The trenches for the tile should be excavated at least 3 feet wide on top and 12 inches on the bottom. After the tiles are laid the trenches must be filled to subgrade level with round field or cobble stones; stones with angular edges are unsuitable for this purpose. Fine gravel, sand, or soil should not be placed over the drains. Bricks and flat stones may be substituted for the tiles, and the trenches filled as above stated.
As tile drains are more liable to injury from frost than those of either brick or stone, their ends at the side ditches should not in very cold climates be exposed directly to the weather, but may terminate in blind drains, or a few lengths of vitrified clay pipe reaching under the road a distance of about 3 to 4 feet from the inner slope of the ditch.
Another method of draining the roadbed offering security from frost is by one or more rows of longitudinal drains. These drains are placed at equal distances from the side ditches and from each other, and discharge into cross drains placed from 250 to 300 feet apart, more or less, depending on the contour of the ground. The cross drains into which they discharge should be of ample dimensions. On these longitudinal lines of tiles the introduction of catch basins at intervals of 50 feet will facilitate the removal of the water. These catch basins may be excavated three or more feet square and as deep as the tiles are laid. After the tiles are laid the pit is filled with gravel and small stones.
Fall of Drains. It is a mistake to give too much fall to small drains, the only effect of which is to produce such a current through them as will wash away or undermine the ground around them, and ultimately cause their own destruction. When a drain is once closed by any obstruction no amount of fall which could be given it will again clear the passage. A drain with a considerable current through it is much more likely to be stopped from foreign matter carried into it, which a less rapid stream could not have transported.
A fall of 1 inch in 5 feet will generally be sufficient, and 1 inch in 30 inches should never be exceeded.
Side Ditches are provided to carry away the subsoil water from the base of the road, and the rain water which falls upon its surface; to do this speedily they must have capacity and inclination proportionate to the amount of water reaching them. The width of the bed should not be less than 18 inches; the depth will vary with circumstances, but should be such that the water surface shall not reach the subgrade, but remain at least 12 inches below the crown of the road. The sides should slope at least 11 to 1.
The longitudinal inclination of the ditch follows the configura tion of the general topography, that is, the lines of natural drainage. When the latter has to be aided artificially, grades from 1 in 500 to 1 in 800 will usually answer.
In absorbent soil less fall is sufficient, and in certain cases level ditches are permissible. The slopes of the ditches must be protected where the grade is considerable. This can be accomplished by sod revetments, riprapping, or paving.
These ditches may be placed either on the road or land side of the fence. In localities where open ditches are undesirable they may be constructed as shown in Figs. 13 to 17, and may be formed of stone or tile pipe, according to the availability of either material. If for any reason two can not be built, build one.
Springs found in the roadbed should be tapped and led into the side ditches.
Drainage of the Surface. The drainage of the roadway surface depends upon the preservation of the cross-section, with regular and uninterrupted fall to the sides, without hollows or ruts in which the water can lie, and also upon the longitudinal fall of the road. If this is not sufficient the road becomes flooded during heavy rainstorms and melting snow, and is considerably damaged.
The removal of surface water from country roads may be effected by the side ditches, into which, when there are no sidewalks, the water flows directly. When there are sidewalks, gutters are formed between the roadway and footpath, as shown in Figs. 13 to 17, and the water is conducted from these gutters into the side ditches by tile pipes laid under the walks at intervals of about 50 feet. The entrance to these pipes should be protected against washing by a rough stone paving. In the case of covered ditches under the footpath the water must be led into them by first passing through a catch basin. These are small masonry vaults covered with iron gratings to prevent the ingress of stones, leaves, etc. Connection from the catch basin is made by a tile pipe about 6 inches in diameter. The mouth of this pipe is placed a few feet above the bottom of the catch basin, and the space below it acts as a depository for the silt carried by the water, and is cleaned out periodically. The catch basins may be placed from 200 to 300 feet apart. They should be made of dimensions sufficient to convey the amount of water which is liable to flow into them during heavy and continuous rains.