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High Speed | Riding on asphalt cushion

The 60km long Nîmes-Montpellier bypass is a first for France and a technical trailblazer

The 60km long Nîmes-Montpellier bypass is a first for France. It’s a high speed railway designed to carry passenger and freight trains.

Now well into construction, by 2017 the project will be busy enhancing the attractiveness of high speed passenger traffic towards southern France and Spain, aiding trans-Continental freight services and, crucially, boosting regional rail services by freeing up capacity on the existing network.

It is also a technical trailblazer – it is understood to be the first high speed rail line to feature an asphaltic layer between the formation level and the ballast.

Nimes-Montpelier

Nimes-Montpelier

Rolling the asphalt layer

Why? It is private finance initiative thinking, through and through – the upfront cost of laying the asphalt is more than outweighed by whole-life maintenance savings.

“Normally you would lay the ballast straight on the formation layer,” says Colas Rail director Said Lahssioui. ”So why are we laying 100km of asphalt at a 9m to 10m width? It wasn’t a contractual requirement.” 

The benefits of the asphaltic are in the improved track bed stability. Forces transmitted through to the formation layer are reduced, reducing differential settlement. This produces a reduction in tamping and, importantly, a reduction in the wear on rails and the catenaries caused by undulations in the ride. This in turn reduces the need for rail grinding – and ultimately rail replacement – and minimises the risk of damage to the catenary structures.

We’re doing it because Oc’via worked out that it is cheaper over 25 years

Said Lahssioui, Colas Rail

“We’re doing it because Oc’via worked out that it is cheaper over 25 years,” Lahssioui answers. And that’s important: “If you do it for yourself, then you have to realise it must be a benefit,” he explains.

Behind the thinking is the the Oc’via consortium, delivering the project as a 25 year public-private partnership. Oc’via’s shareholders are Bouygues, Bouygues subsidiary Colas Rail, Spie Batignolles and Alstom.

It is not a hugely long rail line, and as such it is not demanding a huge investment: €2.28bn (£1.76bn) overall, with £1.15bn of the works being delivered under the public private partnership (PPP) contract (the two stations are being financed and built separately). 

Used on other lines

But it is the only one of the three high speed lines currently under construction in France to be using the asphalt technique. That it happens to be the high speed line featuring Colas Rail should come as no surprise. Its sister highways company is a leader in its sector and a great innovator in asphalt.

Colas Rail meanwhile has more experience than most in high speed rail track laying, having worked on France’s – and the world’s – first high speed line in 1981.

It has now laid the track and installed railway systems on over 40% of all high speed lines in Europe and Africa, and is constantly looking for ways to innovate. And Nîmes-Montpelier, as a PPP, is offering that opportunity.

“PPPs allow us to use the technology we want,” explains Colas Rail director Said Lahssioui. “Nîmes-Montpelier is a good example.”

It is still not as maintenance-free as slab-track, but the system retains the pure speed of construction offered by ballasted rail systems.

So slab track or ballast? As Lahssioui explains, Colas Rail is open to either: “I understand why a client would want the [relatively] maintenance free option of slab-track.” ”And in tunnels it’s the only option,” he adds, noting that the depth of ballast needed drives up tunnel diameters – and hence cost.

“But replacing slab-track after its 25 year life is expensive,” he adds.

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