Why are Britain’s roads melting and its rails warping in the heat?

The extreme temperatures have led to widespread problems and disruption on Britain’s railways, with trains running at slow speeds and main lines closed. Airport runways and some roads have also been shown to be heat sensitive.

Railways

Steel rails expand and tend to warp under the effect of heat, whatever the climate. According to Network Rail, railways around the world are designed to operate in a range of 45C, depending on local conditions. In the UK, steel rails are ‘prestressed’ at summer temperatures of 27°C, while in countries with warmer climates, rails are prestressed at higher temperatures.

Sleepers and ballast must hold the tracks in place during the UK winter and summer. When the temperature reaches 40 C, the rails can reach 60 C and expand and deform. A train traveling fast on rails can speed up this process due to the heat caused by friction, and could be more at risk if it buckles – hence the widespread speed restrictions.

Overhead cables on electrified roads also expand and sag in heat and contract in cold weather. Engineers have solutions, the tension being automatically relieved by a pulley system. But eventually the counterweights hit the ground and the wires sag, making them more likely to get tangled in a pantograph, the device atop the train that draws power from the lines.

Roads

Highways and strategic roads are built with modified asphalt surfaces which – so far – should not start to melt, being resistant beyond 60°C, or an air temperature equivalent to 40° C, according to national highways. However, the base asphalt materials used on local roads – the vast majority – can start to soften at temperatures of 50°C. At this point, Professor Xiangming Zhou, head of civil and environmental engineering at Brunel University, said, “The road may become soft and greasy, and it is difficult for cars to brake.” This is why the municipalities have put on standby sanding trucks, more generally used in icy weather, to cover the roads with sand and dust. Tarmac and asphalt are cheaper and less abrasive to tires than some materials, he says, but because they’re black, they tend to heat up faster in the hot sun.

About 4% of UK roads are made of concrete, which is more popular overseas for highways and motorways and can be stronger, but is not immune to problems with extreme temperatures, as shown in the closure of the A14. The dual carriageway near Cambridge had been built with asphalt over old concrete slabs which expanded and warped in the heat, creating enough of a bump to close the road overnight for emergency repairs.

Rick Green of the Asphalt Industry Alliance says a road that can withstand all temperatures is “a significant challenge for design engineers”. At extremely high temperatures, the surface “does not melt, but the bitumen it contains may soften”, “increasing the risk of deformation”.


airport runways

Again, some may be concrete – but Luton’s asphalt was the problem once temperatures soared into the mid-30s, Zhou says. In the words of the airport, “high surface temperatures caused a small section to heave” – ​​a loop in the runway that engineers repaired within hours, but which still caused major disruption to The passengers. While local roads are often shaded by trees and houses, the runways are fully exposed and subject to additional heat stress from aircraft landing and taking off. Repairs and maintenance are frequent.

Heathrow, which was even hotter than Luton on Monday, also had a runway problem last week when overnight repair work was not completed in time for planes to land. However, it has two runways and has not been forced to stop its operations.

So what is the solution?

Network Rail already spends hundreds of millions of pounds a year on climate change mitigation. However, most of them serve to counter erosion or damage caused by rains or storms. Future infrastructure might be rated for a warmer climate, but then it might be more prone to failure and cracking in cold winter weather when the rails contract. Some track materials, such as concrete sleepers, are more resistant to wider temperature ranges and conditions – and significantly more expensive.

The rails are already painted white in critical places to combat the heat. Countries with extreme weather conditions make much larger seasonal adjustments to track, which is time consuming and expensive. Air conditioning was not a standard feature on older trains still in circulation. Resilience will become an economic and political choice – and a few days of heating outages each year may be considered better than the change bill.

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