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 8.0 kilometres per hour (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.

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.

Laboratory-scale studies of the phenomenon typically employ a wheel or a blade, which is towed behind a pivot point and allowed to bounce over the material studied as it follows a circular path in a pan, holding the material. These studies have investigated a variety of granular and viscous, even fluid, materials. In the laboratory, washboarding has been studied for a range of parameters, including the thickness and grain size of the material for varied wheel sizes, shapes, and masses. Experiments produced ripples for each parameter, above a threshold speed, when the wheel (or blade) began to bounce. Experiments also show that the pattern can move either against the direction of motion or in the direction of motion. They also show that a passive, non-driving wheel suffices to create corrugations and that displacement of material, rather than ejection is the dominant mechanism.

Several articles addressing real-life washboarding on roads cite South Dakota Local Transportation Assistance Program (LTAP) Special Bulletin #29, “Dealing with Washboarding,” by Ken Skorseth. These sources attribute washboarding to three causes: dry granular materials, vehicle speeds, and the quality of the granular material. Other cited causes include vehicle speed, traffic volume, and hard acceleration or braking. Also, the source claims that light vehicles have a bigger effect on producing washboarding than heavy trucks.

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Wikipedia