In the improvement of a country road, where the construction of a good Telford or macadam road can not be undertaken, a surface of gravel may frequently be used to advantage, giving much better results than could be obtained with the surface of earth. Even a light layer of gravel may frequently prove of very great benefit.
Where the subsoil is of a porous nature and well drained, a layer of three or four inches of gravel, or sometimes even less, well compacted, will constitute a very considerable improvement, especially if, as is usual with these light soils, the nature of the mate rial of the road-bed is particularly unsuitable for the wearing-surface, difficult to compact sufficiently to shed water, and likely to become soft ,when wet.
Gravel for use on roads should be of hard, tough material, capable of resisting the abrasion of traffic. Natural gravels May differ widely in the character of the materials composing them, and in many instances are harder and more durable than the native stone of the same locality. Nearly any gravel will be an improvement upon an ordinary road surface, but where an important road is being improved the material should be carefully selected. The size of pebbles composing the gravel is important in considering its value for road purposes. As a general thing they should not be more than z inch, or at most i inches, in greatest dimension. The size should not be too uniform, but the gravel should contain enough small fragments to fill the interstices between the larger pieces, in order that it may pack well in the road. When the gravel is too fine or too uniform, it will not bond properly, and will be difficult to compact into a hard surface. The proper gradation of sizes is the most important characteristic of good gravel. The larger pieces are usually the hardest and most durable part of the gravel. They have resisted the grinding action which has reduced the other material to smaller fragments. It is desirable therefore that there be only enough fine material to fill the interstices in that of larger size. When fine material is in considerable excess the gravel should be screened in order to get the best results. In many instances, it is possible to greatly improve the quality of gravel by screening into two or three sizes and then recombining these in proper proportions to produce the most dense material.
Binder. In order to bind well in the road surface, the small spaces between the fragments of gravel must be filled with fine material. Without this the frag ments composing the gravel will roll upon each other and not pack well. Natural gravel may contain enough fine material or soft material which will crush under the loads coming upon it to cause it to bind well in the road; or it may be necessary to add some material to the gravel surface to act as a binder. Clay, loam, or stone screenings may be used as a binder.
It is desirable to use as little binder as is consistent with the proper bonding of the gravel. When in excess it has a tendency to cause the road to soften in wet weather and to crack in dry weather. This is espe cially noticeable with clay binder. If gravel contains too much fine material, or when the fine material is unevenly distributed through the gravel, it should be passed over a one half inch screen, and the fine part thus removed be used on the surface as a binder.
When gravel contains considerable large material, a screen of i I to i inches mesh may be used to remove such material from the portion of the gravel to be used in the surface layer of the road. If the road is to be sufficiently thick to be constructed in two layers, the larger pebbles screened from the gravel will be suitable for use in the lower course.
Construction. In the construction of a road with gravel surface the road-bed should first be brought to the proper grade, with a form of cross section the same as that to be given the finished road. The gravel is then placed upon it and rolled to a surface, or left to be compacted by the traffic. It is always advantageous when possible to compact the road by rolling. The road-bed should be well rolled before placing the gravel, and the gravel surface afterward. A smooth hard surface may thus be produced, upon which the wheels of loaded vehicles may roll without producing any visible impression.
In preparing a road-bed for gravel surface, when a light coating of gravel is to be used, the surface of the ground is shaped up with the grader in the ordinary manner, but using a section flatter than the finished road is to have. The gravel is then placed and spread so as to have the proper thickness at the middle and diminish the thin edges at the sides of the road. On a good well drained road-bed, this construction with gravel four or five inches thick at the middle of the road may make a very good country road. Good drainage is, however, essential to success with such a road. • On important roads gravel is often used instead of broken stone, in the manner described in Chapter V. In many localities, gravel exists which is superior in hardness and durability to the local stone available for road metal. In such instances gravel is often used for surfacing a stone road.
Maintenance. A new gravel road when first opened to travel may require considerable attention to keep it in good condition. The surface should be watched and all ruts or depressions which may form be at once filled up. When new material must be added in repairing a gravel surface, it should be fine and contain more binding material than the gravel used in the first construction of the road.
After a gravel road is thoroughly compacted by the traffic, less attention is required to keep it in surface until it is worn thin enough to require resurfacing.