Almost any gravel deposit can be utilized in some way if the material is of a durable nature, regardless of other characteristics. The serviceability of a gravel road will depend largely on how nearly the gravel approaches the ideal, but variations in the manipulations will do much to overcome deficiencies in materials.
Thickness of Layer.—The thickness of the layer of gravel required depends both upon the type of soil upon which it is placed and the nature of the traffic to which the road will be subjected. Gravel surfaces should not ordinarily be constructed on highways carrying heavy truck traffic, but if gross loads of three or four tons are the heaviest anticipated, the gravel will be reasonably stable. On such roads, a layer of well compacted gravel ten inches thick will support the loads if a well drained earth foundation is provided. If but little truck traffic is anticipated and loads up to three tons on steel tires are the average, a layer 8 inches thick will be sufficient. In dry climates, a layer six inches thick will be adequate if it can be kept from raveling.
On secondary roads, carrying principally farm-to-market traffic, and not a great volume of that, the above thicknesses may be reduced about one-fourth.
The exact thickness needed for any particular road is a matter for special study on account of the variations in the gravels and in the supporting power of the soil upon which they are placed.
Placing Gravel
Preparation of the Road.—The roadway that is to be surfaced with gravel is first brought to the desired grade and cross section. It would be advantageous if this could be done a year before the gravel is placed so that no settlement of the earth foundation would occur after the gravel surface is completed. But if that is impractical, the grading may be done just prior to placing the gravel, providing appropriate methods are adopted for securing compacted fills.
Trench Method.—Two distinct methods of placing the gravel are in general use, known as the trench method and the surface or feather edge method respectively. The method to adopt for any particular road will depend largely on certain conditions that will be explained later.
In the trench method, a trench of the proper width and depth for receiving the gravel is excavated in the earth road surface and the gravel is placed therein.
The trench is formed by plowing a few furrows and scraping out the loosened earth with a blade grader. The loose material is generally moved out laterally to build up earth berms or "shoulders" alongside the gravel. Into this trench the gravel is dumped in the proper quantity to give the required thickness after being compacted.
The greatest care must be exercised in spreading the gravel to eliminate unevenness where the loads were deposited. An ordinary blade grader is one of the best and most economical implements to use for spreading the gravel. When the gravel has been deposited in the trench for a distance of a thousand feet or more, the spreading is accomplished by dragging the surface repeatedly with the blade grader, the work being continued until all waviness disappears. The gravel is then thoroughly and repeatedly harrowed with a heavy stiff tooth harrow to mix thoroughly the fine and coarse gravel so as to produce as nearly a uniform mixture as may be. The gravel is then finally smoothed with the blade grader.