The evils from improper construction of stone roads are even greater than those resulting from the use of improper material. Macadam never intended that a heterogeneous conglomeration of stones and mud should be called a macadam road. The mistake is often made of depositing broken stone on an old road without first preparing a suitable foundation. The result, in most cases, is that the dirt and mud prevent the stone from packing and by the action of traffic ooze to the surface, while the stones sink deeper and deeper, leaving the road as bad as before.
Another great mistake is often made of spreading large and small stones over a well-graded and well-drained foundation and leaving them thus for traffic to consolidate. The surface of a road left in this manner is often kept in constant turmoil by the larger stones, which work themselves to the surface and are knocked hither and thither by the wheels of vehicles and the feet of animals. These plans of construction cannot be too severely condemned.
The roadbed should be first graded, then carefully surface-drained. The earth should then be excavated to the depth to which material is to be spread on and the foundation properly shaped and sloped each way from the center so as to discharge any water which may percolate through. This curvature should conform to the curvature of the finished road. A shouldering of firm earth or gravel should be left or made on each side to hold the material in place, and should extend to the gutters at the same curvature as the finished road. The foundation should then be rolled until hard and smooth.
Upon this bed spread a layer of five or six inches of broken stone, which stone should be free from any earthy mixture. This layer should be thoroughly rolled until compact and firm. Stone may be hauled from the stone-crusher bins or from the stone piles in ordinary wheelbarrows or from wagons, and should be distributed broadcast over the surface with shovels, and all inequalities leveled up by the use of rakes. If this method of spreading is employed, grade stakes should be used so as to insure a uniformity of thickness. After the stakes are driven the height of the layer is marked on their sides, and if thought necessary a piece of stout cord is stretched from stake to stake, showing the exact height to which the layer should be spread. Spreading carts have been recently invented which not only place the stone where it is needed without the use of shovels, but spread it on in layers of any desired thickness and at the same time several inches wider than the carts themselves.
If the stones have been separated into two or three different sizes, the largest size should compose the bottom layer, the next size the second layer, etc. The surface of each course or layer should be thoroughly and repeatedly rolled and sprinkled until it becomes firm, compact, and smooth. The first layer, however, should not be sprinkled, as the water is liable to soften the foundation. The rolling ought to be done along the side lines first, gradually working toward the center as the job is being completed. In rolling the last course it is well to begin by rolling first the shoulderings or the side roads if such exist.
A coat of three-quarter inch stone and screenings, of sufficient thickness to make a smooth and uniform surface, should compose the last course, and, like the other layers, should be rolled until perfectly firm and smooth. As a final test of perfection, a small stone placed on the surface will be crushed before being driven into the material.
If none of the stones used be larger than will pass through a two-inch ring, they can be spread on in layers as above described without separating them by screens. Water and binding material—stone screenings or good packing gravel—can be added if found necessary for proper consolidation. Earth or clay should never be used for a binding material. Enough water should be sprinkled on to wash in and fill all voids between the broken stones with binding material and to leave such material damp enough to insure a set.
If a road is built of tough, hard stone, and if the binding material has the same characteristics, a steam roller is essential for speedy results. A horse roller may be used to good advantage if the softer varieties of stone are employed. For general purposes a roller weighing from eight to twelve tons is all that is necessary. Heavier weights are difficult to handle upon unimproved surfaces unless they be constructed like the Addison roller, the weight of which can be increased or lightened at will by filling the drum with water or drawing the water out. This roller can be made to weigh as much as eight tons and, like several other very excellent ones now on the market, is provided with anti-friction roller bearings, which lighten the draft considerably.
Every stone road, unless properly built with small stones and just enough binding material to fill the voids, presents a honeycombed appearance. In fact, a measure containing two cubic feet of broken stone will hold in addition one cubic foot of water, and a cubic yard of broken macadam will weigh just about one-half as much as a solid cubic yard of the same kind of stone. Isaac Potter says:
"To insure a solid roadway and to fill the large proportion of voids or interstices between the different pieces of broken stone, some finer material must be introduced into the structure of the roadway, and this material is usually called a binder, or by some road-makers a 'filler.'