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Why do potholes always return to the same spots, even after they are repaired?

Why do potholes always return to the same spots, even after they are repaired?

Norman Dennis, professor of civil engineering, replies:

Above left: Water from melting snow and ice seeps into the pavement and softens it. During repeated cold spells, the water in the pavement refreezes and expands, breaking up the pavement, on and below the surface. Above middle: When the ice melts, it leaves gaps inside the pavement, and the moisture further softens it. The soft, fractured asphalt cannot support the weight of passing vehicles, and begins to break up. Above right: As vehicles continue to pass over the weakened spot, pieces of roadway are kicked out, creating the hole in the highway.

Potholes are always related to water. Potholes often return in places where we put a “bandage” on the pavement by patching the hole, but don’t fix the real source of the problem.

In cold climates, water that seeps through joints and cracks in the asphalt collects under the pavement and freezes to form what are known as ice lenses. These lenses can become quite large, measuring up to two meters in diameter.

When the ice lenses melt, either during the heat of the day or during the spring thaw, the water drains away, leaving a void near the surface. When cars drive over the place where the void exists, the pavement collapses into the void, creating a pothole.

In the South, where the temperatures rarely stay below freezing long enough to create ice lenses, the culprit behind potholes is the soil itself. Soil beneath the asphalt gets wet and softens. The weakened soils cannot carry the wheel loads transmitted by the asphalt, so when cars and trucks drive over the road the asphalt collapses into the area of weakened soil and a pothole forms.

Fixing a pothole correctly requires getting deep under the asphalt surface and providing materials to support the asphalt that drain well like sands and gravels. This way water does not build up under the pavement.

The best way to prevent potholes from forming is to build the entire substucture of the road out of free draining material and keep the pavement surface sealed. However, both of these methods cost lots of money, so instead you will continue to see potholes appearing and being fixed in many of the same places.

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