262 



Bulletin 254. 



may result from loose fitting. It is to be noted in this last named 

 union that the lateral joins the main at the horizontal center of the tile 

 and not in a way to have the bottoms of the tiles on the same level. 



(2) Depth. — The depth of the tile will vary with the character of the 

 soil and the nature of the slope. In very sandy or other porous soil, they 

 may very wisely be placed at a depth of 3)^{> to 4^ feet. In heavy clay 

 soil they should generally be placed much more shallow and where they 

 are to supplant the surface drains and have to deal primarily with surface 

 water, they may be even less than two feet in depth. The most common 

 depth is 2y2 feet. 



!5?5^ 



y 



Fig. 201. — A 11 a t h c r 

 Fig. 200.— a method of method of connecting 



joUiing lateral to the a lateral zcith the 



main line of tile. main line of tile. 



Fig. 202. — The most com- 

 mon method of joining 

 a lateral to a main line 

 of tile. 



Three limiting factors of shallow drains may be mentioned. These 

 are first, the effect of frost. The action of frost to throw tile out of 

 line is much less than is generally supposed and if the drainage of the 

 soil is thorough, very little or no injury to the system may be expected 

 even if the tile are placed as shallow as 15 or 18 inches. It should be 

 clearly understood that the placing of drains so shallow as this is 

 not advocated on any but level heavy clay soil. The eft'ect of the 

 drainage is to greatly reduce not only the extent of absolute heaving, 

 but also the tendency to freeze. If the tile are within the range of 

 much frost action, only hard burned tile should be used. Second, the 

 interference of tillage implements, such as the plow and subsoil plow. 

 Third, plant roots may sometimes enter and fill up tile drains. This 

 is true not alone of shallow drains ; deep drains are also clogged by tree 

 roots and the roots of some crops. This difficulty is due less to the 

 depth of the tile than to the character of the flow of the water in the 

 tile. Roots seek an adecfuate supply of moisture. If the tile carries 

 only the excess of water during wet priods there will be very little 



