Getting Rid of Washboards

It is not always possible to haul new and better quality gravel to reduce our washboarding problems. What can a grader operator do to reduce the problem? Once again, the operator has to do whatever is possible to change the material. Simply blading over washboards and filling the depressions between the ridges is almost useless. The best way to handle this is to cut all of the material loose to a depth of one inch or more below the bottom of the washboard area. This also brings up some fines to mix with the surface material. Then re-lay the material to the proper crown and shape. But remember that one cause of washboarding is dry conditions. This should never be done without good moisture in the material. It may pay to quickly run to the problem areas after a good rain, work them, and then resume normal blading.

Another useful tool is the replaceable bit-type cutting edge. This type of cutting edge tends to have a shallow scarifying effect and makes it easier to cut material loose and mix it. One of the most effective ways we've seen these used is on a front mounted dozer blade. The operator can drop the dozer to cut out a washboard area and use the moldboard to shape the area. The use of a conventional scarifier also works, but be careful about going too deep and bringing up dirt and large rock from the subgrade. This will contaminate the gravel.

Another method in trying to change the gradation of material is to pull in material from the shoulder area of the roadway and mix it with the loose gravel on the surface. This works best in the spring before too much vegetation grows on the shoulder and moisture is present. This material is generally not the best binder, but it does have some benefit in restoring fines to the gravel.

There are also a couple of more advanced methods that work well, but are probably affordable only in high traffic locations. One of these is treatment of the gravel with either calcium or magnesium chloride. These products are not binders, but they are a tremendous aid in keeping gravel in place. They work by simply drawing moisture from the air. The real key to success with these products is to treat gravel that has a very nice gradation, particularly a good natural binding characteristic. The chlorides then take over by keeping the surface slightly damp and the gravel will remain tightly bound.

The other method is to use reclaimed asphalt as part of your surface gravel. This product is not available everywhere, but as more of our pavements reach the end of their lives, they are being recycled in various ways and the material is sometimes available to local agencies. It is usually a high quality product. We have seen the best results with a 50/50 blend of recycled asphalt and virgin gravel. In this mix, the asphalt becomes the binder and the material usually has a good binding characteristic and will resist washboarding, yet it can still be worked with a grader. It should be placed at a minimum compacted depth of three inches. If this is not affordable for a whole section of road, it works well in trouble spots.

Here is another tip. When placing new material on a washboarded area, always cut and rework the area before adding the new material. If this is not done, the washboard pattern in the original surface will invariably reflect right up to the new surface and your problems begin all over again. It is also important to have the road properly crowned and shaped. Sometimes the original material will have to be cast to the side and used as shouldering material since adding a depth of new material will make the finished surface too high.

There are some things we can do on the road to modify the gradation of gravel by scarifying, pulling more fines from the shoulders, etc., but the real key is to make a great effort to get a high quality surface gravel in place, particularly in washboard- prone areas. With high traffic in prolonged dry periods, even this will not guarantee elimination of washboarding, but it will definitely reduce it. There will also be a real bonus in reducing blading requirements, less material loss from whip-off, and less dusting.

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

This article is from the August-September 1998 issue of Technology News, a publication of Iowa State University's Center for Transportation Research and Education.