Road Bumps: Why dirt roads develop a washboard surface

Driving on a dirt road can rattle the bones. Every foot or so, a ridge of dirt up to several inches high lies in wait to jolt passing cars and trucks and their hapless occupants. In many places, road crews battle this “washboard” effect by frequently scraping the roads with bulldozers. But as soon as more vehicles pass, the ridges, phoenixlike, return.

TROUBLED TRAVEL. Vehicles passing constantly over dirt roads create an irksome washboard surface, seen here on a road in Australia. D. Mays

Now, a team of physicists has explained why a washboard forms, and their research has a dispiriting message for road crews: Scrape often, or give up. Washboard is inevitable.

Most previous theories of washboard formation involved relatively complex dynamics. Some focused on the bounce of a vehicle’s suspension and tires. Others suggested that differences in compaction between the bottoms and tops of bumps were essential. Still other theories invoked the tendency of dirt to segregate according to grain size. Many an engineer has tried to design washboard-resistant road surfaces, but the ridges keep rising.

Stephen W. Morris of the University of Toronto and his colleagues Nicolas Taberlet and Jim N. McElwaine of the University of Cambridge in England aimed to find the simplest possible explanation for the phenomenon. They built a circular turntable that they could cover with dirt or sand, and positioned a hard rubber wheel above it.

After smoothing the dirt, they turned the table at varying speeds, allowing the wheel to run over the surface. Then they watched the washboard form.

The researchers varied the experiment in every way they could think of. They compacted the dirt. They used sand grains of varying sizes and mixtures, and they even tried substituting rice. They used wheels of different sizes and weights as well as a flat plow-wheel that didn’t spin. Some of the variations changed the pattern, spreading the ripples or packing them closer together, but the ripples always formed.

The team reports its findings in the Aug. 10 Physical Review Letters.

The researchers found one, and only one, solution: Slow down. A lot. “The critical velocity below which [the surface] would remain flat is about 5 miles per hour,” Taberlet says.

The researchers then created a computer simulation to model the movements of individual grains of sand so that they could see precisely how the ripples formed.

Any bed of dirt or sand, even a very smooth one, has minuscule irregularities that slightly jog a rolling wheel. Each time the wheel hits a bump, the computer simulation showed, it pushes the dirt forward a bit, enlarging the irregularity. Then, as the wheel passes over the top of the bump, the force of its descent pushes dirt forward into the next bump. Repeat these actions a hundred or more times and the familiar pattern of ridges appears.

Douglas Kurtze of Saint Joseph’s University in Philadelphia says that this is the first time anyone has studied washboard formation using a controlled experiment. Although it won’t eliminate washboards, it lets scientists “get down to the essentials of what the mechanism is,” says Kurtze.

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