The thorough drainage of a street involves four elements: (1) the surface drainage, (2) the gutters, (3) the catch basins, and (4) the underdrainage. They will be considered in the reverse, order.
Subdrainage. The underdrainage of a street is the first step toward paving it. Without thorough subdrainage a pave ment is likely to settle here and there, forming unsightly depres sions on the surface, and possibly breaking through. The subsoil may be drained by one or more lines of porous tile as described in § 98-109; but as a rule the surface and underground waters are both collected in the same drain, and therefore it is advisable to lay a line of tile at each side of the street or to construct a larger con duit under the center of the street. Since the pavement is prac tically impervious to water, a third line of tile under the middle of the pavement is unnecessary, however wet and retentive the soil originally.
If there is a grass plat between the pavement and sidewalk, as is usual on residence streets, the tile should be laid under the outer edge of the parking or grass plat; and if there is no parking, the tile should be laid under the gutter. The deeper the tile the
better the drainage and the less the liability of its becoming choked with tree roots. The tile should not be too small since it is to carry both underground and surface water—the latter from a smooth and impervious pavement.
The formula for size of tile for the drainage of earth roads. (§ 103) is worthless for pavements, since in cities a large pro portion of the rain falls upon impervious roofs, pavements, side walks, etc., and nearly all speedily reaches the storm-water sewers. This subject has been very carefully studied in connection with the design of sewers, and the reader is referred to treatises on that subject, for further information concerning the size of drains or storm-water sewers required.*