The cost of a mile of 5-inch tile drain is usually from $200 to $250, exclusive of freight on the tile. If there is any considerable amount of work, the above prices for the smaller tile can be reduced 10 to 20 per cent; and often there is enough discount on the prices given in Table 13, page 75, to cover the railroad freight-charges. A tile drain is a permanent improvement with no expense for main tenance, the benefit being immediate and certain; and therefore it is doubtful if money can be spent on earth roads to better advan tage than in laying tile.
A few other observations seem to confirm the above result for the slope of the surface of saturation. The exact form of the surface of saturation is not known, hut it is known to be a curve slightly convex upward. The inclination varies with the nature of the soil, is most convex near the tile, and is most convex immediately after a rain and gradually thereafter approaches an inclined plane.
The traveled portion is usually not more than 50 feet wide, and therefore a single line of tile to 3 feet below the bottom of the side ditch, if of adequate size, will give nearly perfect drainage; and a second line will not materially improve it. For example, in Fig. 8, if A represents the first line of tile, the surface of the ground water is represented by the lines A B C. If a second line of tile, D, is laid, the water surface will be A B D, and the second line will drain only the comparatively small portion C B D. The diagram shows that a single line well below the surface is far better than two shallow ones. For example, lowering the tile A 6 inches, lowers the water surface to A' C', which represents better drainage than the line A B D with the two lines of tile.
It is generally conceded that for agricultural drainage it is suffi cient to place the lines of tile 100 feet apart, provided they are of reasonable size and at sufficient depth. A tile will give agricul tural drainage 50 feet on either side of it; that is, a tile under only one side ditch will give agricultural drainage of the traveled way. More thorough drainage is required for agricultural than for road purposes, since when damp most soils will pack, which is harmful to agricultural land but beneficial to a road.
The above seems to prove that one line of tile, if of proper size and at sufficient depth, will afford sufficient drainage for road purposes; but nevertheless it is claimed by competent authorities that two lines are sometimes required. In some localities a stratum of hard-pan near the surface makes it necessary to lay the tile so shallow that two lines are really required; and sometimes the tile is so small or so poorly laid that one line is insufficient.
In case of doubt as to whether one or two lines of tile are needed, put in one and watch the results. If both sides of the road are equally good, another tile drain is not needed. In mak ing these observations care should be taken not to overlook any of the factors, as, for example, the difference in the effect of the sun upon the south and the north sides of the road, the effect of shade or of seepage water, the transverse slopes of the surface of the road, etc.
Location of Tile. Some writers on roads recommend a line of tile under the middle of the traveled portion. A tile under the middle of the road is a little more effective than one at the same level under the side ditch; but the former is con siderably more expensive to lay, since it necessitates more digging— whether the tile is laid before or after the road is graded. With the same depth of digging, a tile under the side ditch is more effective than one under the center of the road. Further, if the tile is under the center, there is liability of the settling of the soil in the trench, which will make a depression and probably a mud hole; and if the tile becomes stopped, it is expensive to dig it up, and the doing so interferes with traffic Finally, if the road is ever graveled or macadamized, the disadvantage of having the tile drain under the center of the road are materially increased.