If you consider it, having paved roadways is somewhat of a luxury. Streets have always been around, but they haven’t always been safe, comfortable, or able to accommodate the enormous number and weight of vehicles that use our present system of roadways every day. Whether or not you love how much roads dominate the landscape, you have to marvel at the fact that, in most parts of the modern world, anyone can get in a bus, car, bike, truck, motorcycle, or scooter, and go almost anywhere else in relative ease and comfort. In fact, roads make travel so convenient that not having them - or having them be in poor condition - is a significant source of frustration. There are definitely times when driving does not feel that luxurious, and one of them is something we’ve all experienced once or twice. Hey, I’m Grady, and this is Practical Engineering. Today, we’re talking about potholes in paved roadways.

I remember the excitement of getting my first car as a teenager and finally being able to drive. Sad to say, that was probably the most joy that driving a vehicle will ever give me. Now, it’s kind of a chore. And I hope I’m not out of line by saying this, but I think for most people, driving is a little dull. It’s the thing we do in between where we are and where we’re trying to be. I don’t know about you, but I don’t wake up in the morning excited to jump in the car for my morning commute. Driving is something that most of us take for granted. But, the only reason we’re able to do that - to regard vehicle travel so indifferently - is because roadways are so well designed and constructed.

There are lots of ways to build a road. From yellow bricks to rainbows to simple dirt and water, the combinations of materials and construction techniques are practically endless. And yet, across the world, there’s really one design that makes up the vast majority of our roadways. It consists of one or more layers of angular rock called a base course and then a layer of asphalt concrete (also called blacktop or tarmac). It turns out that this design strikes the perfect balance between being cost-effective while creating a smooth and durable road surface. But, asphalt roadways aren’t invincible, and they do suffer from a few common problems, one of those being potholes.

The formation of a pothole happens in steps. And the first of those steps is the deterioration of the surface pavement. Asphalt stands up to a lot of abuse. Exposure to the constant barrage of traffic in addition to harsh sunlight, rain, snow, sleet, and freezing weather will eventually wear down any material, no matter how strong. When that happens to asphalt, the first sign is cracking. They might seem innocuous, but cracks are the Achilles heel of pavement systems. Why? Because they let in water. And not just let it in, but let it come back out as well. A hole is a lack of substance or material. It’s the only thing that gets bigger the more you take away. If you started without a hole and now you have one, that material had to go somewhere. In the case of a pothole, the material is the soil below the road (called the subgrade), and where it goes has everything to do with water.

As water finds its way into cracks and below the pavement, it can get trapped above the subgrade. Eventually, these soils get waterlogged, softening and weakening, and then the traffic shows up. Cars and trucks are heavy, and they pass over the road at rapid speeds. Because of this, traffic is just a generally destructive environment. It’s a lot for any road to stand up to, let alone one that’s waterlogged and weakened. Asphalt is called a “flexible pavement” because it doesn’t distribute these loads across a large area like something more rigid would. So, every time a tire hits this soft area, it pushes some of the water back out of the pavement. That water carries particles of soil with it.

This is a slow process at first, but every little bit of subgrade eroded from beneath the pavement means less support, and less support means more free volume below the pavement for water to be pumped in and out by traffic. This, in turn, creates more erosion in a positive feedback loop. Eventually, the pavement loses enough support that it fails, breaking off and crumbling, and you’ve got a pothole.

Of course, this whole process is made even worse in climates with freezing weather. Water expands when it freezes, and it does so with tremendous force. Thin layers of water between pavement and base freeze and grow into formations called lenses. When those lenses thaw out, all the ice that was supporting the pavement goes away, creating voids. In addition, the lower layers of soil stay frozen, trapping that meltwater between the pavement and the subgrade and accelerating the erosion. Potholes exist everywhere you have asphalt concrete roadways, but they’re worse in areas with cold climates and much worse in the spring as the ground begins to thaw.

They’re annoying, yes, but they’re not just that. Potholes cause billions of dollars of damage to tires, shocks, and wheels of vehicles. Even worse, they’re dangerous. Cars swerve to miss them, sometimes at high speeds, and if a bike, motorcycle, or scooter hits one, it can be bad news. So, roadway owners spend a lot of time and money fixing them. There is a large variety of types of pothole fixes depending on the materials, cost, and climate conditions. But, they all mostly do the same thing: replace the soil and pavement that was lost and (hopefully) seal the area off from further intrusion of water. That second part is obviously critical but much harder to do. A pothole repair is a bandage after all, and it doesn’t always create a perfect connection to the rest of the roadway. This is why, even after they’re repaired, potholes seem to recur in the same location over and over again.

After understanding how these annoying and sometimes damaging defects occur, the next logical question is, how do we prevent them in the first place. Obviously, we could build our roadways out of more robust and more durable materials. Many highways are paved with concrete for this exact reason. But, roads are unusual in that even a tiny change in design has a significant overall impact on cost. Choosing a pavement that’s even just a centimeter thicker could mean millions of tons of additional asphalt because that centimeter gets multiplied by a vast area. So, we balance the cost of the original pavement with the expense of maintaining it over its lifetime. In the case of asphalt pavement, that maintenance primarily means sealing cracks to prevent intrusion of water. If you can do that and do it regularly, you can extend the life of asphalt pavement for many years.

Since roadways are mostly public infrastructure, their condition (at least to a certain extent) reflects the importance we all place on vehicle travel. In the broadest and most general sense, we choose potholes by choosing how much tax we pay, how much of those taxes we’re willing to budget toward streets, and how large and how many vehicles we drive over them. Pavement is one of the highest value assets owned by a City, County, or DOT. It’s essential, and it’s expensive, which means there’s an entire industry surrounding how to design, build, and maintain roadways as safely and cost-effectively as possible. Politicians, government officials, engineers, and contractors drive on the same roads as everyone else, so they all have a vested interest in keeping those roads as pothole-free as possible so that we all can enjoy the luxury of driving on paved streets in safety and comfort. Thank you for reading, and let me know what you think!