Frequently, as in many cases, of a road along a side slope, there is a well-defined flow of sub-surface water from one side to .the other, and in such case the water may perhaps be intercepted by a single longitudinal drain on the side of the roadway from which the water comes. An example of this is shown in Fig. 3.
When the subsoil is of stiff and retentive material which does not drain readily, an underdrain on one side may not draw the water from under the whole width of the roadway. In this, case it is advisable to use a drain on each side to cut off the water before it reaches the roadway. This may be necessary with a clay soil when the line of ground water is high.
Sometimes a single drain is laid under the middle of the road, as shown in Fig. 4. This is, in general, an undesirable practice; the middle of the roadway is not a convenient place for the drain, and necessitates digging a trench under the roadway which is likely to give considerable trouble in the early maintenance of the road surface. In some instances however a spring of water may come up under the roadway, as in a clay spout, and when this occurs it is desirable to lay a pipe to take the water from the source of supply rather than to drain it through the soil to the side drains.
The most satisfactory and cheapest method of under drainage is commonly by the use of porous drain tile, as used for farm drainage. Where stone is plentiful and handy to the road, a stone drain may be cheap and equally effective with the tile. These types of drains are described in succeeding articles.
Many road builders utilize the side ditches, intended for surface drainage, for underdrainage also by making them deep and narrow. This is not usually an eco nomical practice. A tile drain and shallow gutters will not be more expensive to construct than the deep ditches, while they are much easier and cheaper to maintain. In some instances tiles are laid under the surface ditches and the trenches filled with stones, or gravel, as shown in Fig. 5, thus permitting the surface drainage to seep into the tile. This gives very effective drainage, if the tile be of sufficient capacity, but is expensive to construct.
In considering the advisability of underdrainage and the method of accomplishing it, the fact should be kept in mind that the purpose of underdrainage is to remove ground water, and that efficient drainage of the road surface, can only be accomplished by main taining the surface in smooth condition and of proper form.