Why are Britain’s roads melting and its rails buckling in the heat?  |  infrastructure

Why are Britain’s roads melting and its rails buckling in the heat? | infrastructure

Extreme temperatures have caused widespread problems and disruption on UK railways as trains run at slow speeds and main lines are closed. Airport runways and some roads have also shown that they can be vulnerable to heat.

railways

Steel rails expand when it’s hot and tend to buckle – no matter what the climate. According to Network Rail, the world’s railways are designed to operate within a 45°C range, depending on local conditions. In the UK, steel rails are ‘tempered’ to summer temperatures of 27°C, while rails in countries with hotter climates are tempered to higher temperatures.

Sleepers and ballast must hold rails in place throughout the UK winter and summer. When the temperature reaches 40°C, splints can reach 60°C and expand and bend. A train moving fast over rails can accelerate this process from frictional heat and could be more vulnerable if it buckled – hence the widespread speed restrictions.

The overhead lines on electrified routes also expand and sag when it is hot and contract when it is cold. Engineers have solutions where the tension is automatically relieved by a pulley system. But eventually the counterweights hit the ground and the cables sagged — making it more likely they became tangled in a pantograph, the device on top of the train that draws power from the lines.

streets

Highways and strategic roads are being built with modified asphalt surfaces that — so far — should not begin to melt and are resilient above 60°C, or an equivalent air temperature of 40°C, according to National Highways. However, basic asphalt materials used on local roads – the vast majority – can begin to soften at temperatures of 50°C. At this point, Prof. Xiangming Zhou, head of the Department of Civil and Environmental Engineering at Brunel University, says, “The road can get soft and slippery, and cars can have a hard time braking.” in freezing weather, on standby to cover roads with sand and dust. Tarmac and tarmac are cheaper and less abrasive on tires than some materials, he says, but because they’re black they tend to heat up faster in the scorching sun.

Around 4% of Britain’s roads are built from concrete, which is more popular abroad for motorways and motorways and can be more resilient, but is not immune to extreme temperature problems, as demonstrated by the closure of the A14. The dual carriageway near Cambridge had been built with tarmac over old concrete slabs, which expanded and buckled in the heat, creating a bump enough to cause the road to be closed overnight for emergency repairs.

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


Airport runways

Again, some may be concrete — but Luton’s asphalt was the problem when temperatures soared into the mid-30s, Zhou says. In the airport’s words, “high surface temperatures caused a small section to rise” – a crease in the runway that engineers repaired in hours, but which still caused major disruption to passengers. While local roads are often shaded by trees and houses, runways are fully exposed and subject to further 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 the planes to land. However, it has two runways and has not had to cease operations.

so what is the solution?

Network Rail already spends hundreds of millions of pounds a year on climate action. However, most of this is designed to counteract erosion or damage from precipitation or storms. Future infrastructure could be designed for a warmer climate – but then it could be more prone to failure and cracking in cold winter weather when rails shrink. Some track materials, like concrete sleepers, are more resilient over a wider range of temperatures and conditions – and significantly more expensive.

Rails are already painted white at critical points to counteract heat. Countries with extreme weather do much more extensive seasonal adjustments to track them, which is time-consuming and costly. Air conditioning was not a standard feature on older trains that still ran. Resilience is becoming an economic and political choice – and it may be that a few days of heating outage per year is preferable to the bill for changes.

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