214 



NATURE AND PROPERTIES OF SOILS 



of area and slope on the size of the main near the outlet of 

 any system: 



Table XLI 



GRADES TO A HUNDRED FEET IN DECIMALS OP A FOOT WITH AP- 

 PROXIMATE EQUIVALENTS IN INCHES. 



The grade necessary for the satisfactory operation of a tile 

 drain system varies with the system itself and the portion 

 under consideration. The grade of the main drain may be 

 very low, especially if the laterals deliver their water with a 

 high velocity. In general, the grade will vary from 4 to 20 

 inches to the hundred feet, 8 inches being more or less ideal. 

 The depth of the tiles beneath the surface and the distance 

 between laterals will vary with the soil. With sandy soils 

 the tile may be placed as deep as 3 or 4 feet. With clayey 

 soils the depth must be shallower, ranging from 15 to 30 

 inches, while the interval is reduced as the soil becomes finer 

 in texture. On a clayey soil the distance between the strings 

 is sometimes as low as 35 feet although 50 to 70 feet is com- 

 moner. 



The maintenance cost of a tile drain system is low, the only 

 especial attention needful being at the outlet. The outlet 



