Washboarding

Washboarding or corrugation is the occurrence of periodic, transverse ripples in the surface of gravel and dirt roads. Washboarding occurs in dry, granular road material with repeated traffic, traveling at speeds above 5 mph. Washboarding creates an uncomfortable ride for the occupants of traversing vehicles and hazardous driving conditions for vehicles that travel too fast to maintain traction and control.

Mechanism

Washboarding or corrugation of roads comprises a series of ripples, which occur with the passage of wheels rolling over unpaved roads at speeds sufficient to cause bouncing of the wheel on the initially unrippled surface. Most studies of washboarding pertain to granular materials, including sand and gravel. However, other work suggests that the phenomenon may occur in material, which has some binding of particles, e.g. clayey soils.

Highway department experts in the mid-1920s were aware that traffic volume and speed were primary causes of corrugations on gravel roads and cited the role of drive wheels tossing material as a factor.