DITCHING MACHINES 



259 



is bad, the joints too open, or a section is broken. The 

 joints should be fairly snug, but it is not now considered 

 necessary to use collars in ordinary soils. The textures 

 of soil which give most trouble by entering the joints 

 and stopping flow are very fine sand and silt. These 

 materials flow readily when saturated with water. Con- 

 sequently, in laying tile in these materials, precaution 

 must be taken against this. " Silting-up" is most trouble- 



Fig. 90. Traction ditching machine. A modern machine for constructing 



tile ditches. (See Fig. 91.) 



some immediately after laying the tile, and before the 

 soil structure has become settled and readjusted. When 

 this has taken place, the tendency to silting-up is small, 

 even in fine sand and silt. In clay and coarse sand it is 

 negligible. This difficulty can be checked or controlled 

 by using some filtering medium around the joints. Straw, 

 leaves, chaff, etc., are excellent and undergo slow decay, 

 coincident with which a resistent structure of soil is 



