The essential condition of the stone to produce this binding effect is that it lie sound. • No decayed stone retains the property of binding, though in some few cases, where the material contains iron oxides, it may, by the cementing property of the oxide, undergo a certain binding.
A stone for a road surface should be as little absorptive of moisture as possible in order that it may not suffer injury from the action of frost. Many limestones are objectionable on this account.
The stone used should be uniform in quality, otherwise it will wear unevenly, and depressions will appear where the softer material has been used.
As the under parts of the road covering are not subject to the wear of traffic, and have only the weight of loads to sustain, it is not necessary • that the stone of the lower layer be so hard or so tough as the stone for the surface, hence it is frequently possible by using an inferior stone for that portion of the work, to greatly reduce the cost of construction.
Size of Stones. The stone should be broken into fragments as nearly cubical as possible. The size of the cubes will depend upon the character of the rock. If it be granite or trap, they should not exceedf', inches in their greatest dimensions; if limestone, they should not exceed 2 inches.
The smaller the stones the less the percentage of voids. Small stones compact sooner, require less binding, and make a smoother surface than large ones, but the size of the stone for any particular section of a road must be determined to a certain extent by the amount of traffic which it will have to bear and the character of the ruck used.
It is not necessary nor is it advisable that the stone should be all of the same size; they may be of all sizes under the maximum. In this condition the smaller stones fill the voids between the larger and less binding is required.
Thickness of the Broken Stone. The offices of the broken stone are to endure friction and to shed water; its thickness must be regulated by the quality of the stone, the amount of traffic, and nature of the natural soil bed. tinder heavy traffic it is advisable to make the thickness greater than for light traffic, in order to pro vide for wear and lessen the cost of renewals.
When the roadbed is firm, well drained, and not likely to be affected by ground water, it will always afford a firm foundation for the broken stone, the thickness of which may be made the mini mum for good construction. This thickness is four inches. When
this thickness is employed the stone must be of exceptionally fine quality and the road must be maintained by the " continuous " method. With heavy traffic the thickness should be increased over the minimum a certain amount, say 2 inches, to provide for wear. Where the foundation is unstable and there is a tendency on the part of the loads to break through the covering, the thickness of the stone must be made the maximum, which is 12 inches. In such a case it may be advisable to employ a Telford foundation. Where the covering exceeds six inches in thickness, the excess may be composed of gravel, sand or ledge stone, the choice depending entirely on the cost, for all are equally effective.
Foundation. The preparation of the natural soil over which the road is be constructed, to enable it to sustain the superstruc ture and the weights brought upon it, requires the observance of certain precautions the neglect of which will sooner or later result in the deterioration or possible destruction of the road covering. These precautions vary with the character of the soil.
Soils of a siliceous and calcareous nature do not present any great difficulty, as their porous nature generally affords good natural drainage which secures a dry foundation. Their surface, however, requires to be compacted; this is effected by rolling.
The rolling should be carried out in dry weather, and any de pressions caused by the passage of the roller should be filled with the same class of material as the surrounding soil. The rolling must be repeated until a uniform and solid bed is obtained.
The argillaceous and allied soils, owing to their retentive nature, are very unstable under the action of water and frost, and in their natural condition afford a poor foundation. The prepara tion of such soil is effected by drainage and by the application of a layer of suitable material to entirely separate the surface from the road material. This material may be sand, furnace ashes, or other material of a similar nature, spread in a layer from 3 to 6 inches thick over the surface of the natural soil.