Tile drains for road drainage are constructed in the same manner as for land drainage. Ordinary porous tiles are used as in farm drainage, sizes from 4 to 8 or 10 inches in diameter being commonly employed for this purpose. They are usually in lengths of slightly more than 12 inches, the excess of length being sufficient to allow for probable breakage, so that estimates may be made on the basis of one tile to each foot of length. The tiles should be truly cylindrical with the ends cut off square, and be smooth inside. They are laid in a trench 3 or 4 feet below the surface of the ground, with their ends in contact. They should be carefully placed so that the ends fit closely, and the bottom of the trench should be cut to about the width of the tile, so that they cannot move sideways when the is filled; or better still, a groove may be scooped out in the bottom of the trench to fit the tile.
Grade of Tile. The velocity of water through a tile depends upon the slope of the tile. Considerable water may be carried by a tile laid to an almost level grade. Such grades are, however, rarely necessary in road drainage and are to be avoided whenever possible. It is not desirable, except under unusual conditions, to use a grade less than about 2 inches per no feet. This gives a velocity which may be reasonably expected to keep the tile clear, when properly laid, except for small tiles, although a greater slope is to be preferred when obtainable.
Care should be used in laying tile to place it accu rately to the grade line, particularly when the slope is light. Irregularities are apt to produce depressions in which deposit of silt may take place.
Size of Tile. A tile for road drainage should not be less than 4 inches in diameter. While a smaller tile may often be large enough to carry the water, . the danger of clogging is much greater and the effect of irlequalities in grade are increased for such tile. The size of tile required depends upon the quantity of water to be carried and the slope of the tile. For agricultural drainage it is common to assume that the tile must remove from inch to z inch of water per day over the area which it drains. The rules commonly followed probably give an excessive run-off in most instances, and recent observations indicate that inch would be ample in most instances of ordinary drainage. This method may be applied in road work where the area from which the water is drawn can be determined. The area to be included depends upon the character of the soil and the way the ground lies. On level
ground the drain may be assumed to receive water from a certain distance on each side depending upon the porosity of the soil.
For road drainage the size of tile used should be such as to provide liberal capacity. Comparatively small sizes will usually be required and the differ ences in cost are small. An area of 25 to 5o feet on each side may be considered as contributing water to the tile. In ordinary soil the effect of the tile will reach much farther than this, but the percolation is so slow that the water will reach the tile very gradually. • This method may serve as a guide in selecting the size of tile required, but is not capable of accurate computation and is only of value as an aid to judg ment. Good practice in such work must rest mainly upon the judgment- derived from experience. If the tile be supposed to collect water from about 25 fees on each side, it would drain about an acre for each 870 feet of length, or about 6 acres per mile. Assum ing one-half inch in 24 hours, over the drainage area, as the amount to be provided for, one acre will yield 1815 cubic feet per day, or if cubic feet per minute; and one mile of length, 50 feet wide, will yield about 7i cubic feet per minute.
The water carrying capacity of tile drains has not been accurately determined but it probably does not greatly differ from that for vitrified pipe sewers, and the use of the formulas usually applied to sewers will be sufficiently accurate for practical purposes. The common formula for the flow of water, v = C 'SRS, may for our purpose be transposed into the form V =k D in which V is the velocity in cubic feet per second, D is the diameter of the pipe in inches, S is the slope, and k is a coefficient varying from about 9 for 4-inch pipe to 12 for 12-inch pipe.
The following table for the capacity of tile drains is based upon this formula. It is computed by the use of Kutter's formula, using a coefficient of roughness of .013, which corresponds to the flow in pipe sewers.
Tiles laid upon very flat slopes sometimes may carry a quantity of water greater than the capacity due to the slope; this is caused by the level of ground water standing above the tile, thus causing the water to flow in the tile under a head greater than that due to its slope. Where gravel or other porous material is available such tile will be benefited by a porous filling immediately over the tile. This also assists in keeping the tile clear of