Washboarding

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Washboarding is a process which results in roads (particularly gravel roads or dirt roads) developing a series of regular bumps with short spacing in the road surface.

This effect is called road corrugation in Australia.

In scientific or engineering terms, washboarding is a feedback loop between the natural frequency of vibration of the vehicle suspensions and the long term response of road surfaces to high loads.

Individual potholes are typically created by overly soft road subsurface materials. The weight of passing vehicles causes material to shift away from the softest spots in the road.

Washboarding is then caused from initial potholes by a reaction between the road surface and vehicle suspensions. As cars encounter a pothole, their wheels dip down into the pothole and then back up again, causing a bounce in the car. The car bounces down into the pothole, up out of the pothole, and then comes back down again slightly farther down the road. Where the car comes back down, another soft spot develops in the road due to the higher impact forces at that point.

Over time, this forms another pothole, which then causes another bounce up and down, and eventually another pothole similarly farther down the road. Over extended periods, an initial disturbance or pothole will create a regular pattern, with the spacing depending on the typical vehicle speed on that road and suspension natural frequency.

Other natural processes could cause washboarding, but automobile and other motor vehicle traffic seem to do so much faster than other causes.