Conservation Service, U. S. Department of Agriculture and 

 tile system contractors; however, final decisions are 

 usually made by individual farmers. The materials, construc- 

 tion, and methods of installation are discussed in the 

 subsections that follow. 



Conduit. In the past, the term "tile drain" referred to a 

 drainage system in which red clay tile conduit was used. 

 The term as used locally refers to drainage systems using 

 both concrete and tile. Prior to 1969> concrete pipeline 

 was used almost exclusively in all major tiled areas. 

 Although it usually costs more than red clay tile or plastic 

 conduit, its tongue -and -groove construction resists deforma- 

 tion by the soil more than does the "butt joint" clay tile, 

 which makes it more desirable from a maintenance standpoint. 



The use of plastic as conduit is rapidly becoming popular 

 because it is less expensive than concrete pipeline. Several 

 large drainage systems constructed of plastic were installed 

 in the Gustine-Mendota area in 1969 and 1970. 



Concrete and tile conduit are segmented, usually in sections 

 whose lengths depend on the diameter. The diameter of 

 conduit to be used is chiefly determined by the size of the 

 drainage system and the expected peak discharge. Plastic 

 conduit is available in several different diameters and 

 usually comes in a more or less continuous roll which is 

 perforated throughout its length. Plastic drains installed 

 recently in the Gustine-Mendota area appear to perform as 

 well as the concrete and clay tile systems. 



Filter Materials . To operate satisfactorily, subsurface 

 drainage systems require a filter medium. Sand and gravel 

 placed around the conduit during installation have proven 

 to be quite successful in most major tiled streas . The kind 

 of material and the amount needed seem to be a matter of 

 controversy. Investigation of tile performance by Johnston 

 and Pillsbury (4) has shown that the amount of filter sand 

 has no relationship to the efficiency of drain operation, 

 providing it is no less than 6 and no more than 18 tons per 

 100 lineaj? feet of conduit. 



Soil Conservation Service engineers (5) have reported better 

 results with pea gravel than with graded sand or gravel in 

 tile systems located near Tracy. 



Contractors and drainage specialists have reported instances 

 of tile drainage systems which have ceased to function 

 because improper filter materials were used. Fiberglass, 

 used as envelope material in certain experimental plots near 

 Firebaugh, has been found in a deteriorated condition after 

 only a few years in the soil . Inspection of excavated 



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