-
-
"Crossroads," the newsletter of the Wisconsin Transportation Information Center,
shared answers to questions on gravel roads in their Summer 1997 issue. Below is
that article, which they entitled "Gravel road Q & A."
Ken Skorseth, a gravel road expert from South Dakota, talked about maintaining gravel
roads at a T.I.C. [Transportation Information Center] workshop presented over the
statewide Educational Telephone Network. Here are some of the questions from
participants in the workshop and his answers.
Is it a good idea to recycle blacktop and mix in sand and fines to make road
gravel?
Old asphalt that has been run through a crusher is well sized. I have seen some
excellent results if you simply place it as surface gravel. Don't try to place it in a thin lift
because it sometimes takes on the characteristics of asphalt pavement again,
developing potholes and resisting blading except during a rain. Four inches is the
minimum lift depth. Alternatively, a 50-50 blend with virgin gravel is excellent because
it can be bladed more easily, but has excellent binding characteristics. It reduced
maintenance by over 50%. Place this in a lift of three inches over a three-inch base.
What blend of different size aggregates do you recommend?
You want a blend of gravel, sand, and silt/clay. In the base you want 40-80% hard
stone graded from 1/4 to 3 inches in diameter, but on the surface use smaller size
stone. The standard state specs call for 3/4 inch aggregate for surface gravel (crushed
stone) with 20-60% sand (less than 1/4 inch) and 8-15% fines. The specifications are
in Sec. 304.2.6 of 1996 State Standard Specifications.
What experience have you had paving gravel roads?
A good gravel road is not necessarily a good base for asphalt. There are problems
with excess fines, crown, and depth. While surface gravel needs a good percentage of
fines to give it a binding characteristic, base material needs cleaner material with fewer
fines.
Gravel roads have a greater crown than you want on an asphalt surface. Unless you
reshape them you can have problems. For example, I've seen some sealcoated roads
where after ice storms cars would slide off.
An average gravel road has about four inches of surface gravel. This is not adequate
base for an asphalt surface. If you have any truck traffic at all, you need a minimum of
six inches of base, and preferably eight. Otherwise you will have tremendous breakup
problems.
What good binding material can we add to the 3/4 inch gravel for our roads?
Clay is the best natural binder, but be careful. It can be easy to mistake silts and clay
when you're out in the field. Take the material to the lab. Clay, which you want, will
have tremendous cohesion which shows up on lab tests as a good plasticity index
(P.I.).
When preparing a subgrade we ran across clay pockets. To what depth should
we excavate to remove the clay and replace it with cleaner material such as
sand?
You may not need to excavate if the road is a minimum of two feet above the
surrounding terrain and you have good ditches. Otherwise, excavate six inches and
install a geotextile. Cover it with any type of gravel, then put two to three inches of
surface gavel over that. The geotextile prevents the clay from pumping up through the
gravel. If you're not using geotextile, excavate down two feet.
[Reprinted with permission from the Summer 1997 "Crossroads," a publication of the
Wisconsin Transportation Information Center]
