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High Speed | Which track bed is best?

Which of the two basic generic forms of trackbed – ballast or slab track – is the client for Britain’s next high speed railway line likely to opt for? 

“The question is a good one,” says a spokesperson for High Speed 2 (HS2) HS2 Ltd. A given is that from [the London terminus at] Euston to the north portal of the Chiltern tunnel, rails will be carried on slab track, as this form is particularly suited to tunnelled infrastructure.

“North of that, for more than 100km, we’ve not yet made up our minds. The evaluation process is ongoing but has to complete in time for the main civils work tender documents being issued later this spring.”

Expressed simply, ballasted track comprises rail fixed to sleepers supported by crushed stone. Slab track (of which there is a growing number of variants) has rails on resilient pads supported by a  concrete base. Both have their proponents. Both can be used in succession (see box), although transitions from one form to the other have to be engineered with care.

“Ballast is the traditional way of supporting railway track and the industry is perhaps reticent to embrace slab track solutions that are non-proven in the long term,” comments Mott MacDonald technical director and professional head of track engineering Alan Cudlipp. 

tgv ballast track

TGV ballast track

France’s new Tours-Bordeaux LGV features ballasted trackbed

“There is a long standing knowledge of ballasted track.”

The benefits of opting for ballast include generally lower construction costs, with the ballasted track form being quicker to construct than slab track, he says. Ballast displays an inherent resilience to ground conditions, with maintenance via tamping to manage alignment plus ballast cleaning (a process to recycle ballast and restore coherence) carried out relatively quickly. If settlement occurs, it is much easier to correct than the alternative form of trackbed. 

“Ballast is not perfect,” Cudlipp says. “It requires higher levels of inspection and maintenance which is a problem for clients pushing for a ‘24 hour railway’. The track geometry can become misaligned, vertically and horizontally, through settlement and poor drainage as well as the normal passage of traffic. This has to be considered particularly when developing high speed systems, where the need for geometrical compliance is more acute.”

Slab track may lack the pedigree of ballast but it still displays an impressive array of advantages

Slab track may lack the pedigree of ballast but it still displays an impressive array of advantages. Such systems are designed and built to exacting tolerances to provide high levels of passenger comfort. They are not generally subject to misalignment and also require lower levels of maintenance. As Cudlipp points out, “The 24 hour railway concept is easier to achieve with slab track.” Construction takes longer than ballasted track and initial costs are higher, offset by life time costs that are generally lower. “And it embraces the ultimate aim of a ‘fit and forget’ philosophy,” he says.

 Aecom director for transport asset management Matthew Brough points out that, as slab track systems comprise “engineered” layers, this has benefits in terms of optimising track stiffness and in limiting substructure deformation. On the negative side, if slab settlement does occur, or renewal is required, there are no simple solutions. 

“Slab tracks can be viewed as potentially providing very real problems in the longer term, and there are specific issues to contend with at ballasted track transitions,” he says. “Where a slab fails, interventions can be a lot more significant. There are high cost and time penalties for remediating or renewing slabs within the restrictions of an operational railway, as well as increased maintenance where transitions occur. Slab tracks are designed and built to stay put, but if they don’t, they are less easy to restore.

Where slab tracks do stay put, they require fewer interventions and so present less risk to operatives. 

“These are the real benefits of slab tracks,” says Brough. “If the asset owner wants zero maintenance, does not want workers out on the line, seeks maximum safety, ok, then go for slab track.” 

There are various slab track forms: the Swiss Sonneville LVT trackbed used successfully in the Channel Tunnel springs to mind, as does the embedded rail slab system developed by the UK’s Balfour Beatty. 

Developments are constantly underway. Spanish construction group Acciona is working on a modular, precast concrete based slab track which is particularly rapid to install. 

“The components are of reduced dimensions and to very high tolerances,” says the company’s internal innovation and technology manager Javier Bonilla. “The system is flexible and cost efficient; and is currently nearing homologation.” 

Innovation does not stop with slab track. According to Colas Rail’s track alliance director Said Lahssioui, the company has developed a ballast system which incorporates a 100mm thick asphalt underlayer designed to reduce forces passing into the trackbed’s subformation. 

“The object is to minimise interventions and the cost of maintenance,” says Lahssioui. “The system is currently being used in France for the Contournement Nîmes-Montpellier, a 60km long high speed railway line being built to bypass the two towns.”

Longevity is an important consideration of what track form to opt for. The life of ballast is 25 to 30 years, with sleepers and rails lasting about twice that. The claim for slab track systems is that although they may be more expensive from a capital expenditure perspective, they will last 60 years-plus. 

High Speed 1

High Speed 1, formerly known as the Channel Tunnel Rail Link (CTRL), passes over and through highly variable geology from marshland silt to London Clay and chalk, on its 109km way from St Pancras to the Channel Tunnel. 

Combined length of the line’s tunnels and viaducts amounts to about 29km. 

The varying sub base conditions meant – according to former CTRL director of public communications Bernard Gambrill –  the choice of trackbed was similarly diverse.

“It was horses for courses,” Gambrill says.

The London tunnels were ballastless track (slab track) out of St Pancras; piled slab track (“like a viaduct in the ground”) was used to traverse the highly unstable East Thames marshes; with concrete and ballastbeing the solution for tunnel beneath the North Downs in Kent.

Underbridges, viaducts and open line sections were ballasted with concrete sleepers throughout. 

“The sub-base transitions caused great pause for thought,” says Gambrill.

 

 

 

 

 

Readers' comments (1)

  • Interestingly, there has been some mention in the national newspapers of research, carried out by Prof Peter Woodward, which concludes there may be some problems for ballasted track on embankments, with trains running at 360kph. This research was carried out at the behest of the HS2 Design Panel. I must admit I was somewhat surprised to find the proposed design speed was this high, considering that the highest speeds used abroad are at, or below, 320kmh.

    It will be interesting to see which way HS2 decides to go, taking into account cost and noise implications.

    Unsuitable or offensive? Report this comment

  • Well spotted. We've got a response from High Speed 2 on that story (in the Sunday Telegraph). It assures us that it is "fully aware of all technical aspects of travelling at high speed" and that there are there are "definitely no safety issues associated with the design of HS2". To suggest otherwise is wrong, it says. HS2 technical director professor Andrew McNaughton said: "We have a world class team of engineers including some of the most prominent in their field working on HS2. We also have shared experience from high speed train services around the world built up over decades. HS2 is being designed and developed with safety as the key priority. Mitigation measures designed to cope with all phenomena occurring when trains travel at these speeds have been costed into the project."

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