01.08.15
Resilience of concrete rail support systems
Source: RTM Aug/Sep 15
Dr William Coombs and Dr Joseph Ghaffari Motlagh from the School of Engineering and Computing Sciences at Durham University are undertaking a two-year research project to understand the structural resilience of concrete rail support systems.
The UK’s rail infrastructure currently supports 1.3 billion passenger journeys and 100 million tonnes of freight each year. The freight transport alone contributes £870m to the UK economy. The sector has seen 40% and 60% increases in passengers and freight over the last 10 years, respectively, and it is expected that passenger numbers will double and freight increase by 140% over the next 30 years [1].
As the network has developed, the magnitude and frequency of loads applied to the rail support systems have increased significantly, even over the last 10 years. It is estimated that between 2009 and 2014, the UK’s rail infrastructure operator, Network Rail (NR), invested £3.75bn in track to maintain the network [2]. The vast majority of this 32,000km of rail infrastructure is supported by pre-stressed concrete sleepers (PCSs) and crossing bearers (CBs). These concrete members provide lateral restraint and vertical support to the running steel rails. The PCSs and CBs are in turn supported on three sides by track ballast; crushed stone 30-50mm in diameter.
Since their introduction in the 1950s, PCSs have superseded traditional wooden sleepers in new track and NR replaces approximately 200,000 timber sleepers each year with PCSs. However, despite the reliance of the UK’s rail network on these concrete structures, and the simplicity of their geometry, surprisingly their structural behaviour is poorly understood. NR’s CP5 delivery plan estimated that the used-life of the sleepers supporting the network for the 2013-14 period is 60.9% and that 5,918 track failures would occur during the same timeframe [3].
PCSs are attached to the running-rail (and other components, such as the electrified third rail) through cast-in-place inclusions. Two types commonly used are steel shoulders, designed to clamp the rail through a reaction spring, and threaded plastic inserts to allow in-situ bolting of components. The plastic inserts are soft relative to the surrounding concrete (with 10-40 times lower Young’s moduli) and create local stress concentrations exaggerated by high longitudinal pre-stressing. These concentrations have the potential to initiate fractures that can significantly reduce the support system’s designed service life.
In order to maintain a safe, reliable and resilient rail network, it is essential to understand how the concrete support systems can be designed to future-proof them against ever-increasing structural demands.
Research funded by the Engineering and Physical Sciences Research Council (EPSRC) being conducted at Durham University is concerned with 3D dynamic analysis to assess the structural robustness of PCSs and CBs using a novel computational mechanics framework and associated numerical analysis technique. If successful, this research will provide a transformative, complete and consistent framework for modelling fracture in a variety of engineering materials.
The approach is transformative because previous researchers have decoupled the continuum model controlling the failure of a material from the enveloping numerical framework controlling the initiation and propagation of fracture. However, a recent publication* has shown that it is possible to efficiently propagate fractures through brittle media by starting from a sound thermodynamic basis. This research will go further and apply the developed techniques in constitutive and fracture modelling to a scientifically neglected, yet absolutely critical, component of the rail network potentially transforming the way in which rail infrastructure is supported.
The vast majority of engineering numerical analyses use the mesh-based finite-element method (FEM). However, the FEM suffers from a number of drawbacks when trying to model fracture propagation. Its main limitation lies in the direct coupling of the material deformation with the mesh used to integrate the ‘problem domain’. A consequence of this is the computationally cumbersome task of re-meshing and transferring of internal variables associated with material deformation as the crack front advances [4].
The research at Durham is using an alternative mesh-free method: the material point method (MPM) [5]. The beauty of the MPM is that it decouples the material deformation from the background mesh. This allows the background mesh to be arbitrarily defined as a propagating fracture advances through the problem domain and facilitates simple mesh splitting.
The creativity of this proposal lies in the application of up-to-the-minute ideas in computational mechanics analysis techniques to a poorly understood yet critical component of the UK’s rail infrastructure.
The research team will provide a new fracture simulation approach. The research will also provide a framework within which to develop new resilient rail-support system fastening methods. The impact of this research can be split into a number of areas, namely: (i) safer railways and increased service performance, (ii) reduction of produced C02 and operational costs through increased service life and (iii) understanding of the impact of higher speed (such as HS2) and load traffic on the existing ageing infrastructure (that is, will the sleepers fail through fracture).
It is hoped that this EPSRC-funded research will impact on the manufacture of railway infrastructure in the UK and help develop robust and resilient PCS/CB-rail securement methods.
References
[1] Britain relies on rail, Network Rail.
[2] CP4 update 2012, Network Rail.
[3] CP5 delivery plan, Network Rail.
[4] L. Kaczmarczyk et al. (2014), Int. J. Numer. Meth. Eng., 97(7):531-550.
[5] D. Sulsky et al. (1994), Comp. Meth. Appl. Mech. Eng., 118:179-196.