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Potholes

It’s spring in Regina, and we all know what that means: snow is melting, water and funny-looking guys in shorts are running, and the potholes are in bloom.

Everyone knows that Regina has a pothole problem, and for once, what “everyone knows” is right. But don’t blame the city. Especially, don’t blame Harlan Ritchie, Manager of Roadways Engineering, or Dennis Lawrysyn, Manager of Roadways Maintenance, because they’re the ones who gave me information for this column. It’s not their fault! (The potholes, that is, not this column.)

Our roads are built on thick, gooey clay. Gooey clay is gooey for one reason: it traps and holds moisture. Potholes occur for one reason: moisture trapped and held under the road surface. Get the connection?

It might comfort you to know that through most of history, roads have been pretty pitiful: well-planned and well-maintained, some of them, like the vast Incan system or that of the Ch’in dynasty in China, but still just fancy dirt trails. The exception was the Romans, who revolutionized road- building with the Via Appia: they paved it.

Some Roman roads are still around precisely because they recognized the damage water could cause, and insisted on adequate drainage. A ditch was excavated on each side of a road, and the earth was used to make a bank called the agger, constructed on foundations of broken stone, bricks, and pottery cemented with lime. Large blocks of hard stone or lava were carefully fitted together to form the top surface of the road, called ” pavimentum” in Latin — the root of our word “pavement.”

After the Romans, unfortunately, roads went back to being muddy and rutted until the 18th century, when John McAdam, British commissioner for highways, invented roads that could carry relatively high-speed wheeled traffic. Three 10-centimetre-thick layers of small stones were rammed down, first by hand, then by horse-drawn rollers. This, and then traffic, ground the edges off the stones, making a watertight surface. “Macadam” roads served well until the advent of the automobile, whose rubber tires would loosen the stone surface, throwing up dust and rocks. Something else was needed on top.

The French already had it: asphalt, a sticky black substance made up mainly of carbon and hydrogen that occurs naturally and can also be produced from petroleum.

Asphalt has been the pavement of choice in most places ever since, including Regina, where roads are built on a base anywhere from 50 to 80 centimetres thick, depending on the traffic load. The bottom layer is clean sand, which serves as a drainage layer . Over that goes a layer of dirty sand, then a layer of coarser aggregate, and finally the asphalt .

The city has only been using this “deep granular base” for 10 years or so; older streets were generally built on a concrete base only about 30 centimetres thick. That thinner base means they’re more susceptible to potholes.

Potholes begin as cracks. There are two kinds of cracking: old pavement cracks because its dry and brittle, and younger pavement cracks because of the expansion and contraction of the asphalt as the temperature changes with the seasons.

The first cracks usually appear in new pavement after about two years. Some run parallel to the roadway, but others, occurring about every 30 metres, run across it. After another couple of years, a second set of cracks shows up, giving you cracks about every 15 metres. This process continues until somebody seals the cracks.

Crack sealing is designed to keep out moisture, and extends the life of the pavement about five years. (New pavement is good for about 18 to 20 years, if properly maintained.) If you can’t keep water out — and sometimes you can’t, depending on when the crack occurs and how successful the seal is- – it builds up under the pavement, the gooey clay helping to trap it. If this happens at a time when it’s freezing at night and thawing during the day, it’s bad news. The water expands as it freezes, heaving the pavement up and widening any crack its trapped in, then thaws, letting in more water, then freezes, pushing everything apart again, then thaws, and so on.

Water from the clay is also drawn up into the area where freezing is occurring, adding to the problem.

Sometimes you get more moisture on one side of a crack than the other, which raises one side higher and also leaves it less supported underneath. Drive a few dozen cars over it, and eventually a chunk will break way: presto, a pothole! More freezing and thawing and a few more cars, and it can become a veritable canyon.

Is there a solution? Well, Winnipeg has basically the same kind of soil, but has fewer potholes. That’s because many of their streets are paved with concrete. Concrete doesn’t expand and contract as much as asphalt because it’s white, reflecting the sun’s energy, unlike black asphalt. As well, concrete streets are steel-reinforced and can stand up to 4,000 pounds of tension, compared to 300 to 400 pounds for asphalt.

Concrete has been tested here and worked well, but it’s too expensive for a city this size. To paraphrase, “Smoothness costs money. How smooth do you want it to be?”

But technology marches on. There are new asphalt additives that may help our streets in the future. There are new machines for repairing streets.

In the meantime, as you jounce along in your automobile, console yourself with this thought: if we didn’t have potholes, we’d have to change the name of our football team.

Permanent link to this article: https://edwardwillett.com/1995/03/potholes/

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