Road Management & Engineering Journal
Road Management & Engineering Journal
June, 2000
TranSafety, Inc.
(360) 683-6276
Fax: (360) 335-6402

Preventing Washboarding

(This article is reproduced, with permission, from the August-September 1998 issue of Technology News, a publication of Iowa State University's Center for Transportation Research and Education. Technology News adapted the article from Special Bulletin #29 in the summer 1998 issue of The Connection, the technology transfer newsletter published by the South Dakota Local Technical Assistance Program.)

Ken Skorseth, Field Operations Manager, South Dakota Local Technical Assistance Program

One of the most aggravating gravel maintenance problems that plagues motor grader operators, managers, and elected officials is corrugation or "washboarding." This problem generally brings more complaints from the public than any other gravel maintenance problem. It not only produces an uncomfortable ride, but moderate to severe washboarding can cause a driver to have less control of his or her vehicle. It actually becomes a safety problem.


One myth is that motor graders cause washboarding. They do not! It is true that graders can cut certain distortions into a gravel surface. When an operator runs a grader too fast, the machine can begin to lope or bounce. The humps and dips this causes will be farther apart and will be cut at an angle across the roadway, the same angle that the moldboard is adjusted to while blading.

It is impossible to deal with the problem of washboarding if you don't clearly understand what the main causes are.

1.   Lack of moisture

When frequent rainfall occurs, washboarding is greatly reduced. But that is not guaranteed, and in high traffic areas just a few days without rain can really cause problems. Prolonged dry weather can cause washboarding in almost any situation, even with relatively low traffic.

2.   Traffic

People's driving habits can really aggravate washboarding. Hard acceleration or hard braking are the greatest problems. Consequently washboarding will appear at locations such as intersections, coming into or going out of sharp curves, business entrances, and sometimes even at driveways. As vehicle tires lose a firm grip on the road and begin to spin or skid just a little, a slight amount of gravel will be displaced. After this is repeated a number of times, the materials will align itself into the washboard pattern. A U.S. Forest Service study has shown that light vehicles with small wheels and light suspensions cause more washboarding than trucks.

3.   Poor quality gravel

There are several things to consider in determining quality. Washboarding will almost certainly develop it the surface gravel has poor gradation, little or no binding characteristic, and a low percentage of fractured stone.

What can we change?

We cannot predict rainfall, and in some areas prolonged dry weather can be expected. With the exception of a few special situations, it is cost prohibitive to haul water. The amount of moisture available is something we cannot change.

It is all but impossible to change the driving habits of people either. Some departments have made an effort to educate the public in this matter. The results have been disappointing. People are generally in a hurry and will continue to accelerate hard, drive fast, and apply their brakes firmly.

Of the three major causes, it becomes obvious that gravel is the only one that we can change. In prolonged dry weather, almost any section of road with a high traffic count will develop some corrugation, but good gravel will definitely reduce the problem.

What is good gravel?

Good surface gravel should have a nice blend of stone, sand, and fines. Generally, the maximum size stone should be 3/4 inch. Crushed gravel that has a high percentage of fractured stone will have much better aggregate interlock and will stay in place on the road surface better than rock with a naturally rounded shape. This also gives the road better strength. There must also be a good mix of sand-size particles and fines. The ideal blend produces a gravel that will compact into a dense, tight mass with an almost impervious surface. This will reduce washboarding dramatically.

Perhaps the least understood factor in obtaining good surface gravel is the right percentage and quality of fine material. This is the percentage of material that passes the #200 sieve. In order to resist washboarding, the gravel must have a good cohesiveness or binding characteristic. There are commercial binders available, but in South Dakota we generally rely on natural clays. A true clay, when it is separated down to individual particles, will be so fine that you cannot see the individual particles with the naked eye. These particles, when exposed to moisture, will cling together tightly, and this is what we want in our gravel.

However, some fines can also fall into the silt category and will not give the cohesive characteristic needed. The only way to determine which type of fines you have is to do one further test. The result of that test is called the plasticity index (PI). This test must be performed in a lab, but it is very valuable. Fine material that has a low PI or is actually nonplastic will not perform as well in the field. While you may be able to compact it into a dense mass, it will loosen more quickly under traffic and will cause more dust in dry weather.

The sampling and testing of gravel is the only sure way to determine gravel quality. Gravel that is short of stone will not have strength in wet weather. Too much stone will make the gravel hard to compact and it will "float" in dry weather, piling up between the wheel tracks and along the shoulders. Too few fines will not allow the gravel to form a crust, but excess fines will make the road slick in wet weather. Testing is the answer to reduce these problems.

Work to obtain good gravel

Obtaining good gravel in the field is the real challenge. Yet this is the place to begin fighting washboard problems. Start by establishing good specifications. We generally see close control of materials used in the base and the asphalt or concrete on our major constructions projects. However, when surface material is produced for the "plain old gravel road," very little attention is given to the specification. We have seen everything from no specification at all to a few cases where very good specification is established. The difference in how the material performs on the road is dramatic!

The real keys are to increase your knowledge of materials and then follow through by specifying what you want. Make this clear before you let bids for crushing and/or supplying gravel. Communicate with your supplier. Some pits or quarries do not have a good natural blend of material. In some cases, material such as clay or stone may have to be hauled in and blended at the plant. However, material can often be improved by simply working the pit differently.

Sometimes changes have to be made while the material is being produced. We are aware of one situation where clay on the surface of a pit became too wet to process through the crusher. The contractor and the buyers agreed on an arrangement to rent an ag tractor and chisel plow to use for drying the clay quickly in order to process it into the gravel. This increased the cost of the material, but they knew the long-term benefit would be better gravel that would require less blading, would remain bound and stay in place longer, and would reduce washboarding.

Don't overemphasize a cheap initial cost for material. You will pay either way: by purchasing cheaper material up front, spending more to maintain and replace it over the years, and taking more complaints from the public, or by paying more for quality material that requires less maintenance, lasts longer, and generates fewer complaints. Remember also that trucking is often 70 percent or more of the total cost of gravel placed on the road. Spending more to increase the quality of the gravel itself does not change the total cost as much as you might think.

We also understand that truly good quality gravel is very hard to obtain in certain areas. At the very least, you should consider hauling the best material you can find to real washboard trouble spots. Use regular material available to you for the rest of the road system. For example, one township in South Dakota used millings (recycled asphalt) near busy intersections and found that this material reduced washboarding dramatically. They certainly could not afford this for the whole road system, but they found it cost effective for troublesome areas.

Corrugations that extend from one side of the road to the other create headaches for drivers, motor grader operators, and road superintendents.

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