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Road Management & Engineering Journal |
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June, 2000 TranSafety, Inc. (360) 683-6276 Fax: (360) 335-6402 |
Ken Skorseth, Field Operations Manager, South Dakota Local Technical Assistance Program
This article is part of a continuing series of tips for motor grader operators.
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.
![]() Where the dust flies like this, it's probably too dry to do extensive maintenance on gravel roads. |
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.