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Slab track or ballast Alan Cudlipp

Among the 3M civil engineering documents in Network Rail’s National Records Centre is a drawing of early trackforms, signed off by George Stephenson in 1839. It shows rails fixed either to narrow, buried sleepers or independent stone blocks, two feet square.

Like everything else, track moves on. Today we have two basic trackforms: rails fixed to sleepers supported by crushed stone – the ballast option – and rails sitting on resilient pads supported by a concrete base known as slab track. In a high speed context, what tips the balance between ballasted and slab track?

Train operators and infrastructure owners both have performance outputs to meet. Their ability to do so relies, in part, on the appropriate trackform being specified as this has significant implications for the design, construction, operation, maintenance and performance of the railway system.

Conventional and high speed railways present varying challenges but with higher speeds and increased traffic levels, the track system is subjected to greater and more frequent forces which hasten its deterioration. If left unchecked, whole system performance will be adversely impacted. This can manifest itself at the wheel/rail interface, leading to excessive maintenance needs for both track and train, as well as resulting in poor ride quality.

Influencing the selection of ballasted or slab track are key indicators such as technical performance, safety, environment, programme, cost and longevity. Experience across Europe and Asia demonstrates no clear consistency of choice for high speed routes. Slab track is however the preferred trackform in Germany for lines of 250kph or more; at those speeds, China, now with a high speed network of more than 19,000km, also mandates slab track for its passenger lines.

In the UK, High Speed 1 offers valuable insight in terms of construction and ongoing maintenance into the pros and cons of each trackbed system. Running for 109km between the Channel Tunnel and London St Pancras, the route passes over and through highly variable geology, from marshland silt to London clay and chalk. The combined length of the tunnels and viaducts amounts to about 29km.

Changing sub-base conditions drove a diverse choice of trackbed. Ballast with concrete sleepers was generally specified for open sections of route, and across viaducts and underbridges. However, slab track was used through the London tunnels while the highly unstable East Thames marshes were traversed using piled slab track, likened to a ‘viaduct in the ground’. North Downs Tunnel in Kent has ballast on a concrete sub-base.

Pros and cons: ballast

Historically, ballast has become the established way of supporting track in the UK and that position will be maintained for mixed-traffic routes; there is longstanding knowledge and experience of it. Slab track is considered unproven in the long term, so some industry reticence exists when it comes to embracing it.

Ballasted trackform offers lower capital costs and quicker construction than slab track. It has an inherent resilience to ground conditions and is much easier to correct in the event of settlement. Alignment management can be achieved through tamping and track quality improvements by ballast cleaning.

An impressive array of high-output machinery is available to renew ballasted track – either as a complete trackform or just the ballast – minimising cost, maximising limited possession opportunities and offering linespeed handbacks. But the required inspection and maintenance regime can be intensive; this presents difficulties in pushing towards a ‘24-hour railway’, particularly when faced with high tonnages and the resulting effects on track quality.

Track geometry can become misaligned, both horizontally and vertically, through settlement, poor drainage and the normal passage of trains. This is a key factor for consideration in the context of high speed systems where the need for geometrical compliance is more acute. Achieving this on a heavily-trafficked high speed route would involve the on-track machine fleet – tampers, dynamic track stabilisers, ballast regulators and the like – being out working on a very regular basis.

However, frequent tamping and regulating of ballast will create ballast attrition and the subsequent degradation has to be addressed through ballast renewals; these have a cycle of less than 20 years. Although rails and sleepers can last for up to 40 years, it is questionable whether this regime could be sustained in the long term.

Pros and cons: slab track

While it may lack the pedigree of ballast, slab track offers high levels of passenger comfort thanks to the exacting design and construction tolerances. This also contributes to a lower overall maintenance requirement which makes the ‘24-hour railway’ aspiration easier to fulfil.

Construction takes longer than with ballasted trackforms and the capital costs are higher, a function of additional groundworks, extensive concrete precasting and noise mitigation. However this is offset by the reduced size of both the maintenance plant fleet and on-track workforce, as well as fewer track renewals, possessions and subsequent speed restrictions. It is as close to ‘fit and forget’ as possible on a railway. And, by virtue of reduced construction depth, track slab is a recognised solution for improving structure gauge through tunnels.

Slab track systems optimise track stiffness and limit substructure deformation. If slab settlement or heave occurs – or renewal is required – the solutions are generally neither quick nor simple, but adjustable fastenings allowing for restoration of the rail’s correct vertical alignment have been developed in Germany and are in use on the country’s high speed network.
The permanent way forward

With higher tonnages, a trend may be emerging towards slab track, which offers the ability to engineer resilience and stiffness to suit specific needs, whether they are geological or operational. The predicted tonnages for HS2 represent a step change from anything that has gone before. But ballast and slab both have advantages and shortcomings. The skill lies in selecting the right solution for each location.

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