Article on road materials
Questions with Answers from a Gravel Road Expert
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.
(1) 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.
(2) 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.
(3) 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.).
(4) 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]