What causes potholes?

If you drive often enough, you will have wondered: what causes potholes? Every driver has their own pothole story to tell, all negative – no one is ambivalent. In fact, the issue resonates so deeply in the UK that there is even a National Pothole Day dedicated to raising awareness and lobbying for more action on pothole repair. Potholes are a problem for highway engineers and road users worldwide, affecting all climate areas and types of roads.

In this guide, we will explore how potholes are formed, the factors that contribute to these persistent road potholes, and what can be done to prevent them.

If you’re particularly interested in one section, use the links below to navigate to it:

• What are potholes?
• How do potholes form?
• The 5 stages of pothole formation
• What factors lead to potholes?
• How to prevent potholes from forming Preventing potholes with Tensar’s solutions

Looking to prevent pothole formation on an upcoming road construction project? Tensar provides design support and a range of solutions that can help – get in touch today.

What are potholes?

Potholes are areas of a road surface where the surface layer, usually asphalt, has developed cracks then broken away under the repeated load of traffic passing over, forming a hole with rough vertical sides. A pothole may begin with a few small cracks but can expand from just a few centimetres to up to a metre wide and 10 or more centimetres deep if it isn't fixed right away. Potholes can occur in all road types, but are a problem particularly associated with asphalt surfacing.

Why are potholes called potholes?

So, why are potholes called potholes? The term "pothole" has an interesting history. The word ‘pot’ was used in late 14th-century English and is still common in Scotland and Northern England, where it refers to a deep hole, often linked to a mine or cave. By 1826, ‘pothole’ was used to describe geological features in glaciers and rocks. It wasn’t until 1909 that the term evolved to describe the potholes we know today.

Are potholes dangerous?

Potholes can be highly dangerous, particularly to cyclists and motorcyclists. According to the UK Department for Transportation Statistics, in the ten-year period up to 2021, there were 7847 casualties, including 452 cyclists and 725 motorcyclists killed who were or seriously injured in accidents where the primary cause was a poor or defective road surface. In addition, many accidents involving both road users and pedestrians occur from drivers swerving to avoid potholes.

How do potholes form?

Potholes are formed by three elements: surface cracks, water and traffic. Small surface cracks form and expand over time with the action of traffic. Water then seeps through the surface of the cracked pavement, causing further deterioration – in cold climates, this can be exacerbated by freeze-thaw action.

Ultimately, potholes arise from a complex interaction of these elements, and understanding this can help in creating strategies for pothole repair and prevention.

The five stages of pothole formation

So, how do potholes form? The process of pothole formation is complex, involving several stages that contribute to the development of these persistent road hazards. Below, we delve into the five key stages that explain how potholes form and what causes potholes to grow from small cracks into large, disruptive holes.

Stage 1 – Cracking of the road surface

The initial stage in pothole formation begins with the cracking of the road surface, typically due to fatigue in the asphalt. Fatigue cracking of the asphalt occurs due to an inadequate pavement structure. If the pavement layers lack the necessary strength and stiffness to handle the traffic loads, the pavement will flex under the pressure of passing vehicles. The repeated strain at the bottom of the asphalt results in the initiation of fatigue cracks in the asphalt layers. These are likely to appear as ‘alligator cracking’ where the surface breaks down into irregular blocks.

Other factors may also contribute to pavement cracking, including:

● Poor drainage: allowing the water to weaken the pavement structure.
● Ageing of the asphalt: makes the road surface more brittle and prone to cracking.
● Shrinkage cracking: caused by thermal movements or expansive clay subgrade soils.
● Reflective cracking: where a new asphalt layer has been laid over existing cracks and eventually reflects through to the surface. This can be minimised using an asphalt reinforcement interlayer such as Tensar AX5-GN.
● Utility works: where poorly backfilled trenches can lead to surface cracking due to settlement.

Stage 2 – Water enters the pavement

Water seeps down through the pavement layers, saturating the aggregate subbase and base layers and softening the subgrade soil below. As it seeps deeper, it softens the underlying subgrade soil, further compromising the structrual integrity of the road.

Stage 3 – Traffic breaks down the pavement      

Traffic loading begins to break up the fractured asphalt surface. Small blocks of asphalt are thrown out, initiating a pothole that then expands and deepens. Traffic-induced pumping action of water at the subgrade/subbase interface causes fine soil particles from the subgrade to contaminate the aggregate type, resulting in further pavement weakening and further increases in pavement flexing under traffic.

For effective solutions to improve road durability and reduce potholes, our subgrade stabilisation solutions will strengthen the foundation layers and enhance overall pavement performance.

Stage 4 – Freeze-thaw weather worsens potholes

In colder climates, freeze-thaw cycles significantly speed up pothole formation. When temperatures drop, water in the pavement freezes and expands, opening up cracks and bursting the asphalt around the pothole. This leads to faster development and deeper potholes.

Stage 5 – Standing water completes the pothole formation

Water ponding in the potholes after rainfall scours and ejects material under the action of traffic, rapidly expanding and deepening the pothole.

What factors lead to potholes?

Poor road maintenance is often proposed to be the cause of potholes, and this can be the case. Periodic resurfacing or sealing is needed to prevent ageing of the asphalt that can lead to break-up of the surface. Regular maintenance of side drains is also a requirement if the road foundation is to be protected. 

However, in most cases it is the pavement structure that is the root cause of potholes. The construction may have been inadequate from the outset, but it is more likely that increased traffic loading now exceeds the original design or that issues with drainage below the road foundation have locally weakened the pavement. These problems require more than surface road repair and budgets may not stretch to reconstruction. 

Why are there so many potholes in the UK?

The UK's pothole problem is a persistent issue, and several factors contribute to the prevalence of road potholes across the country. An additional £200 million has been allocated for highway maintenance in the 2023 to 2024 fiscal year. This comes on top of the £2.7 billion in local highway maintenance funding outlined in the October 2021 spending review for the 2022 to 2025 tax years, as reported by Gov.uk. Potholes are going to be a feature for decades to come.

One of the major contributors to the pothole problem is poorly reinstated utility excavations. Each year, over 2 million utility excavations are conducted on UK roads. Despite legal obligations for utility companies to restore the road to a certain standard, these repairs are not always up to par, leading to weakened surfaces that eventually develop into potholes.

Is the UK in a worse position compared to other European countries? Potholes are universally present. However, the UK does have some specific contributory issues apart from budget allocation:

● UK roads carry much higher traffic loads than most other countries.
● The UK's dense network of buried utilities adds to the risk of road damage.
● Winter temperatures in the UK tend to be just above freezing for much of the time but with frequent dips below zero. The result is that there are more freeze-thaw cycles in the UK compared to some Northern European countries where temperatures remain below zero for longer periods.

Which part of the road is most vulnerable to potholes? 

Potholes require the passage of vehicles to initiate and develop, so they will tend to occur under the wheelpaths. The exception will be where poorly backfilled utility works are the cause.The presence of water in the subgrade is another contributory factor. This is more likely to occur below the outer wheelpath closer to the road edge. 

And why do potholes come back in the same places?

Pothole repair is usually a surface- only repair. The pothole is trimmed back and the hole filled with asphalt, compacted and sealed around the edges. This will not have addressed the underlying problem where fatigue cracking due to inadequate pavement thickness or poor drainage is the cause of the pothole. In such cases the pothole can be expected to re-appear, sometimes very quickly.

Why do potholes form most often in spring?

Freeze-thaw cycles are a significant accelerator in the formation of potholes. During the winter months, temperatures regularly dip below zero. Each cycle creates more damage to potholes, widening and deepening them. The result is that come spring, our roads are in a significantly worse state.

How to prevent potholes from forming

While much attention is focussed on the need to repair potholes – a never-ending cycle – highway engineers recognise the importance of constructing highways that can prevent potholes from forming in the first place. The key to achieving this is to build roads that are not going to develop fatigue cracking. The challenge is how to future proof road construction without inflating costs. This includes:

Increase pavement stiffness

Boosting the thickness of the asphalt base or granular subbase layers enhances the road’s ability to withstand traffic loads and reduces the likelihood of cracking.

Accurate traffic load predictions

Designing roads based on accurate traffic load estimates helps ensure they can handle long-term use without premature failure.

Optimise pavement design

Instead of simply adding more material, design engineers can use a mechanically stabilised layer to strengthen the road while minimising costs and environmental impact. Tensar offers solutions like Tensar InterAx and Glasstex to enhance road durability with this method.

By implementing these strategies, we can build more resilient roads whilst reducing the need for constant pothole repairs.

Tensar’s role in preventing pothole formation 

To reduce the fatigue-related cracking that leads to pothole development, we need to increase the strength and stiffness of pavements without inflating construction costs. By incorporating Tensar InterAx geogrids into the road foundation layers to stabilise and strengthen the granular material, the strength and stiffness of the pavement is enhanced. In this way, traffic-carrying capacity is increased      without the need to increase layer thickness. This has a positive impact on construction cost and carbon emissions.  

By increasing the traffic-carrying capacity of new pavements by incorporating Tensar InterAx geogrid, pavement flexing and associated fatigue-related cracking is reduced. In this way, the future development of fatigue-initiated potholes can be avoided.

In turn, when it comes to the maintenance operation, where there is no option to replace the whole pavement structure, the new asphalt overlay can be enhanced with an asphalt interlayer. By incorporating Tensar AX5-GN geocomposite at the bottom of the new asphalt entire package to stabilise and strengthen the bound asphalt course, the fatigue cracking prevention and fatigue life of a pavement is provided. 

What next?

We have seen how most potholes are initiated by cracking of the road surface under the action of traffic. This fatigue-related cracking occurs when the pavement construction is insufficient for the true volume and loading from traffic using the road. Once potholes initiate, they rapidly expand under the action of traffic whenever water is present. This is further accelerated by freezing and thawing of water seeping into the pavement layers.

While significant sums will still need to be spent repairing potholes well into the future, the development of potholes in new road construction can be eliminated by optimising the pavement design using mechanically stabilised road foundations and pavement layers that incorporate a stabilising geogrid such as Tensar InterAx.

Tensar+ design software enables you to design with Tensar InterAx, allowing you to compare material and evaluate project specs. For existing pavements, the recurrence of potholes in asphalt overlays can be delayed by the incorporation of a reinforcing asphalt interlayer such as Tensar AX5-GN. 

To find out more, visit our geogrids for road, pavements and trafficked areas page. If you’d like to submit a project or an enquiry, head to our support page.     

Found this post useful? You might also want to take a look at:

● Types of road cracks and how to avoid them
●  What is rutting in roads? 
● Designing road foundations to DMRB-CD225 

You can also download our full white paper on the causes of potholes here.