Water sitting on a gravel surface causes many problems when it is worked on by passing vehicles. Even a small amount of water sitting in a depression on a traveled way can cause a pothole to develop or can become mixed up with the road surface to create another hazard to travel — mud. Therefore, the primary maintenance concern for most gravel roads and driveways is shaping the surface to get water to run off and away as quickly and effectively as possible.
The more a road surface is sloped, the quicker it will shed water and less likely it will become potholed or muddy, within reason. Maintenance and safety considerations limit the recommended pitch for the surface of unpaved roads to a range of 3/8 to 3/4 inch per foot of lane width. Rounding off to 1/2 inch per foot for practical reasons, this means that a 20 foot wide roadbed (two 10 foot lanes) should have a centerline that is five inches higher than the edge of the road. That is 1/2 inch of rise for each foot of width from the center to the edge. If properly maintained, this “crown” is adequate to inhibit the formation of potholes and other surface breakdown. By contrast, paved roads generally require only half this pitch, or ¼” per foot or 2 ½” for a 10′ lane. Other paved surfaces like driveways and parking lots require even less pitch to shed water, about one inch per ten feet. The integrity of the paving material is generally adequate to keep potholes from forming, even if there is an occasional “birdbath”.
For the most part, typical older rural roads have a crown that is gently rounded in the center whether they are paved or unpaved. Paved roads that have this shape began as gravel roads and were paved by the old fashioned method of mixing the asphalt in place over the gravel rather than having it placed by a modern paving machine. Because of the stabilizing effect of the pavement, this rounded shape usually presents little problem. Gravel roads with this “round crown,” are a different story, however.
With most gravel roads, the 1/2″/ft crown should continue over the entire road width which means the surface needs to be pitched at this grade across the entire traveled way from one shoulder to the other. There are a few situations, such as curves on higher volume roads, that warrant maintaining this pitch in just one plane across the entire road, making the whole surface drain toward the inside of the curve because the outside of the road is higher. Most roads, however, break the pitch in the middle, requiring the “A” shaped or “teepee” crown shown in the accompanying diagram and not the rounded crown that is so common.
What happens to a road surface that has a “rounded crown” rather than the recommended “A” shaped crown? In a word, Potholes! Where do they occur? In the middle of the road. Why? Lack of drainage on a very critical area!
Looking at the cross section of a road with a rounded crown it becomes obvious that the center of the road is actually flat rather than crowned. The more gentle the crown of the road is, the greater the area that is flat, or nearly flat. If traffic would stay off this centerline area this lack of pitch would not be a problem but that isn’t usually the case with low volume, unpaved country roads.
Many gravel roads started as wagon paths and have had little done to them except to add gravel and, perhaps, have a ditch cut on each side. Most are no more than a lane and a half in width. This means that most cars always drive with one set of wheels (the left ones) in the center of the road. When a vehicle approaches from the opposite direction, both drivers simply swerve slightly to accommodate the other and then return the left wheels to the center of the road. Even if the road was built as two full lanes, poor ditch maintenance or brush encroachment often pushes drivers toward the centerline. Lack of a yellow line seems to have the same effect.
This scenario means that the center of the road receives twice the traffic of the ditch side if every car’s left wheels travel this section. Couple this with a lack of pitch to shed water, and centerline potholes are inevitable. Proper pitch on the part of the road closer to the ditch can certainly help the passenger side, but considering that cars almost always have drivers on the left side, it would make sense to ensure a smooth ride for them also.
CONSTRUCTING THE CROWN
Ernie Danforth, former grader training instructor for the State of Maine and occasionally New Hampshire and Vermont, had mastered the art of the “A” shaped crown. His newly graded roads looked rather extreme to the uninitiated, but Ernie knew that what he was doing was the only way to prevent center line potholes. He did all of his work strictly with a grader and knew how to be successful with this technique.
Fortunately, most road graders permit positioning the blade in such a manner that constructing a proper crown is fairly straightforward when the operator does know what he’s doing. This is not the case with many of the other devices used to regrade roads when a grader is not available, such as front or rear mounted or tow-behind rakes or grader blades.
Ernie never wanted anyone to work on his roads with a rake because he knew that many “operators” would shave his crown right back toward the ditch! In too many cases Ernie’s concern was justified. Many operators, even those who pull shoulder material to the center to establish the crown as they should, then work rocks out of the surface without paying any attention to maintaining proper crown. By following just one recommendation they could maintain the “A” crown no matter whether the tool be in front or behind them.
After material has been pulled to the center of the road to reestablish the crown, the rule to follow is never to straddle the centerline with the wheels of the vehicle to which the tool is attached unless the rake or blade can be adjusted in the same manner as a road grader. Violating this rule generally means cutting down the centerline crown while attempting to smooth the surface. It is possible, however, to remove rocks and debris and cast them all the way from one side of a road to the other with almost any vehicle without destroying the crown.
GETTING RID OF THE ROCKS WITHOUT LOSING THE CROWN…
Simply angle the tool (preferably a rake for this finishing process) towards the center of the road, casting debris toward the opposite side. Keep the vehicle just to the side of center, never letting any of the wheels supporting the tool cross the middle of the road. Usually this means running the vehicle and attachment supporting wheels right down the centerline which also compacts the gravel that has just been pulled in from the sides. This will rake rocks and debris over the center and start them down the opposite side of the crown. Then come back with the vehicle just to the other side of center (again without straddling it) and angle the rocks toward the shoulder.
Rocks and debris can also be worked toward either shoulder or the middle of the road for pickup without taking off the crown simply by keeping the operating vehicle to either side of the middle, treating each side as if it were a completely separate grade which, in fact, is the goal of the entire operation.
MEASURING THE CROWN…
With all the high tech equipment available today for precision leveling, the simplest method of checking for proper crown is still a straight piece of wood or metal that is ten feet long and a carpenter’s level. Set one end of your “gauge” in the middle of the road and prop the other end up until level using a rock or the toe of your boot and measure the distance from the end of the gauge to the ground. If it’s five inches, you’ve got your crown!
5 thoughts on “Getting your Crown in Shape”
Great! Now, how to find a contractor with this training and, hopefully some experience.
We’ve had our share of; “Oh, sure. I can do that.” With a leveled surface result. Of course, this leads to a repeat of business and an opportunity to pitch for a paved driveway.
America was sold a bill of goods by the blacktop industry. Getting rid of gravel roads begins with getting rid of the training of maintenance.
Great article, very informative. What is your opinion on using drainage tile in conjunction with a soft or gentle crown?
If I understand your question correctly you are addressing two totally different issues. The crown is to handle rain as it is falling, drainage tile is to address groundwater.
I have recurring ruts and “mudpits” that take forever (or never) to dry out. I graded it to the solid surface and I think that If I just add dirt and shape it to a crown that it will still devolve in to a muddy mess. My thinking is a crown together with drainage tile so that when it rains, the water runs off, but also when the water table gets too high it is diverted. This way, the soil never gets saturated enough to lose its shape and dries quickly. Thoughts?
Great article, , but it still leaves me with a basic question: Does a “tar and chip” driveway need a crown, and if so, what is the grade in terms of incline per foot? Ours is about 80 yards long, with a fairly steep hill on one side. Would it be OK to have no crown if the entire road surface was inclined slightly to one side (as opposed to both sides falling away from the center)? Any advice would be MUCH appreciated!