158 



LAND DRAINAGE 



the tile, for 3 to 8 feet, with the upper joint very open, 

 and then to fill the ditch with broken stones of 3 to 4 

 inches in diameter, or with cobble stones of the same 

 dimension. Another is to construct a shallow silt-basin 

 of rather large diameter to receive the excess of sur- 

 face water, with a siphon to remove this water to the 

 tile below. A shallow open ditch gathers the surface 



FIG. 67. Plan for permitting excess of surface water to reach tile 

 drain by filling a section of ditch with crushed stone or cobble stone. 



water and delivers it to the silt-basin. (See Figs. 67 

 and 68.) 



209. Tile in muck soil should be laid deep. Muck 

 soils settle rapidly after they are tile-drained. This 

 settling is due, in part, to a natural shrinkage because of 

 the reduction of moisture present, and probably to chemi- 

 cal changes which take place because of the more ready 

 access of air to the organic material. These changes are 

 commonly spoken of as oxidation. In the course of a few 

 years, the surface of a tiled muck area may have settled 

 until the tile lies within the frost zone, and not infre- 

 quently lies so near the surface as to be disturbed by the 

 plow-point. The effect of freezing is to shale the tile, if of 

 the ordinary clay kind, even to the extent of causing them 



