Why Do Roads Corrugate?

Washboard roads are a bane to travelers worldwide. This includes, of course, Alaska, where the options for getting away from it all would be limited indeed if gravel roads are excluded from one's itinerary.

Corrugations that produce washboard roads are not limited to those with sand or gravel surfaces, but are also found in asphalt pavements and even in railroad tracks, although on a less severe scale.

It would seem that such a common phenomenon should be readily understood and explainable, but the fact is that the process which produces them was a subject of controversy among engineers for many years. One of the most popular theories was that a tire, even as it rolls, pushes material ahead of it in a heap. Once the pile reaches a certain size, the tire rolls over it and starts the process again. As it developed, this is incorrect.

The January 1963 issue of Scientific American contains an article by Dr. Keith B. Mather, now Vice Chancellor for Research and Advanced Studies at the University of Alaska, which puts the matter to rest once and for all.

Working at the University of Melbourne, Mather observed that vehicles passing over the unsurfaced roads of Australia's "outback" did not produce dust uniformly even on uncorrugated roads, but rather in little spurts arising from rapid bouncing of the wheels.

This led to the construction of a laboratory apparatus which would permit the observation of wheel and road interactions under controlled conditions. The first experiments utilized a five-inch wheel mounted on the end of a shaft which pivoted about the center of a sand track 24 inches in diameter. Locomotive forces were provided by pushing the arm around the track with a finger. Unexpectedly, this soon produced little corrugations several inches apart in the sand.

Encouraged by these results, Mather then proceeded to construct a somewhat more elaborate system equipped with a variable speed electric motor, which drove the axle, a spring-mounted wheel and a revolution counter. Parameters such as weight, size of wheel and stiffness of spring were made independently adjustable.

Among the more significant findings were that:

1. If the wheel moves slowly, no corrugations were formed, but a deep rut instead;
2. it did not matter whether the wheel was driving or idling, at sufficient speeds, washboarding occurred;
3. the trough-to-trough distances between ruts increased linearly with increasing speed; and
4. sand was not pushed ahead of the wheel and then overridden to begin another cycle, as had been commonly believed.

The most important contribution to understanding washboarding lay in the observation of how the corrugations are actually formed. When the wheel reaches a certain critical speed, it begins to move in short hops, bounding on random irregularities of the surface. Hitting an obstacle, even a small one, propels the wheel into the air for a certain distance. When it lands further down the track, it sprays sand forward and to the side, thus creating the beginning of a crater. Each time it digs itself in at a crater it has to ride out again and thus repeats the pattern. If traffic were to move at widely diversified speeds, different "hop-lengths" might tend to cancel each other out, but depending on road conditions, all traffic tends to travel in a rather closely constrained speed range, thus compounding the problem with each successive vehicle.

Corrugated roads would be all but eliminated if people followed two simple rules. First, they must let most of the air out of their tires (hard tires corrugate roads faster), and second, they must be willing to travel at less than ten miles per hour. Since it seems unlikely that either of these guidelines would be followed, the only thing that remains is to construct all rural highways out of three-foot-thick reinforced concrete--not a likely prospect for the foreseeable future.