extensive field measurements. These were then used to validate a FEM model that considers the relevant track components, earthworks and bridge; accounts for the train-track interaction using contact elements; and is very accurate in reproducing the measurements. Results showed that the backfill design fulfils its purpose in that it provides a stiffness transition from the embankment to the bridge. The dynamic component of the train-track interaction remained low. The performance of the model makes it a very useful tool to further study the railway track at critical locations, such as transition zones.

The initial geometry of a railway track continually degrades over its life-cycle. Changes in the track alignment give rise to variations in the dynamic axle load which accelerate track degradation, with consequences for maintenance and availability of the line. This behaviour is particularly evident at some critical locations that are associated with abrupt changes in the track's vertical stiffness, such as transitions to bridges or other structures. In order to mitigate this problem, careful design and construction is required, for which several recommendations have been suggested in the literature. However, studies based on the maintenance records of existing high-speed lines have shown that the problem of track degradation associated with stiffness variations is far from being solved. This paper presents a short review on the design of transition zones. A case study on the design and construction of a transition zone on a new Portuguese railway line is analysed. Results of conventional laboratory and cyclic load triaxial testing on granular materials and in situ mechanical characterization of the layers are presented. Relevant aspects regarding the construction are addressed and discussed. The results obtained at the substructure level seem to indicate that the design of the transition zone was successful in minimizing settlement and achieving a gradual stiffness increase as a bridge is approached.

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a b s t r a c tRailway tracks on earthworks with poor geotechnical performance may undergo important differential settlements that amplify the dynamic response of the train-track system, inducing rapid track degradation, lower passenger comfort and higher derailment risk. Because the track substructure is normally inaccessible, these problems are not detected early, are very complex to solve and disrupt normal train operation. To study the influence of settlement profiles on the train-track system, the authors performed a parametric study comprising non-linear dynamic analyses using a FEM model to simulate different scenarios. Some profiles caused important amplifications in the train-track system that were associated to rapid track degradation and derailment risk, while others were associated to lower passenger comfort. Hanging sleepers were found to be strongly associated to critical situations of track degradation. The results were integrated and used in a framework to establish performance indicators that can be useful for planning railway track maintenance works.

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Abstract:The railway track system is a crucial infrastructure for the transportation of people and goods in modern societies. With the increase in railway traffic, the availability of the track for monitoring and maintenance purposes is becoming significantly reduced. Therefore, continuous non-destructive monitoring tools for track diagnoses take on even greater importance. In this context, Ground Penetrating Radar (GPR) technique results yield valuable information on track condition, mainly in the identification of the degradation of its physical and mechanical characteristics caused by subsurface malfunctions. Nevertheless, the application of GPR to assess the ballast condition is a challenging task because the material electromagnetic properties are sensitive to both the ballast grading and water content. This work presents a novel approach, fast and practical for surveying and analysing long sections of transport infrastructure, based mainly on expedite frequency domain analysis of the GPR signal. Examples are presented with the identification of track events, ballast interventions and potential locations of malfunctions. The approach, developed to identify changes in the track infrastructure, allows for a user-friendly visualisation of the track condition, even for GPR non-professionals such as railways engineers, and may further be used to correlate with track geometric parameters. It aims to automatically detect sudden variations in the GPR signals, obtained with successive surveys over long stretches of railway lines, thus providing valuable information in asset management activities of infrastructure managers.

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Performance of traditional railway structure depends significantly on the behaviour of its support layers, particularly the ballast. This layer's rock particles are selected to ensure high mechanical strength, but traffic and mechanical maintenance break and wear the particles. Consequently, the layer incurs permanent deformations that degrade its strength and increase deformability and permeability. Particle physical characteristics, in particular those related to size and shape, influence their fragmentation and wear and must be studied accordingly. In addition, structural numerical models that represent individual particles, such as the discrete element method, have been increasingly used to model the infrastructure and therefore detailed geometrical characterization in the form of 3D digital models of the particles are necessary. This work contributes to this goal by investigating a contact-based cost-effective method that digitizes particle form and allows the determination of their geometric parameters. This method is described, compared with well-established laser scanning technique and then applied to study degradation of particles in Los Angeles and microDeval fragmentation tests.