epth that precludes damage to the road. Commonly, the tile will be
laid diagonally across the road some distance above the section where
the effect of the water is noted, and will be turned parallel to the
road at the ditch line and carried under one of the side ditches to an
outlet.

=Tile Drains.=--Where the soil and climatic conditions are such that
the roadway at times becomes unstable because of underground water
rising to a level not far below the road surface, the ground water
level is lowered by means of tile underdrains. The function of the
tile drains in such cases is precisely the same as when employed in
land drainage; to lower the ground water level.

=Laying Tile.=--The tile lines are usually laid in trenches parallel
to the center line of the road near the ditch line and at least 4 feet
deep so as to keep the ground water level well down. They must be
carefully laid to line and grade. A good outlet must be provided and
the last few joints of pipe should be bell-and-spigot sewer pipe with
the joints filled with cement mortar. The opening of the tile should
be covered with a coarse screen to prevent animals from nesting in the
tile.

It is frequently necessary to lay a line of tile at the toe of the
slope in cuts to intercept water that will percolate under the road
from the banks at the sides. In some cases, it is desirable to
back-fill the tile trench with gravel or broken stone to insure rapid
penetration of surface water to the tile. In other instances, it is
advantageous to place catch basins about every three or four hundred
feet. These may be of concrete or of tile placed on end or may be
blind catch basins formed by filling a section of the trench with
broken stone. When a blind catch basin is used, the top should be
built up into a mound, and for a tile or concrete catch basin, a
grating of the beehive type should be used, so that flow to the tile
will not be obstructed by weeds and other trash that is carried to the
catch basin.

=Culverts.=--Culverts and bridges are a part of the drainage system
and the distinction between the two is merely a matter of size.
Generally, structures of spans less than about eight feet are classed
as culverts, but the practice is not uniform. In this discussion
culverts will be defined as of spans of 8 feet or less.

Numerous culverts are required to afford passage for storm water and
small streams crosswise of the road, and their aggregate cost is a
large ite