01.04.13
TRACK21: Railway track for the 21st century
Source: Rail Technology Magazine March/April 2013
Professor William Powrie FREng, principal investigator of TRACK21 at the University of Southampton, describes the latest track research and its implications for the industry.
TRACK21 is a collaborative research programme grant funded by the UK Engineering and Physical Sciences Research Council (EPSRC) that aims to improve how existing railway track is maintained and how new lines are designed and built.
The grant brings together researchers from the Universities of Southampton, Birmingham and Nottingham and industry partners including Network Rail, HS2, LUL, RSSB, Balfour Beatty, Tata and Pandrol.
The key research challenge addressed by TRACK21 is to develop improved understandings of the complex mechanisms of railway track behaviour governing stiffness, robustness, longevity, noise and vibration. The goal is to reduce deterioration rates and maintenance requirements substantially, while at the same time mitigating the environmental impacts of noise, vibration and materials use. These are perhaps the most significant challenges facing railway systems today. If successful, the research will lead to reduced costs and improved reliability, with resulting environmental and customer service benefits.
Work over the first 30 months of the grant has focused on developing a better understanding of the way track behaves in response to the increasing demands of heavier, faster and more frequent trains through a combination of field monitoring, laboratory testing and particle scale numerical modelling. A summary of progress in selected areas is given below.
Ballast migration
Figure 1 shows the phenomenon of ballast migration. This may occur on canted curved track traversed by high speed (~200 km/hour) trains, and involves the gradual migration of the ballast down the cant so that the high end of the sleeper is exposed and the ballast gathers in a heap against the low rail.
A mechanism has been proposed to explain this behaviour, and is described in full in a paper soon to be published in the Journal of Rail and Rapid Transit (Priest et al, 2012).
Transition zones
Transition zones, where the track passes from ordinary ground onto a rigid substructure such as a bridge or a culvert, are potentially problematic in terms of ongoing differential displacement and increased maintenance requirements. The strategies adopted to try to mitigate these effects are sometimes spectacularly unsuccessful (e.g. Coelho et al, 2011) and further research into more effective designs is required.
Within TRACK21, critical zones (including switches, transitions and underbridges) have been identified at various locations mainly on the southern region of the UK network, and their performance is being monitored both from the track and using on-train instrumentation developed at the University of Birmingham.
Earthworks in cyclic loading
Seasonal cyclic shrinkage and swelling of clay embankments resulting from vegetation and climate effects can make it difficult for rail infrastructure owners to maintain the required track geometry. Fatigue might also lead to the gradual failure of such embankments over several decades.
These issues are being investigated in cyclic triaxial tests in which 70mm diameter specimens of Lias Clay embankment fill from a site near Bristol are being subjected to cyclic variations in pore water pressure of 100kPa with the total stress held constant. Further tests are being carried out in which the total stresses are being cycled at a much higher frequency, mimicking train passage.
In both cases, the laboratory tests are being complemented by field and full scale studies. Additional collaborators include Mott MacDonald, GeoObservations and Arup.
Effects of principal stress rotation on different types of track sub-grade
A torsional hollow cylinder apparatus has been set up and a testing procedure developed, following Powrie et al (2007), to investigate the effect of the principal stress rotation associated with train passage on a variety of railway foundation soil types. Particular emphasis is being placed on the effect of clay content and the time interval between loading events.
Current data suggest a reduction in the susceptibility of a sub-base material with increasing clay content, up to a clay content of about 16%. Above this, the reduced permeability starts to have an adverse effect.
The effectiveness of ballast and sleeper modifications
The replacement of traditional timber sleepers by reinforced concrete has resulted in a much harder interface, with smaller and possibly fewer sleeper to ballast contacts and the increased likelihood of ballast particle breakage rather than embedment into the softer sleeper material.
The University of Southampton sleeper testing rig (figure 2; Le Pen and Powrie, 2011) has been upgraded to enable the long-term performance of different combinations of sleeper material (timber, plastic, steel) and shape (traditional, duo-bloc, inverted U) to be investigated; and also the effectiveness of under sleeper pads in reducing sleeper/ballast contact forces.
Rig tests typically involve up to three million load cycles, representing a cumulative load of 60 Megatonnes and perhaps between a year and a decade’s real use with no maintenance interventions.
Lateral as well as vertical loads can be applied, simulating the effects of curving at a cant deficiency and/or sidewind loading.
Settlement data from a typical test comprising three million loading cycles are shown in figure 3.
Pressure-sensitive paper placed between the underside of a standard reinforced concrete sleeper and the underlying ballast gives an interesting demonstration that the sleeper is in fact supported on a relatively small number of quite mobile particle contacts (figure 4).
The influence of contamination with finer particles such as sand is also being investigated. Complementary tests are in progress in the Nottingham sleeper testing rig (Brown et al, 2007).
Noise and vibration
In addition to the load-deformation response, the effect of any track system modifications on noise and vibration must also be considered.
The dynamic stiffness of the ballast bed at higher frequencies influences both the transmission of vibration into the ground or supporting structure and acoustic radiation from the track.
A new test rig is being used to measure dynamic track stiffnesses at frequencies between 50 and 1000Hz under a range of preloads.
Development of structure in railway ballast
The development of structure in railway ballast is being investigated by the use of computed tomography (CT scanning) to assess the arrangement and orientation of particles in samples of trafficked ballast recovered from below sleepers during track renewal operations (figure 5). Figure 6 shows a typical CT scan.
Initial analysis suggests that the development of structure in ballast is associated primarily with densification and an increase in the number of particle to particle contacts (coordination number), rather than gross particle reorientations.
Particle scale discrete element modelling
Further insights are being gained through numerical discrete element analyses at the particle scale, using particles representative of real ballast generated by means of the Potential Particle approach (Harkness, 2009).
A typical numerical model of a triaxial test specimen made up of ballast particles is shown in figure 7.
The results of TRACK21 will be used to inform further work on the suitability and development of ballasted track for very high speed trains (300- 400 km/hour).
This will include consideration of track geometry, ballast flight and mitigation of critical velocity effects.
The work described in this article has been supported by the UK Engineering and Physical Sciences Research Council (EPSRC) through the Programme Grant TRACK21, the Rail Safety and Standards Board (RSSB), and Network Rail through the Future Infrastructure Systems Collaborative Research Centre at Southampton.
References
Brown, S.F., Brodrick, B.V., Thom, N.H. and Mc- Dowell, G.R. (2007) The Nottingham Railway Test Facility. Proceedings of the Institution of Civil Engineers:Transport 160(TR2), 59-65.
Coelho, B., Hölscher, P., Priest, J. A., Powrie, W. and Barends, F. (2011) An assessment of transition zone performance. Proc. I.Mech.E. J. Rail and Rapid Transit. 225(2), 129-139.
Harkness, J. (2009). Potential particles for the modelling of interlocking media in three dimensions. International Journal For Numerical Methods In Engineering 80, 1573-1594.
Le Pen, L. and Powrie, W. (2011) Contribution of base, crib and shoulder ballast to the lateral sliding resistance of railway track: a geotechnical perspective. Proc. I.Mech.E. J. Rail and Rapid Transit. 225(2), 113-128.
Priest, J.A., Powrie, W., Le Pen, L., Mak, P., Burstow, M. C. (2012) The effect of enhanced curving forces on the behaviour of canted ballasted track. Proc. I.Mech.E. J. Rail and Rapid Transit. In press.
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