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How do potholes form? Find out on this Moment of Science.

By the time you hear this program, the record snowfalls of the past winter will be only a white, wet memory. But if you drive regularly on roads and highways, you’ll see that the snowy past has left its calling card in the present form of pot holes. Roads are built to last and often repaired, but these miniature craters seem to crop up every year. What gives?

Blame it on winter weather. Roads are built in layers, with pavement resting on top of a aggregate sub-base composed of sand, pebbles, and dirt. When it’s dry, pavement can withstand the daily pounding of cars and trucks without much damage. But snow, ice, and rain can wreak havoc on a road surface and its sub-base.

During the winter, when snow and ice periodically melt, water seeps into the pavement and sub-base and softens them. When the liquid refreezes it expands, exerting pressure on the sub-base and surface pavement. Repeated melting and refreezing can weaken various spots on a road, leaving it vulnerable.

When the weather warms up and the ice finally melts, the water further softens the pavement. As it evaporates, the water leaves gaps in the pavement, sort of like an air bubble beneath the surface of the road. When vehicles pass over these pockets, the pavement begins to break, eventually giving way and crumbling down into the pocket. The larger the air pocket, the bigger the pothole.

Although potholes will be an unfortunate feature of our roads for the foreseeable future, researchers and engineers are working to develop more effective water resistant roads and stronger pavement.

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