LAND DRAINAGE 



639 



its way through the openings. All stone drains are 

 likely to be of short duration because of obstructions that 

 develop in the channel by the accumulation of sediment, 

 often promoted by the burrowing of animals. The 

 throat of a ditch, to receive stone or brush, should be 

 relatively large (see Fig. 64). 



FIG. 64. The most common types of drainage tile and other materials 

 used for land drainage. (1), cobblestones with smaller pieces of 

 stone on top ; (2) , flat stones placed face to face and parallel to line 

 of ditch ; (3) and (4) , throated drains constructed of flat stones 

 used in different ways ; (5), pole drain; (6), triangular box drain; 

 (7), square box drain. Note construction for admission of water 

 along lower edge ; (8) , horseshoe tile laid on a board ; (9) , horse- 

 shoe tile, bottom attached; (10), single sole tile with round open- 

 ing; (11), double sole tile; (12), hexagonal tile; (13), round tile; 

 (14), Y-shaped junction piece; (15), elbow piece. 



539. Tile drains. Modern underdrainage is usually 

 omplished by means of short sections of pipe of burned 

 clay or concrete, placed in the ground sufficiently deep to 

 lower the water table in the subsoil to the desired depth 

 within two or three days. They are given an accurate 

 grade, and this, coupled with the smooth, hard channel 

 which is not subject to erosion, makes them a very effi- 



