32 BULLETIN 724, XJ. S. DEPARTMENT OF AGRICULTURE. 
gutter, it may be located in the shoulder under the edge of the sur- 
facing. The principal objections to locating the drain under the 
center of the road are that it is very difficult to back fill the trench 
in which the tile is laid so that no settlement will take place, and in 
case of injury to the tile repairs are hard to make and always inter- 
fere more or less with traffic. Where a center location is selected, 
corresponding care to insure proper alignment and grade and to 
prevent silting of the stone or pipe line should be taken. Where 
the road is located on a side hill it is usually satisfactorily subdrained 
by one line of tile placed on the up-hill side so as to intercept the 
flow of the ground water. 
SIZE OF TILE. 
The proper size of tile for any particular location depends upon the 
rate of the run-off and the slope of the tile. Where the topography 
is flat and no surface water is allowed to enter the tile directly it may 
be assumed safety that the maximum amount of water drained off in 
24 hours would coyer the area drained by the tile to a depth of 
about one-fourth inch, or that the maximum run-off would be about 
35 cubic feet per hour for each acre drained. Where the topography 
is more or less broken or where springs empty into the tile, this 
maximum run-off may be increased very materially, and the probable 
influence of such factors as these always should be considered in fix- 
ing the size of the tile. 
In order to determine the area a given tile will drain it is necessary 
to know how far out on each side of the tile the level of the ground 
water will be affected. In loose sandy soils this distance may be as- 
sumed to be about 100 feet, while in stiff clays that tend to retain 
water the level of the ground water may be practically unaffected at 
a distance out of only 20 or 25 feet. For most soils that require sub- 
drainage it probably is safe to assume that the area drained on each 
side of the tile is from 30 to 50 feet wide. These distances are essen- 
tially affected by the depth at which the tile is laid. To develop the 
highest service in sandy soils the depth should be from 3 to I feet, 
while in stiff clays the depth should not exceed 2-J to 3 feet. Where 
deep freezing is likely to occur and heaving or disalignment of the 
tile can be expected, it should be laid deeper, of course, and the num- 
ber of lines increased if necessary to reduce the ground water peak 
between them. 
Table 4 shows the sizes of tile required to carry various amounts of 
water. An examination of this table will show that rarely under con- 
ditions ordinarily encountered in road work need the diameter of the 
tile be greater than 6 inches, and that for most conditions a diameter 
of 4 inches is sufficient. Xo matter how small mav be the amount of 
