The pebbles in a gravel road are simply imbedded in a paste and can be easily displaced. It is for this reason, among others, that such roads are subject to internal destruction.
The binding power of clay depends in a large measure upon the state of the weather. During rainy periods a gravel road be comes soft and muddy, while in very dry weather the clay will con tract and crack, thus releasing the pebbles, and giving a loose surface. The most favorable conditions are obtained in moderately damp or dry weather, during which a gravel road offers several advantages for light traffic, the character of the drainage, etc., largely determining durability, cost, maintenance, etc.
Repair. Gravel roads constructed as above described will need but little repairs for some years, but daily attention is required to make these. A garden rake should be kept at hand to draw any loose gravel into the wheel tracks, and for filling any depres sions that may occur.
In making repairs, it is best to apply a small quantity of gravel at a time, unless it is a spot which has actually cut through. Two inches of gravel at once is more profitable than a larger amount. Where a thick coating is applied at once it does not all pack, and if, after the surface is solid, a cut be made, loose gravel will be found; this holds water and makes the road heave and become spouty under the action of frost. It will cost no more to apply six inches of gravel at three different times than to do it at once.
At every one-eighth of a mile a few cubic yards of gravel should be stored to be used in filling depressions and ruts as fast as they appear, and there should be at least one laborer to every five miles of road.
Broken Stone Roads. Broken stone roads are formed by pla cing small angular fragments of stone on the surface of the earth roadbed and compacting into a solid mass by rolling. This class of road covering is generally called a Macadam or Telford road from the name of the two men who first introduced this type into England.
The name of Telford is associated with a rough stone founda tion, which he did not always use, but which closely resembled that which had been previously used in France. Macadam disregarded this foundation, contending that the subsoil, however bad, would carry any weight if made dry by drainage and kept dry by an im pervious covering. The names of both have ever since been associated with the class of road which each favored, as well as with roads on which all their precepts have been disregarded.
Quality of Stones. The materials used for broken-stone pave ments must of necessity vary very much according to the locality. Owing to the cost of haulage, local stone must generally be used, especially if the traffic be only moderate. If, however, the traffic is heavy, it will sometimes be found better and more economical to obtain a superior material, even at a higher cost, than the local stone; and in cases where the traffic is very great, the best material that can be obtained is the most economical.
The qualities required in a good road stone are hardness and and toughness and ability to resist the disintegrating action of the weather. These qualities are seldom found together in the same stone. Igneous and siliceous rocks, although frequently hard and tough, do not consolidate so well nor so quick as limestone, owing to the sandy detritus formed by the two first having no cohesion, whilst the limestone has a detritus which acts like mortar in binding the stones together.
A stone of good binding nature will frequently wear much better than one without, although it is not so hard. A limestone road well made and of good cross-section will be more impervious than any other, owing to this cause, and will not disintegrate so soon in dry weather, owing partly to this and partly to the well known quality which all limestone has of absorbing moisture from the atmosphere. Mere hardness without toughness is not of much use, as a stone may be very hard but so brittle as to be crushed to powder under a heavy load, while a stone not so hard but having a greater degree of toughness will be uninjured.' By a stone of good binding quality is meant one that, when moistened by water and subjected to the pressure of loaded wheels or rollers, will bind or cement together. This quality is possessed to a greater or less extent by nearly all rocks when in a state of dis integration. The binding is caused by the action of water upon the chemical constituents of the stone contained in the detritus produced by crushing the stone, and by the friction of the fragments on each other while being compacted; its strength varies with the different species of rock, but it exists in some measure with them all, being greatest with limestone and least with gneiss.