Article on road design fundimentals
What is Needed for All-Weather Roads and Driveways?
Modern agriculture, especially dairy farming, is rapidly making the old-fashioned dirt road obsolete. It doesn’t take 20/20 vision to see the role of bulk milk trucks, fuel trucks, feed trucks and school buses in our farm economy and their effect on our roads and driveways.
Let’s consider briefly some of the elements of building a good all-weather rural road. Here is a check list for your consideration.
1. Provide a Good Location
Highway engineers have said that “the only permanent part of a road is line and grade.” Permanent is a pretty strong word, but our first consideration should be that our road or drive is properly located. In selecting a location, let’s not think only of Grandfather’s route; remember that Grandfather may have been satisfied with 15 miles per hour travel. Instead, we should look ahead and take out sharp curves, ease steep grades and keep sight distance at a maximum.
2. Adequate Clearing of Right-of-Way
To reconstruct a road and leave the brush is extremely shortsighted. Among the dangers of a poorly-cleared roadside are these:
brush, weeds or even trees on the right-of-way may limit sight distance and impair safety of travel.
brush or high weeds may cause severe snow drifting during the winter months, and
the roots of woody vegetation act much like a sponge and hamper drainage of the roadway
3. Keep a Raised Grade Line Where Possible
One of the best insurance policies for good road performance is a raised grade line. If you’ll keep a close eye on road failures next spring, you’ll find that most of the failures occur in road cuts or on flat cross sections. A raised grade line improves drainage, minimizes frost damages and, because of its streamlined cross section, discourages the drifting of snow in the roadway.
4. Don’t Skimp on Road Width:
Using economy as an excuse, some local governmental units are still building and maintaining roads 12 to 14 feet wide. This is likely to be false economy. If all-weather travel is requisite, if vehicles larger than passenger cars use the road, or if the daily traffic volume exceeds 50 to 100 vehicles per day, wider roads are essential.
These wider roads provide room for storage of plowed snow during the winter months, permit safe passing of other vehicles without “two wheels in the ditch” and minimize shoulder rutting caused by vehicles traveling too close to the road edge. Roads carrying 50 to 200 vehicles per day should be at least 16 feet wide with 2-foot shoulders. If traffic exceeds 200 vehicles per day, the road width should be at least 18 feet with 5-foot shoulders.
5. Keep Surface Water Moving
The basement soil or subgrade should be crowned to a slope of ½ inch per foot toward the road edge. The road surface should be crowned ½ inch per foot if it is a gravel road or ¼ inch per foot if it is to receive a bituminous surfacing. The road cross slope should be uninterrupted from the center of the road to the ditch line. Rutted or built up shoulders will trap surface water and softening of the roadway may result.
6. Intercept Subsurface Seepage
In cut sections or other sidehill areas where seepage is encountered, subdrains may be used to intercept the water before it reaches the roadway. Subdrains should be placed in the shoulder area rather than the ditch bottom and should be backfilled with a graded coarse sand to minimize intrusion of fine soil into the backfill and pipe.
7. Provide an Adequate Foundation Thickness
Generally there are two types of road construction. A rigid pavement consists of a concrete slab. A flexible pavement consists of a gravel or stone foundation with or without a bituminous surfacing. This discussion only concerns flexible pavement.
The thickness of a gravel or crushed stone foundation depends on the size and number of heavy wheel loads to be carried and the character of the subgrade soil.
General recommendations are summarized as follows:
Thickness (unbound layers only)
Soil Type Wheel Load
School buses or delivery trucks only Large trucks or a high percentage of trucks
Very soft silt or clay 15 inches 21 inches
Sandy silt or clay 10 inches 15 inches
Clean sand or gravel 6 inches 9 inches
It may be necessary to alter these general design thicknesses where severe frost action is a problem such as in most of New York State. In such a location, the minimum depth of the unbound layer should be twelve (12”) inches. Ideally, relatively clean foundation material should extend to the depth of frost penetration. Some engineers provide less frost protection, recommending that a clean granular foundation material be provided to a depth of one-half the expected frost penetration.
Our discussion of foundation courses concerns the importing of a suitable gravel or crushed stone. Where such materials are not readily available, it is possible to strengthen the subgrade soil by incorporating such materials as asphalt, Portland Cement, lime, and so forth.
8. Consider a Sand Subbase
Once the needed foundation thickness is established, we must decide what types of materials we will use to build the foundation. If an open graded stone or gravel is used for the full depth of foundation, a very undesirable condition may develop. Under wheel loads, a soft muddy subgrade may ooze into the base and reduce its effective thickness. This condition may be prevented if the lower 4-6 inches of the foundation course is built with a clean, well-graded sand, Such a sand subbase serves to filter out a saturated subgrade. It is advisable to extend the sand subbase through the shoulder area.
9. Select a Suitable Gravel:
The proper selection of a suitable local gravel for the base or upper part of the foundation course is important. The gradation, or distribution of the gravel, sand, and silt-clay sizes, is a good measure of quality. We can readily examine local gravel sources to determine the approximate percent, by weight, of (1) gravel, (2) sand, and (3) silt and clay.
General Specifications for Gradation
For gravel roads Gravel 50-70%, Sand 25-40%, Silt and clay 8-15% Max. of 8%
For bases to receive a bituminous surfacing Gravel 50-70%, Sand 25-40%, Silt and clay 8-15% Max. of 8%
Notice that less silt and clay is permitted in bases to receive a bituminous surfacing. When there is a large amount of heavy trucks, the silt-clay content of bases should not exceed 5 percent in the top 6 inches. The gravel foundation should be thoroughly compacted in lifts not exceeding 6 inches for best compaction. The material should be moist during rolling operations.
10. Dust Palliatives & Bituminous Surfacings
An untreated gravel road may be expected to dust and wear about ¾ inch per year. On roads carrying over 50 vehicles per day, it will be advisable to control dusting and wear with a dust palliative. Suitable palliatives include calcium chloride, rock salt, bituminous materials, lignates, and so forth. The palliatives require periodic re-treatment. When traffic exceeds perhaps 100-200 vehicles per day, we should consider a heavier bituminous surfacing. The choice of type of bituminous surfacing depends on expected traffic voices and the character of locally available aggregates.
This article, originally written by our first Director over forty years ago, is still applicable today. We have updated a few details. For instance, we no longer use tar as a surfacing material.
Updated and reprinted by permission from the July 10, 1960 issue of Hoard’s Dairyman. Copyright 1960, W.D. Hoard and Sons, Co., Fort Atkinson, WI.
From Cornell's better roads program.