In a slightly rolling country, the side ditch frequently has no outlet, and the water is allowed to accumulate at the foot of the slope and there remain until it is absorbed by the ground or seeps into a tile drain. The difficulty could be remedied by providing an inlet from the open ditch to the tile. This may be a well, walled with plank or masonry without mortar (except near the top) and having a grating in the side or top through which the water may pass. The well should be large enough to allow a man to enter it to clean it, and should extend a foot or more below the bottom of the tile. Earth roads in villages and towns are usually better provided with such inlets than country roads, but both could be materially improved at comparatively small expense by attention to this matter.
If it can be prevented, no attempt should be made to carry water long distances in side ditches; for large bodies of water are hard to handle, and are liable to become very destructive. Side ditches should discharge frequently into the natural watercourses, though to compass this, it may in some cases be necessary to carry the water from the high side to the low side of the road. This is sometimes done by digging a gutter or by building a dam diagonally across the road, but both are very objectionable. A better way is to lay a tile or put in a culvert (see Fig. 55, page 210), the amount of water determining which shall be done.
It is sometimes necessary to carry water a considerable distance in the side ditches, as, for example, when the road is in excavation. This requires deep ditches, which are undesirable and dangerous; and if the grade is considerable, the ditches wash rapidly. In such cases, it is wise to lay a line of tile under the side ditch, and turn the water from the surface ditch into the tile drain at intervals. This can be accomplished readily by inserting in the line of porous tile a Y section of vitrified sewer pipe, with the short arm opening up hill. Of course, the short arm, i. e., the vertical arm, need not be as large as the body. If necessary, two or three lengths of porous tile may be added at the upper end of the Y to make connection with the bottom of the open ditch. Earth, sods, or stones can be piled around the upper end of the tile to make a dam and to hold the tile in place.
Some road engineers lay a line of tile under the side ditch, and fill the trench with broken stone, thus making the tile carry both the surface water and the underdrainage. This practice probably
affords better surface drainage, but it costs more than to allow the surface water to flow away in the side ditches. This construction is sometimes defended on the ground that the broken stone prevents the wheels from striking the tile when vehicles are forced into the ditches in passing. This danger does not seem very great, and would not occur at all if the tile were laid at the proper depth; but this is sometimes impossible owing to a hard substratum.
As a rule side ditches will not have too much fall, but sometimes a ditch straight down a hill will have so much as to wash rapidly, in which case it is an advantage to put in an obstruction of stone or brush. In extreme cases the bottom of the ditch is paved with stones.
Surface Drainage. The drainage of the surface of a road is very important, and is provided for by making the surface crown ing and keeping it smooth. It should be remembered that water upon the surface of the road can not be carried away by the under drains, since the water can reach them only after it has penetrated and softened the road surface. The slope from the center to the side should be enough to carry the water freely and. quickly to the side ditch; and if the surface is kept free from ruts and holes, less crown will suffice than if no attention is given to keeping the surface smooth. If there is not enough crown, the water can not easily reach the side ditches; and hence the road soon becomes water soaked.
On the other hand, the crown may be too great. If the side slopes are so steep that traffic keeps continually in the middle, the road will be worn hollow and retain the water instead of shedding it promptly to the side ditches. If the crown is too great, it is difficult for vehicles to turn out in passing each other. Again, if the earth is piled too high in the middle, the side slopes will be washed into the side ditches, which not only damages the road but also fills up the ditches. Further, if the side slopes are steep, the top of the wheel will be farther from the center of the road than the bottom, and the mud picked up by the bottom of the wheel will be carried to the top of the wheel and then dropped farther from the center of the road than it was before, each vehicle acting like a plow and moving the earth from the center toward the side of the road. With the ordinary method of caring for earth roads, more water stands on a very convex road than on a flatter one.