650 SOILS: PROPERTIES AND MANAGEMENT 



ends are put in line and as close together as conditions 

 seem to indicate is necessary. Any covering or filling 

 material is then put in place. The tile should be placed in 

 the trench as soon as the latter is finished, and the trench 

 should then be at least partially filled with earth in order 

 to avoid danger from freezing or from the caving-in of 

 the walls. The first lot of earth usually from the sur- 

 face is carefully placed about the tiles and packed 

 in so as to hold them in position. This is called the 

 blinding, or back-filling. The later filling may be accom- 

 plished in any convenient way. 



552. Size of tile. The size of tile must be deter- 

 mined by the amount and rate at which the water must be 

 removed, the grade of the drain, and the nature of the 

 soil. The small lateral drains whose function is to collect 

 the water from the soil will usually be of three or four 

 inches internal diameter. Drains smaller than this 

 should not be used because of their inclination to become 

 clogged. Small tiles are relatively more affected than 

 larger tiles by the inevitable slight imperfections in the 

 grade. The high friction of the walls in small tiles to 

 the moving water reduces the rapidity of flow and en- 

 courages the accumulation of sediment. In soil some- 

 what of the nature of quicksand, and where the grade is 

 less than one foot in a hundred, no tile smaller than four 

 inches in diameter should be used. As the drainage 

 water is collected by the different lines, the size of the 

 tiles must increase correspondingly. 



553. Amount of water to be removed from land. - 

 Many things affect the amount of water to be removed 

 from a given area of land. The more important of these 

 are the rainfall, the occurrence of springs, surface accu- 

 mulation, the storage capacity of the soil, and rate of 



