If possible at reasonable expense, the grade should be at least 2 inches per 100 feet; and should never be less than inch per 100 feet unless absolutely necessary. On level or nearly level ground, the fall may be increased by laying the tile at the upper end shallower than at the lower.
Size of Tile. The following formula has frequently been employed to determine the size of tile: in which Q is the discharge in cubic feet per second, f the fall in a distance 1 (both in feet), and D the inside diameter of the tile in feet. The above formula may be reduced to the following more useful form: in which V is the discharge in cubic feet per 24 hours, and d is the diameter of the tile in inches. Water 1 inch deep over an acre of land amounts to 3,830 cubic feet; and therefore if we divide the constant in equation (2) by 3,830, we get the following formula: in which A is the number of acres for which a tile having a diameter of d inches and a fall of f feet in a length of 1 feet will remove 1 inch in depth of water in 24 hours.
Equation (1) is the formula ordinarily employed for the flow of water through smooth cast-iron pipe, and is only roughly applicable to tile. It probably gives too great a capacity for tile. However, all the factors of the problem are too uncertain to justify an attempt at mathematical accuracy. For example, we can not know with any certainty the maximum rate of rainfall, the duration of the maximum rate, the permeability of the soil, the amount of water retained by the soil, the effect of surface water flowing onto the road from higher ground, the area to be drained, etc. The above formula is useful only in a locality where there is no local experience with tile; and its chief value consists in showing the relation between capacity and grade, and the effect of a variation in the diameter of the tile.
The object of underdraining a road is to prevent the plane of saturation from rising so near the surface as to soften the foundation of the road even during a wet time, and therefore the provision for underdrainage should be liberal; but what will be adequate in any particular case depends upon the amount of traffic, the local conditions, the soil, etc. The best practice in agricultural drainage provides for the removal of 0.5 to 1 inch of water per day;
but since the side ditches will assist in removing rain water from the road, it is probable that a provision for the removal of half an inch per day is sufficient for the underdrainage of a road. If there is an underflow of water from higher ground, or if the ground is "springy,". then the ordinary provisions for underdrainage should be increased.
It is not wise to lay a smaller tile than a 4-inch one, and probably not smaller than a 5-inch. Tiles can not be laid in exact line, and any tilting up of one end reduces the cross section. Again, if there is a sag in the line equal to the inside diameter, the tile will shortly become entirely stopped by the deposit of silt in the de pression.
It is sometimes wiser to lay a larger tile than to increase the fall. Again, it may be better to lay a large tile near the surface with smaller fall than to lay small tile deeper with a greater fall. Ordinarily, the deeper the tile the better the drainage, although 31 or 4 feet deep is usually sufficient.
Laying the Tile. It is unwise to enter upon any detailed discussion of the art of laying the tile. The individual tiles should be laid in line both vertically and horizontally, with as small joints at the end as practicable. Care should also be taken that the tile is laid to a true grade, particularly if the fall is small, for if there is a sag it will become filled with sediment, or if there is a crest silt will be deposited just above it. The drain should have a free and adequate outlet. The end of the line of tile should be protected by masonry, by plank nailed to posts, or by replacing three or four tiles at the lower end by an iron pipe or by a wooden box.