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Tracking support

RAIL: Slab track trial - A British developed concrete slab track system is on trial in Spain. Steve Turner went to find out more.

Arguments for and against slab track have raged for years, with industry experts locked on the question of whether sleepers and ballast should be replaced by continuous concrete slab.

Supporters say that initial higher capital costs are soon recouped by reduced maintenance outlay, increased performance and reduced train delays.

The cost argument is disputed by sleeper supporters, who also say that tracks need to be flexible.

Now, however, slab track technology is taking a giant step forward with an embedded slab track system invented by Balfour Beatty director of engineering and development Charles Penney which, he believes, provides a simple, low maintenance and virtually flat rail bed, suitable for high speed, heavy haul and mixed traffic railways, And so convinced is Spanish Railways (Gestor de Infraestructuras Ferroviarias) of the potential that last year it ordered a section of embedded slab track for the test track it is building to trial a range of track and vehicle alternatives as part of its huge programme of high speed line building.

With detailed development taking place over the past 24 months, the system uses a new symmetrical 74kg/m rail section, continuously supported in an elastomeric pad and fibre reinforced plastic shell. This is then grouted into a low profile concrete slab, with seals to protect against water ingress. The pad type can be varied depending on the client's wish for reduced noise or vibration.

Special track systems manager at Balfour Beatty Rail Projects Vernon Turnbull believes 'the system offers increased safety and performance, increased axle load capacity and better ride quality'.

The risk of derailment due to track failure is reduced by virtue of the embedded rail and a derailment guard. And in the event of a wheel climb scenario, derailment is contained and damage limited.

Rail breaks are said to be reduced as the rail is effectively supported along its length. And being contained with the slab, any rail breaks are more easily managed, Turnbull explains, with the option of imposing a speed restriction until maintenance is convenient.

All round support also increases rail head stability, ensuring that the contact patch, where the rail is in contact with the wheel, remains in a more constant position, preventing it shifting toward the gauge corner.

Lateral buckling is also ruled out and there is reduced propensity for corrugation.

The current track design track allows for 30t axle loads travelling at 120km/h and 22t at 330km/h.

The construction system that Balfour Beatty has devised is also ingenious.

The reinforced slab can be cast insitu, slipformed or precast, with slots left out for the rail. This can be supplied in long lengths, or shorter more manageable lengths which are welded on site. The rail, along with the pad and shell, is then picked up by an alignment frame, designed at Balfour Beatty's Beeston test facility near Derby.

A series of screw adjustments on the frame allows exact positioning of the rail, which is held in place in the recess while it is grouted in and sealed.

As the system is so simple, project manager on the Spanish contract Mike Robertshaw explains, it can be installed to minimal tolerance by unskilled operatives, so reducing costs and the need for a skilled workforce.

The system also has environmental benefits. The slab has a trapezoidal invert which falls towards its centre, and gullies at regular intervals along the centreline allow controlled drainage of water. This contrasts with the seepage through ballast and formation which can be a problem with conventional track beds.

With only four components, once in place the system requires virtually no maintenance, Turnbull says. Risks to track inspectors and workers should be dramatically reduced, he adds.

Turnbull hopes the Spanish Railways trial will lead on to the first operational use of the system. Many of the lines are to be built through tunnels and one of the requirements is that the track bed shall be level to allow access to emergency vehicles.

He has nothing but admiration for the way the Spanish have have been willing to take a quantified risk on a new system.

'They know their engineering needs, have assessed the potential solutions and risks, decided that the best way of doing this is by trialling it, and with little delay have placed an order for trial' - an example that perhaps the UK could learn from.

Balfour Beatty presented the safety plan for the system to Railtrack last October and is awaiting comments. Meanwhile Balfour Beatty is pressing ahead with finalising its safety case.

Turnbull accepts that the initial costs are higher than ballasted track. He estimates about 40% more than a traditional sleeper and ballast system. But bearing in mind the system has a life expectancy of more than 30 years, using a standard cost analysis he predicts that after nine years it starts to become profitable.

Balfour Beatty called on a range of technical expertise from across the industry in developing the system.

Turnbull believes that in the current climate of public dissatisfaction with the railways, it is the responsibility of the industry to embrace new technology as a solution to the problems on the network. He might have a point.

Concrete converts

Spain is not the only country to be trialling concrete slabtrack alternatives to traditional ballast and sleepers. Following the example of Japan, which has been using the system on its high speed train network for more than 25 years, Germany's new high speed Cologne to Frankfurt line will use concrete slabtrack along its entire length, as will the Dutch high speed link between Amsterdam and the Belgian border.

Even the French, who opted for conventional trackbed on country's high speed rail network are looking at slabtrack. Engineering giant Alstrom has built a test track in La Rochelle to evaluate the automatic insertion of rail base plates into the freshly slipformed track bed, at the heart of its concrete slabtrack system, Appitrack (NCE 21st Century Concrete supplement March 2001)

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