32 BULLETIN 724, U. S. DEPARTMENT OF AGEICULTUEE. 



gutter, it ma}' 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 injur^^ 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 safely that the maximum amount of water drained off in 

 24 hours would cover 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 giA'-en 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 gTound 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 wdiich the tile is laid. To develop the 

 highest service in sandy soils the depth should be from 3 to 4 feet, 

 while in stiff' clays the depth should not exceed 2^ 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 shoAvs the sizes of tile retjuired to carry various amounts of 

 water. An examination of this table Avill 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. No matter how small mav be the amount of 



