An information paper on road potholes and repairs

Some history 

Potholes have been a scourge for thousands of years. The word “pot” comes from Middle English meaning deep pit, hollow or vessel. The term “pothole” was first applied (as far as we know) to holes in sealed roads about as soon as sealed roads became common, around 1909. 

Unsealed roads were nearly always rough and pockmarked with holes and gullies making for uncomfortable travel at even horse drawn speeds. 

In 1816 a dramatic improvement in roads was brought about by Scottish civil engineer John McAdam who perfected a road base made from layers of successively finer sharp-edged stones which interlocked with each other to make a stable and durable road base. The top surface was prone to damage from traffic lifting the small stones and the entire structure was weakened by water filtering down and softening, or washing away, the sub soil. Good drainage was an essential element. Not much has changed since. 

A seemingly ideal solution to these two issues came later, and used tar to seal the surface, greatly reducing surface wear and water ingress. This “tarred McAdam”, or tarmac for short, did not catch on whilst horses were the predominate motive power as their hooves dug up the relatively soft tar. 

But once rubber tyred motorised transport caught on tarmac roads became the way to go, although tar was replaced with better tar like materials by then. 

Where do potholes come from? 

In many cases they seemingly come from nowhere and quite rapidly. They can literally form before your eyes in minutes. 

One cause of potholes is when the road base is not supported and is unable to bridge the soft or voided area below it, causing a section of road to drop and the upper surface to form a dip or a bowl, which then gets bigger. You will see this when trenches have been dug across roads and not properly backfilled and compacted but it can happen when the sub soil is too weak or there is water flowing or pooling under the road. 

The most common reason is water entering the road structure from above. 

The first step is cracking in the top surface. This may be fatigue cracks because the structure is not stiff enough, which is a design and construction issue. It may be because the asphalt surface is old and “drying up” and becoming brittle or because of repeated thermal expansion and contraction. 

Once there is a crack in the wearing surface water can seep into the structure, eventually softening the subsoil and causing the structure to collapse. 

As the surface cracks chunks get lifted up, more water gets in, causing surface water to pool which increases the water going into the structure and hastening the damage. At some point a catastrophic collapse can occur turning a shallow puddle into a sizable pothole in minutes. A mini sink hole. 

Potholes are most likely in the wheel tracks, which is inconvenient, or at the road shoulders where the road structure and drainage does not extend far enough away from the road surface’s edge, as often happens when the road has been widened a little without a full rebuild. 

How to stop potholes 

The first line of defence is in the road design and construction. Drainage, soil types, camber, expected usage, local geology and material choice are all vital. As is building it to the design! Trying 

to save money at this stage will cost plenty later on. Often the budget does not extend to the highest standard. Concrete roads are great but expensive. 

Sealing cracks early is a major preventative tool, which is why you often see wriggly black lines of tar on roads, local roads in particular. This is a costly process, but effective if done early. Re-sealing roads before the surface gets aged is another great preventative measure but even more expensive. 

Drainage and water flows over, around, and under the road also need routine inspection and correction as required. 

Much of the damage is unseen until it is suddenly exposed as a pothole, but (expensive) ground penetrating radar can “see” what is going on below the surface allowing rebuilding of the road structure before it all collapses. 

Why is this not done? 

The short answer is money. There is just not enough money allocated. 

Australia’s roads are maintained by three levels of government. Federal roads (national highways) are funded by Federal government with the work sub-contracted to the States and Territories. State roads are funded by the States (used to be VicRoads in Victoria) and local roads by your local shire council. 

None of these levels have endless funds for road building and repair. Any extra spent on roads means something else has to go without. 

Road building and road maintenance are usually two different parts of the budget (capital and recurring respectively) and rarely is there any transfer of the savings made during building to pay for the inevitable extra repairs later. 

Almost always, a pothole is not repaired by filling it with cold mix, it is just a problem put off. But that is often all that can be done quickly and within the budget. To dig out and repair the road structure, perhaps fixing the drainage issue or structural inadequacy which caused the pothole in the first place, takes longer and costs a lot more. One pothole fixed properly, 10 left untouched. 

Will the current pandemic of potholes be fixed soon? 

Probably not. NSW are still catching up 6 years after a similar experience. 

The culmination of many years, possibly decades, of inadequate repair budgets will take a long time and about 3 times current funding to fix. “Cost efficient” road building in the past will add to the repair costs in the future as those roads start to crumble prematurely. Above average rain falls places stress on the road drainage and exploits any surface cracks. Prolonged dry spells after all this rain may cause further damage as the subsoil shrinks. 

Does size matter? 

Road structure damage rises with the 4th power of axle mass – double the mass means 16 times the damage. If a typical passenger car with an axle mass of 850 kg does 1 unit of damage, a larger heavier car (say a modern ute or EV) with an axle mass of 1300 kg will do 5.4 units of damage but a truck axle at 8.5 tonnes will do 10,000 units of damage. So, no, a bit of extra weight in a passenger car is not an issue to the structure. 

The road surface can wear a little faster with heavier vehicles but the main concern here is scrubbing the surface during corning at roundabouts etc. 

What can we do now? 

Our habit of voting for whichever party promises to tax less and spend less in government, at whichever level, probably does not help. Any increase in road expenditure will probably mean other budget/service items getting less money and they may be more important. 

A dedicated road repair levy (of which we already have two but neither are dedicated to road repairs now!) of about $0.04 per km, or $0.40 per litre of fuel, or $600 per vehicle per year, would finance the repairs. 

Reporting patches of broken asphalt before they become potholes can help, as may keeping pressure on your local representative (of the relevant government level) to address the road damage in your area. They are probably well aware of the issue but constant reminders may work. 

Driver Safety

Leaving a longer gap between vehicles to allow time to spot and react to a pothole would certainly help reduce the damage. As does watching the behaviour of the vehicle in front for sudden movements which may indicate hitting or dodging a pothole. 

The views in this paper are those of the author, Lawrence Glynn, and may not align with those of the AOMC. 

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