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Gone to pot: City wages war on plentiful potholes

Potholes near the Worthington intersection of Ninth and Shell streets are pictured.(Tim Middagh / Daily Globe)

WORTHINGTON — Nobody likes to hit a pothole. The unexpected jolt of hitting one hard enough is not only a pain for the driver but a danger to the vehicle, potentially causing damage to a car’s tires, rims, steering, exhaust, undercarriage or suspension system.

They’re a nuisance for the city, too. Worthington Public Works Director Todd Wietzema said it takes about a week of non-stop filling potholes to put a band-aid on the problem every spring and summer.

“From the time that our roads clear off in the spring through the first month of spring when it warms up in the 40s, we spend most of our time running around trying to do potholes,” Wietzema said.

“It probably takes two men 15 minutes to fill it and roll it. It doesn’t take very long to fill them, but you go 30-40 feet down the road and there’s another one.”

Of course, no matter how many potholes you fill, they always find a way to come back. Potholes are caused by the expansion and contraction of ground water after the water has entered into the ground under the pavement. When water freezes, it expands, and when it thaws, roads become vulnerable to damage. In Minnesota, potholes are inevitable.

Asphalt, being less sturdy and more flexible than concrete, is generally more susceptible to potholes. In the fight to reduce potholes, one of the city’s changes has been paving streets with concrete.

For a long time, cities paved residential roads almost exclusively with asphalt. But asphalt prices have risen significantly over the last two decades, which means concrete, which lasts around 15 years longer than asphalt on average and requires significantly less maintenance, has become a more attractive option.

“One will last 20 years, the other 35, so we look at where we’re at relative to life expectancy,” said Dwayne Haffield, city engineer. “Asphalt prices used to be considerably less money, so even if you had to do it twice, it was OK — that’s just not so true anymore, at this time. And that could change — if cement and raw materials became very expensive, we would have to reevaluate again.”

The city’s feasibility report for a 2016 reconstruction project on parts of Darling Drive, Hagge Street and Schaap Drive found that covering the roads with asphalt would cost $672,700, whereas concrete would cost $844,150. However, asphalt costs significantly more to maintain, as it requires mill and overlays, seal coats and more fills to potholes.

But the pothole fix isn’t as simple as constructing concrete roads everywhere. For one, concrete roads vary in effectiveness based on the characteristics of underlying soil.

“What we’re looking for with concrete is uniform support underneath, because concrete relies on a wider area on distribution,” Haffield said. “If you start to get movement of soil that’s coming in and out of frost, wetter seasons, drier seasons, different soil types can move differently. So if we get a lot of differential movement, maybe concrete isn’t the best choice, because asphalt can actually take more variation and do its thing.”

When the soil underneath isn’t uniform, concrete roads can have major issues.

“A concrete pavement being rigid — when a wheel load is on a panel, it’s being distributed over a wider area,” Haffield said.

“If there is a single soft spot underneath there, it will bridge that, so you don’t get the potholes, but what can happen — let’s say if there’s a lot of soil movement — you start to break down panels, and they start to have joint problems, so you may not have potholes, you have a different problem.”

The city will rehab parts of Fifth Avenue this summer — the concrete panels have fractured because of movement underneath. Joints, or gaps in between concrete panels, also present an issue. If they are not too wide, they can be left alone, but wide joints need to be filled.

Concrete paving also takes much longer to complete. McMillan Street is going asphalt because as the job doesn’t take as long, the city can have traffic restored more quickly. The Oxford Street reconstruction project, on the other hand, will likely be concrete, as the city can close lanes one at a time without ruining traffic.

In another effort to stop the pothole takeover, the engineering department has made specific changes to the bituminous design to drain moisture out from beneath the asphalt — a version of the method was implemented in the late 1980s.

“Potholes are caused by excess moisture, so we really targeting rapid drainage of the gravel, that way the moisture that causes the potholes is not there,” Haffield said. “We have subsurface drainage layers underneath, so water coming up from the bottom is cut off and doesn’t get up into the gravel.

“Today, we’re really pretty happy with their performance — streets that were getting turned over much more rapid are not now.”

Despite the city’s efforts, there are certainly still potholes on the ground, and there always will be. Nonetheless, the city hopes to fill as many as they can by Summer.

“First of May, middle of May, we’ll run around and do a lot more extensive patching — right now we’re band-aiding it through the early spring,” Wietzema said.