Field trials are expected to start near Marseille in the coming months of a lower-cost modular slab trackform developed by Systra and construction group Stradal as a potential replacement for conventional ballasted track. Technical consultant Jean-Claude Zabée explains the concept to Nick Kingsley.
What will the next generation of main line track look like? Over the past five decades, ballastless trackforms have emerged as a safe and effective alternative to time-honoured ballast and sleepers, especially in tunnels and urban environments where noise reduction and vibration management are key requirements.
However, there has been little appetite to date for replacing life-expired ballasted track with slab as part of routine renewals, and there seems to be a consensus among some track design specialists that the ballastless track designs currently offered to the market are still too complex and costly for mass adoption. This assumption is about to be challenged, with a new track concept to be trialled on a short section of freight-only railway within the port of Marseille. Consultancy Systra and construction group Stradal expect to deploy their novel slab track design for initial real-world trials in the first quarter of this year.
Formally announced at InnoTrans last September, Systra Slab Track is being offered as a highly modular trackform which is expected to be competitive with traditional ballasted approaches on a whole-life cost basis.
The two companies developed the design after assessing the pros and cons of each track type being deployed commercially today (TableI). ‘We looked at the market and we saw a gap for a lower-cost opportunity which is more flexible and more resilient than current slab track options’, explains Jean-Claude Zabée, a former Deputy Director at Systra who is now a Principal Consultant for the joint venture with CRH subsidiary Stradal. ‘Our objective was simply to see if we could come up with a ballastless trackform that could come close to matching the cost of ballast in terms of capex and opex.’
A further clear aim was to develop a trackform that could be used as a direct replacement for ballasted track during renewals, and which would confer significant advantages in terms of performance and cost of maintenance.
Zabée believes that many railways have stuck solidly with ballast for ‘cultural reasons’, but that there could be opportunities for different approaches as production methods evolve. ‘The point with our slab design is that it will be completely manufactured off-site, and it will be installed extremely rapidly along the existing formation of an existing railway’, he explains. ‘We are talking about renewals being done in days instead of several weeks.’
How it works
The trackform is designed around the use of modular, pre-assembled reinforced concrete slabs which can be transported to the worksite in sections around 4·5 m long (Fig1). Rather than resting on the subgrade itself, each section is supported at each end on a precast base element. Also made of concrete, these are typically 1 300 mm long and 2 500 mm wide, assuming a 1 435 mm track gauge (Fig2). Polyurethane pads are placed between the slab and the base to absorb any vibration and other forces, while steel shims are added to maintain the track geometry (Fig3).
Between these supports, the slab itself is positioned around 150 mm above the subgrade; this gap is intended to allow sand, water and other materials to flow away from the track, contributing to a low-maintenance approach. The slabs are held in position both laterally and longitudinally by polyurethane stops bearing on an upstand in the supporting base, adding further resilience to the trackform.
Twin block supports are embedded in each section of slab to carry the rails, using conventional baseplates and rail fastenings; the design is intended to be compatible with standard track components.
The track geometry is specified at the outset to ensure each slab is produced for the precise position where it is to be installed. To this end, Zabée explains that each location has to be ‘geometrically defined’ before tracklaying can begin. The standard slab is sized to accommodate a minimum curve radius of 150 m and maximum cant of 180 mm. While the top of rail is nominally 700 mm above the subgrade level, the steel shims permit up to 20 mm of lateral and 100 mm of vertical adjustment of the slab in relation to the supporting base elements, as required at a given location (Fig4). This is in addition to any more localised adjustment within the individual rail fastenings.
Fig5 shows the transition between the precast slab structure and a ballasted trackform. A ‘half size’ base would carry the end of the final slab and also act as a form of retaining wall to support the ballast shoulder. This would be supplemented by sheet piling between the two trackforms, and by guide rails and stiffeners running along the length of the transition zone. Gabions placed alongside the formation would be used to further support the track geometry through the area.
Conversion from ballast
A number of factors could influence the ease of converting existing ballasted track to the Systra slab technology, but Zabée is confident that the concept would be adaptable enough to suit most applications. In all cases, he says, the conversion would start with a detailed geotechnical survey of the section of railway to be modified. This would inform a base set of calculations to determine the parameters of the slab trackform. Soil conditions would also be assessed for load bearing capacity, groundwater and frost depth, and soil type. Factors relating to the operation of the line would also then have to be added to the assessment, such as the presence of overhead electrification or whether the line is single or double track.
From there, Systra and Stradal say that a range of installation options would be available. These could include a ‘spread footing’ approach where the slabs are laid with or without horizontal anchorage using short drilled piles or spuds. Alternatively, soil reinforcement may be necessary before the slabs can be put in position. In some cases, piles may need to be driven or drilled into the ground before the new trackwork is installed.
Nevertheless, Systra believes that in most cases the installation would be straightforward enough to allow the existing ballast to be left in situ on the subgrade. This could help to protect the formation against frost, erosion and vegetation growth. Typically any residual ballast would be no more than 50 mm deep below the slab supports and at a maximum depth of 200 mm alongside the track.
One particular use case that the partners are targeting is railways in sandy or desert conditions. Wind-blown sand is often a significant challenge for network operators, and Systra and Stradal suggest that their trackform could aid the sand dispersal process, since it would be harder for the sand to lodge physically on the track. The wind would also blow sand through the small apertures beneath the slabs.
If this proves to be successful, it could reduce the cost of erecting lineside shields and barriers to protect against wind-blown sand in key areas, in turn offering the potential to cut the total cost of ownership for desert railways.
A further option being explored by the promoters is the replacement of the subgrade ballast by a layer of asphalt.
High performance design
According to Zabée, pre-installation tests have already shown that the Systra slab design could be capable of handling main line traffic up to a maximum axleload of 25 tonnes, with trains running at speeds of at least 200 km/h, and the potential for higher speeds in the longer term.
The initial tests in Marseille will be conducted on a section of freight-only route in the port area where the maximum speed is around 30 km/h, but the partners are already working on plans for a larger scale trial on a conventional line in France. This would include its use by passenger trains, Zabée confirms.
In terms of component life cycles, Systra estimates that the reinforced concrete elements could last for up to 50 years with an average loading of 43·8 million gross tonnes per annum and no less than 200 years on a more lightly used line carrying 10·2 mtpa. A lifespan of 30 years is envisaged for the polyurethane elements, subject to specific environmental factors such as UV light exposure and ambient temperature on site. In any case, Systra says that replacing these elements would be relatively straightforward and would not require wholesale dismantling of the track superstructure.
Sustainability credentials
A major consideration in the use of a concrete-intensive track design is the carbon embedded during the production process. Zabée insists that Systra’s studies so far show that the ballastless option has a ‘globally comparable’ carbon footprint to ballasted track, assuming the use of concrete sleepers. ‘Ballast can be a very carbon intensive material, especially because of the need to transport a lot of it’, he explains.
Zabée cites projects such as the Grand Paris Express automated metro network as being important in exploring how the carbon footprint of cement and concrete production for rail use can be reduced. He sees a raft of other societal benefits from a transition to slab track, notably a reduction in lineside dust and noise emissions. This would have significant benefits for maintenance teams and lineside residents alike, he believes.
Looking ahead, Systra and Stradal are working in partnership with Vossloh Cogifer to develop a variant of the precast slab concept to support switches and crossings.
