André Paixão

Design and construction of backfills for railway track 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.

Under sleeper pads in transition zones at railway underpasses: numerical modelling and experimental validation

In railway lines, transition zones between different track support conditions normally evidence higher degradation rates, thus requiring additional maintenance to ensure safety and service quality. Studies based on numerical simulations indicate that under sleeper pads (USP) can minimise those degradation rates. The study presented herein focuses on the influence of USP on the dynamic behaviour of transitions to underpasses, in an attempt to fill the gap between numerical and field studies. To that aim, the authors used finite element method models, calibrated and validated with field measurements. These models take into account the train–track interaction and include all relevant track components and backfill geomaterials. This study shows that soft USP have a significant influence over the tracks dynamic behaviour: amplifying rail displacements and sleeper accelerations, and inducing abrupt variations in the track vertical stiffness and oscillations in train–track forces. To benefit from the use of USP, the authors highlight the need to carefully design stiffness properties of USP and determine their arrangement at transitions. An improved design for the transition zone is proposed.

On the use of under sleeper pads in transition zones at railway underpasses: experimental field testing

The railway track at transition zones to bridges or to other structures usually requires considerably more interventions to maintain the same open track service quality and safety levels. Under sleeper pads (USP) have been reported to contribute to reduce degradation and frequency of maintenance operations in open ballasted tracks, and recent numerical studies have suggested its use in transition zones. In this study, the influence of using USP at transition zones to underpasses was investigated. The structural behaviour of two transitions to similar box culverts was compared. USP were added to the sleepers in one of the transitions. Track characterisation using non-destructive tests is presented and analysed. Measurements of track response from passing trains are also presented and discussed. Results indicate that USP influence the dynamic behaviour of the track, increasing its vertical flexibility and amplifying both rail displacements and sleeper accelerations.

A numerical study on the influence of backfill settlements in the train/track interaction at transition zones to railway bridges

Railway track degrades more quickly in the transition zones between embankments and bridges, mostly due to the development of settlements in the backfill. This amplifies the dynamic train/track interaction forces and accelerates the deterioration of the components of both the track and the vehicles. Consequently, it induces additional maintenance costs, reduces passenger comfort and, ultimately, it may affect train safety at transition zones. To address this problem and to obtain more insight into the influence of the settlement profiles of the backfill in the train/track system, nonlinear dynamic analyses were performed using a transition zone model to simulate different settlement profiles of the backfill. In some cases, the results showed significant amplifications of axle accelerations and loads. In general, the greater the number of hanging sleepers and their gap sizes, the higher the vertical accelerations of the sleepers and the larger the vertical contact forces between the sleepers and the ballast layer. Settlement profiles associated with backfills with a wedge shape seem to limit these negative aspects.

An integrated monitoring system for continuous evaluation of railway tracks for efficient asset management

The maintenance of railway tracks is one of the aspects of the railway infrastructure management activity that most influences the technical and economic performance of this transportation system. It is common practice to support the maintenance actions based on the monitoring of the infrastructure condition, for example by carrying out visual inspections and using dedicated track inspection vehicles. Despite the fact that this traditional approach to evaluate the track, based on its condition, is quite effective, it does not provide useful information on the causes that lead to such reduction in performance. Consequently, the maintenance tends to be more corrective and less preventive. Considering the limitations of the available methods, a R&D project is underway in Portugal aiming at designing, developing and demonstrating the applicability of an innovative and integrated approach to assess the performance of railway tracks, which is also expected to contribute to identifying the causes of some track malfunctions. In this paper we discuss the relevant aspects of this approach and present preliminary results of the tasks currently underway, namely the design and development of the prototype system, including the verification of the measurement chain and its validation in laboratory environment.