188 PRACTICAL DRAINAGE 



since the greatest tendency is for the sand and silt to enter 

 at the bottom and if paper is wrapped tightly entirely 

 around the joints the water itself will be shut out. For 

 genuine quicksand, perhaps the best material is cheese- 

 cloth, which should be doubled once or twice and wrapped 

 carefully about the joint. This material soon decomposes, 

 but in the meantime the soil becomes compacted so that 

 the purpose is served., 



"The more pervious materials should be placed adjacent 

 to the tile. The backfilling may be done with a plow 

 with three or more horses and a long pole evener, or with 

 a scraper, road grader, or go-devil. Recently power 

 backfillers have been placed on the market. All the soil 

 should be returned to the trench and be banked up over it, 

 so that future settling will not leave a depression over 

 the drain. 



"In machine trenching it generally is necessary to draw 

 a portable shield after the machine in which the tile may 

 be laid and blinded before caving takes place." 



Outlets and Silt Basins. The efficiency of a drainage 

 system may be greatly lessened by an ineffective outlet. 

 When the water leaves the drain it should flow away 

 freely and not be allowed to back up in the mouth of the 

 drain, since this condition causes silt to deposit and finally 

 clog the drain. The effectiveness of the drainage system 

 throughout its entire length may be lessened by standing 

 water at the outlet. If the fall of the land does not per- 

 mit of rapid flow from the outlet it may be necessary to 

 let the water run into a pit and then pump it out. This 

 method is in successful operation at Kearney Park, Cali- 

 fornia, in the system described by Weir (13). Here the 

 pumps are turned on by an automatic switch operated 

 by a float. 



