What’s Causing Those Potholes?

July 7th, 2023

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Potholes are so annoying!  I know we need to be grateful that most of our roads are sealed and aren’t rutted, but a pothole was bad news. They were bad news even when roads weren’t sealed and ruts were common.  The shallow ones bump you so hard that not even the world’s best suspension system can cancel it out (unless you dodge them), and the large ones can damage your car (more on that below).  If you try to dodge a pothole, you can put yourself and/or other drivers at risk/ In the case of some modern cars that have driver aids that were designed for and tested on perfect roads, things like lane change assistance might throw a wobbly if you detect a pothole in the road ahead and adjust your driving line carefully to avoid it (the systems are smart enough to shut up if your movements are abrupt).

Why do potholes appear?  Why does what used to be a perfectly good piece of road suddenly look like a teeny asteroid hit it?  Are heavy trucks and road trains to blame?  And what can you do about them?

The thing that causes potholes is nature striking back.  They are caused by one of the most powerful elemental forces in the world: water.  As you’ve probably seen at some point in your life, whether it’s a catastrophic landslide, a cliff eaten away by the action of the sea or just a rut in your garden after someone left the hose on for too long, water sweeps away and acts on dirt. And it’s water that causes potholes.

Now, it’s not the case that a pothole will appear where a puddle has been.  It’s true that both potholes and puddles will form in parts of the road that have slumped or become rutted, but one doesn’t directly cause the other.  There are other factors at play.  The condition of the road is one of them and the amount (and weight) of traffic is another.

Water will get into the soil beneath the road and start loosening the particles of dirt, meaning that ruts and holes will form.  This has always been the case ever since roads were invented.  If anything, the whole point of road surfacing is to have something that doesn’t form ruts and holes every time it rains so that wheels can run over it smoothly.  The different layers of a modern paved road are designed to ensure that water drains away well (and that the road will hold its shape despite heavy trafficking – but that’s good story for another time) with the asphalt over the top forming a mostly waterproof and resilient seal.  However, nature will always prevail, and water will get in.

Once the water has got in, the most common thing that happens next to create a pothole in Australia is that the water will start washing away small, fine particles of dirt, then larger particles, and then a bit more.  This will weaken the ground beneath the asphalt surfacing, as there’s less holding it up.  As traffic goes over it, the asphalt surface will be pushed down a little, not so much that you’d feel it but still a little.  And this compresses the water, which increases the pressure it exerts on the surrounding particles of dirt.  Eventually, a characteristic pattern of cracks will appear on the surface of the asphalt, known as alligator cracking because the pattern looks like the skin of a big old croc.

Eventually, the friction from tyres rolling over the asphalt will break some of the surfacing loose, exposing what’s underneath. The hole will soon get wider and wider, and you’ll get a fair dinkum pothole, and it will get worse and worse the more the water gets in.

Water in its liquid state is the primary cause of potholes in Australia, although in parts of the country where you get frosts – and in other parts of the world where winters get particularly savage – another factor is at play. Water expands as it freezes, so any water in a tiny crack of the pavement or beneath the surface will expand. The asphalt, however, will become more rigid and brittle, so the expanding ice will break the asphalt and crack it more, which accelerates the process of a pothole forming.

Generally, the wetter things get, the more quickly potholes will form.  This trend has often been noticed; in fact, Shakespeare makes a passing reference to it in one of his plays, where a character compares a stupid, pointless and completely undeserved action to fixing highways in summer.

There is nothing that you personally as a driver can do to fix a pothole. That’s the job of the local roading authority. In an ideal world, these people should inspect the roads and take action to resurface and to improve the drainage as soon as they notice signs of alligator cracking.  However, in practice, we tend to see that the problem gets a temporary fix in the form of asphalt being slapped into the hole to fill it up.  This works for a short time, but if poor drainage is what has caused the water to get in and pool beneath the road, another pothole will appear before long.

Ideally, you should drive around a pothole rather than letting your wheel run through them.  If you drive through one, it can cause a lot of damage.  Tyres are the most vulnerable. The most immediate and dramatic type of damage is if the rough edge of the pothole punctures or rips the tyre. However, there are more subtle types of damage.  Going through a pothole can also cause sidewall bulges by forcing the liner apart from the sidewall – and these bulges can blow out very easily.  If the rims are damaged or the alignment is thrown out by going over a pothole too fast, this will make the tyre wear out more quickly and unevenly.

The damage doesn’t stop there.  The shock of going through a pothole will also put a strain on the suspension and steering as well as on the general alignment of the wheels (they’re all interconnected).  This won’t happen straight away, but it will be made worse by continually going over rough roads and hitting potholes (e.g., one that’s on the road you take to work during rush hour, meaning that you have no choice other than driving over it).  In the worst case, which is going into a very deep pothole that the local authorities should really have done something about ages ago, the undercarriage and exhaust system can be scraped and dented as well if it hits the undamaged surface of the road.

In the case of EVs, damage to the underside of the vehicle is particularly serious, as this is where the battery is.  The battery is protected by an underfloor protector, which is like a suit of armour for your EV’s battery.  However, if this underfloor protector is badly damaged, the battery becomes vulnerable and could go into thermal runaway (i.e., catch fire).

Obviously, if you see a pothole, you should avoid it.  If the traffic is light and the road is wide, this isn’t a problem.  However, in heavy traffic, going through that pothole may be unavoidable, as the results of hitting another vehicle would be much worse than the results of going through a pothole.  However, the damage will be less if less force is involved, so dump some of that kinetic energy by slowing down, preferably well before you get to the pothole so you don’t bang on the brakes (however, banging on the brakes will be easier on your car than driving through a pothole, especially if you have brakes with all the driver aids). 

Lastly, the question as to whether trucks are to blame for potholes. The answer “yes but”.  Yes, trucks are heavy and the extra weight wears out the asphalt more quickly.  However, cars are getting bigger and heavier in general, and EVs are particularly heavy compared with their ICE equivalents.  However, the roads are still built to the old specifications for lighter vehicles, and don’t stand up.  What’s more, budget cuts and cheapskate roading authorities mean that roads may be built to meet the bare minimum specifications rather than exceeding the standards for extra durability and resilience.  Perhaps it’s time for the standards to be revise to meet the current vehicle fleet, especially if the government wants greater uptake of the heavier EVs.