DRAINAGE.
In the construction of roads, drainage is of the first importance. The ability of earth to sustain a load depends in a large measure upon the amount of moisture retained by it. Most earths :form a good firm foundation so long as they are kept dry, but when wet they lose their sustaining power, becoming soft and incoherent.
The drainage of roadways is of two kinds, viz., surface and sub surface. The first provides for the speedy removal of all water falling on the surface of the road; the second provides for the removal of the underground water found in the body of the road, a thorough removal of which is of the utmost importance and essential to the life of the road. A road covering placed on a wet undrained bottom will be destroyed by both water and frost, and will always be trouble some and expensive to maintain; perfect subsoil drainage is a neces sity and will be found economical in the end even if in securing it considerable expense is required.
The methods employed for securing the subsoil drainage must be varied according to the character of the natural soil, each kind of soil requiring different treatment.
The natural soil may be divided into the following classes: silicious, argillaceous, and calcareous; rock, swamps, and morasses.
The silicious and calcareous soils, the sandy loans and rock, present no great difficulty in securing a dry and solid foundation. Ordinarily they are not retentive of water and therefore require no underdrains; ditches on each side of the road will generally be found sufficient.
The argillaceous soils and softer marls require more care; they retain water and are difficult to compact, except at the surface; and they are very unstable under the action of water and frost.
The drainage of these soils may be effected by transverse drains and deep side ditches of ample width. The transverse drains are placed across the road, not at right angles but in the form of an inverted V with the point directed up hill; the depth at the angle point should not be less than 18 inches below the subgrade surface, and each branch should descend from the apex to the side ditches with a fall of not less than 1 inch in 5 feet. The distance apart of
these drains will depend upon the wetness of the soil; in the case of very wet soil they should he at intervals of 15 feet, which may be increased to 25 feet as the ground becomes drier and firmer.
The transverse drains are best formed of unglazed circular tile of a diameter not less than 3 inches, jointed with loose collars. The tiles are made from terra cotta or burnt clay, are porous, and are superior to all other kinds of drains. They carry off the water with greater ease, rarely if ever get choked up, and only require a slight inclination to keep the water moving through them.
The tiles are made in a variety of forms, as horseshoe, sole, double sole, and round, the name being derived from the shape of the cross-sections. Round tile is superior to all other forms. The inside diameter of these tiles varies from to 6 inches, but they are manufactured as large as 24 inches. Pieces of the larger pipe serve as collars for the smaller ones. They are made in lengths of 12, 14 and 24 inches, and in thickness of shell from of an inch to 1 inch. The collar which encircles the joint of the small tile allows a large opening, and at the same time prevents sand and silt from entering the drain. Perishable material should not be used for jointing. When laid in the ditch they should be held in place by small stones. Connections should be made by proper '-branches.
The outlets may be formed by building a dwarf wall of brick or stone, whichever is the cheapest or most convenient in the locality. The outlet should be covered with an iron grating to prevent vermin entering the drain pipes, building nests and thus choking up the waterway. (See Fig. 12.) Silt-basins should be constructed at all junctions and wherever else they may be considered necessary; they may be made from a single 6-inch pipe (Fig. 11) or constructed of brick masonry.