A road in Surrey could soon send its summer heat 100 metres beneath the surface, store it underground and bring it back during winter, turning geothermal energy into a defense against potholes

A pothole is usually treated as a surface problem: broken asphalt, standing water, traffic and a repair crew waiting nearby. But much of the damage begins somewhere less visible, beneath the road. When water enters tiny cracks and the temperature repeatedly crosses the freezing point, the ground and pavement can expand, contract and weaken. Researchers at the University of Surrey are now asking whether controlling that hidden thermal cycle could stop some potholes before they ever appear.

The idea is called a thermo-active road. Instead of allowing asphalt to absorb and release heat passively, engineers place water-filled pipes within the pavement and the layers beneath it. Those pipes become a kind of underground thermal circuit. In summer, they remove heat from the road and move it into the ground. In winter, the stored heat can be brought back toward the surface, keeping the pavement warmer and reducing the conditions that encourage ice and freeze-thaw damage.

That matters because asphalt does not simply deteriorate because it gets old. Its mechanical behaviour changes with temperature and moisture. Hot pavement becomes softer and more susceptible to deformation under repeated traffic loads, while cold conditions can make it more brittle. Water adds another complication. Once it penetrates cracks or reaches vulnerable layers below the surface, freezing can generate stresses that gradually turn small defects into larger structural failures.

Why freezing water is so damaging to roads

The freeze-thaw problem is deceptively simple. Water occupies cracks, pores and gaps in pavement materials. When temperatures fall below freezing, some of that water turns into ice and expands. The resulting pressure can push against surrounding material, while repeated freezing and thawing weakens the bonds between particles and enlarges existing defects. Traffic then acts on an already compromised structure, helping transform cracks into potholes.

A single cold night does not necessarily destroy a road. The bigger problem is repetition. A pavement that repeatedly moves through freezing and thawing conditions experiences a sequence of expansion, contraction and moisture redistribution. Surrey's researchers are specifically interested in breaking that cycle by regulating pavement temperatures rather than waiting until visible damage requires repair. The university describes freeze-thaw and traffic loading as important contributors to pothole formation.

The same thermal control could address the opposite problem in summer. Asphalt exposed to strong sunlight can become extremely hot, and high temperatures reduce its stiffness. Heavy vehicles repeatedly pressing against a softer pavement can contribute to rutting and permanent deformation. Recent research on thermo-active roads notes that pavement surface temperatures can exceed 60°C in summer conditions, making thermal management relevant not only to winter cracking but also to summer rutting.

The road becomes a giant thermal collector

The Surrey system turns that seasonal temperature swing into something useful. During hot weather, fluid circulating through pipes beneath the road absorbs heat from the pavement. Rather than allowing all of that heat to remain in the asphalt and surrounding air, the system transfers it underground, where it can be stored and recovered later.

This is where the technology differs from simply heating a road with electricity. The ground acts as a thermal reservoir. Surrey's work uses shallow geothermal principles, with heat moving between the pavement, circulating fluid and subsurface materials. The university's wider research programme is examining how pipe depth, soil properties, fluid flow and other engineering variables determine how efficiently heat can be harvested and recovered.

The current project at Surrey is intended as a proof of concept for a “thermal battery” road. The Transport Research and Innovation Grants project is designed to capture heat from road surfaces during summer and redeploy it for anti-icing in winter. Connected Places Catapult says the system uses water-filled pipes embedded in the pavement and underground storage, with laboratory work already used to investigate how flow rates, temperatures and soil properties affect performance.

A 100-metre borehole could store summer heat

At the Surrey trial site, installation is taking place in the Senate House car park on the university's Stag Hill campus. The system described for the trial uses a network of heat-exchange pipes beneath the road surface and a borehole reaching roughly 100 metres underground. Sensors will allow researchers to watch how heat moves through the pavement and ground instead of relying only on measurements taken at the surface.

That monitoring is crucial because storing heat underground is not as simple as putting energy into a tank. Soil and rock have different thermal conductivities and heat capacities, and moisture can strongly affect how efficiently heat travels through them. The geometry of the pipes matters too. A pipe that is too shallow may collect heat effectively but lose it quickly, while deeper storage can retain thermal energy differently.

Surrey's numerical modelling has already shown how sensitive the system can be to those variables. In one modelling study, increasing storage-pipe depth by 1.6 metres increased energy storage by 1.75 times. Simulations also found that stored energy increased from 0.56 to 0.86 megawatt-hours as assumed soil thermal conductivity rose from 0.4 to 2.0 watts per metre-kelvin.

The surprising role of summer heat

The most counterintuitive part of the idea is that summer heat, normally considered a problem for roads, becomes a resource. Instead of simply trying to cool the pavement and lose that energy to the atmosphere, the system can capture some of it and move it underground.

That creates a seasonal exchange. The road effectively becomes both a solar heat collector and a winter heating surface. During summer, heat is extracted from the pavement. During winter, the stored energy can move in the opposite direction. The goal is not to make the road hot or cold, but to keep its temperature within a range that reduces harmful extremes.

The effect could extend beyond pavement durability. The Surrey team plans to measure air temperatures above the test section to determine whether cooling the pavement during hot periods can also influence the local air immediately above it. That question is important because roads are major heat-absorbing surfaces in built-up areas, and pavement temperatures can contribute to the urban heat environment. Recent thermo-active road research has specifically identified pavement cooling as a potential way to reduce thermal stress and pavement overheating.

Could this actually stop potholes?

That is the question the trial cannot answer yet. It would be premature to describe geothermal roads as a cure for Britain's potholes. Roads fail for many reasons, including poor drainage, weak foundations, heavy traffic, construction defects, ageing materials and repeated weather exposure. Thermal regulation can address only part of that equation.

What makes the Surrey approach significant is that it targets damage before it becomes visible. Instead of repairing the consequence, engineers are trying to modify one of the physical conditions that contributes to deterioration. The university's five-year research programme, running from 2023 to 2028, includes laboratory testing, numerical modelling and plans for larger field trials to determine whether the approach can work beyond controlled experiments.

There is already evidence that the underlying physics is workable. Surrey researchers have validated a three-dimensional model against experimental data from a full-scale field test in Toddington, UK. More recent laboratory research has examined heat harvesting, transfer and underground storage under controlled conditions, finding that pipe spacing, fluid flow and soil thermal properties strongly influence efficiency. Storage efficiency above 70% was achievable in high-diffusivity soils in that research.

The biggest challenge may be economics

The engineering challenge is only half the story. Roads cover enormous areas, and installing pipes beneath existing highways could be expensive and disruptive. Any system that requires widespread excavation would undermine one of its main advantages. Surrey researchers therefore see integration with road construction and resurfacing as an important route for future deployment.

The university has also described the technology as potentially retrofit-capable in its current Transport Research and Innovation Grants project. Standard plastic pipework and water circulation equipment are central to the concept, rather than exotic components that would need an entirely new manufacturing industry.

There is also a question of long-term thermal balance. If a system continuously removes heat from the ground without replacing it, or continually injects heat without sufficient recovery, underground temperatures can gradually shift. Research into geothermal pavement systems has highlighted this issue and the need to design storage and extraction carefully over long operating periods.

From repairing potholes to managing road temperature

The real promise of thermo-active roads is therefore broader than pothole prevention. They represent a different way of thinking about highway infrastructure. A conventional road simply absorbs sunlight, releases heat, freezes, thaws and eventually deteriorates. A thermo-active road could become an engineered thermal system that responds to the seasons.

If Surrey's field measurements show that the technology can reliably moderate pavement temperatures without excessive energy use or maintenance costs, the implications could reach well beyond one university car park. Roads could potentially harvest heat when it is abundant, store it underground and use it when winter conditions demand it.

For motorists, the result would be almost invisible. There would be no dramatic machine filling a pothole before it appears. The important intervention would happen beneath the wheels, where temperature, water and soil interact long before a crack becomes a hole. That is precisely what makes the Surrey trial worth watching: it is not trying to make pothole repairs faster. It is asking whether engineers can make some of those repairs unnecessary in the first place.