258 



THE PRINCIPLES OF SOIL MANAGEMENT 



finer the soil the greater this retentive effect, and con- 

 sequently the less demand there is for drains capable of 

 carrying all of the rainfall in a given short time. Drains 

 run full for only a very small part of the year, and 

 therefore the normal laws of hydraulics are not entirely 

 applicable to them. In a general way, doubling the fall 

 increases the carrying capacity of any given size of tile 

 by one-third. Where the fall is less than 1 per cent, it is 



unwise to use tile smaller 

 than three inches in di- 

 ameter, because of their 

 strong tendency to clog. 

 Water enters tile al- 

 most entirely through 

 the joints between the 

 sections. Short lengths 

 are therefore better than 

 long ones. Through the 

 walls of even soft brick 

 tile very little water is 

 able to percolate. There 

 is, therefore, no appreci- 

 able advantage in using 

 soft tile, while there are 

 many disadvantages, such as their weakness and lia- 

 bility to go to pieces rapidly under alternate wetting 

 and drying, especially if permitted to freeze when 

 saturated with water. 



Dense, hard-burned tile are most safe to use under 

 average soil conditions. 



"Silting-up" of drains results where the alignment 



Fig. 89. Hand tools used in tile- 

 drain construction. 1, Grade cord; 2, pick; 

 3, long-handle, round-point shovel; 4 

 and 7, types of grading shovel for finish- 

 ing the bottom of the ditch; 5, spade; 6, 

 tile hook, used in placing tile in ditch from 

 the bank; 8, grade stakes. 



