The underdrainage of a road not only removes the water, but prevents, or greatly reduces, the destructive effect of frost. The injurious effect of frost is caused entirely by the presence of water, and the more water there is in the road-bud the greater the injury to the road. The water expands on freezing, the surface of the road is upheaved, and the soil is made porous; when thawing takes place, the ground is left honeycombed and spongy, ready to settle and sink, and under traffic the road "breaks up." If the road is kept dry, it will not break up. Underdrainage can not pre vent the surface of the road from becoming saturated with water during a rain, but it is the best means of removing the surplus water, thus drying the surface and preventing the subsequent heav ing by frost.
That frost is harmless where there is no moisture, is shown on a large scale in the semi-arid regions west of the Mississippi river. The ground there is normally so dry that during the winter, when it is frozen, cracks form half an inch or more wide, owing to the dry ing and consequent contraction of the soil, which shows that there is no expansion by the freezing of the water in the soil; and there fore in this region there is no heaving or disturbance by frost. Houses are often built on the very surface of the ground, and no trouble is ever experienced by the action of frost.
The Tile. The best and cheapest method of securing under drainage is to lay a line of porous or farm tile 3 or 4 feet deep on one or both sides of the roadway. The ordinary farm tile is entirely satisfactory for road drainage. It should be uniformly burned, straight, round in cross section, smooth inside, and have the ends cut off square. Tile may be had from 3 to 30 inches in diameter. The smaller sizes are usually a little over a foot long,—the excess length being designed to compensate for breakage; and the larger sizes are nominally 2 or 2 feet long, but usually a little longer. The cost of tile free on board at the factory is usually about as in Table 13, page 75. Y's for connections can be had at most factories, but they cost four or five times as much as an ordinary tile. With patience and a little experience ordinary tile can be cut to make fairly good connections.
Before the introduction of tile for agricultural drainage, it was customary to secure underdrainage by digging a trench and deposit ing in the bottom of it logs or bundles of brush, or a layer of broken stone; or a channel for the water was formed by setting a line of stones on each side of the trench and joining the two with a third line resting on these two. Apparently it is still the practice in some
localities to use such substitutes for ordinary drain tile. Tiles are better, since they are more easily laid and are less liable to get clogged. Tiles are cheaper in first cost, even when shipped consid erable distances by rail, than any reasonably good substitute, and the drains are much more durable.
Tiles are laid simply with their ends in contact, care being taken to turn them until the ends fit reasonably close. In some localities there is apparently fear that the tile will become stopped by fine particles of soil entering at the joints, and consequently it is specified that the joint shall be covered with tarred paper or something of the sort; but in the Mississippi Valley, where immense quantities of tile have been laid, no such difficulty has been encountered. With a very slight fall or even no fall at all, tiles will keep clean, if a free outlet is provided, and they are not obstructed by roots of trees— particularly willow.
In some localities it is apparently customary to use collars around the ends of the tile to keep them in line. If the bottom of the trench is made but little wider than the diameter of the tile, or if a groove is scooped out in the bottom of the trench to fit the tile, no difficulty need be apprehended from this source.
The Fall. There is no danger of the grade of the tile being too great, and the only problem is to secure sufficient fall. A num ber of authorities on farm drainage and also several engineering manuals assert that a fall of 2/ or 3 inches per 100 feet is the lowest limit that should be attempted under the most favorable conditions; but practical experience has abundantly proved that a much smaller fall will give good drainage. In central Illinois and northern Indiana are many lines of tile having falls of only to .1 of an inch per 100 feet which are giving satisfactory drainage; and not un frequently the ordinary porous tiles laid absolutely level directly upon the earth in the bottom of the trench, without collars or other covering over the joints, have given good drainage without trouble from the deposit of sediment. Of course, extremely flat grades are less desirable than steeper ones, since larger tiles must be used, and greater care must be exercised in laying them, and there is more risk of the drain becoming obstructed; but these extremely flat grades are sometimes all that can be obtained, and such drains abundantly justify the expense of their construction.