Tests 133 



tablk xvi. downward pressure on pipes in ditches in pounds 



PER IvINEAL FOOT 



Breadth Depth of ditch filling in feet 



i^Tilf "^ of ditch \ 



of pipe 3 2.5 5 10 15 



Inches Inches Pounds Pounds I Pounds Pounds 



1? ^S 480 840 I 1240 1 1450 



16 30 600 , 1130 1680 1990 



20 35 710 I 1350 2160 2660 



Comparing these loads with the strength of the pipe as shown 

 in column 4 of Table XII, it is apparent that while the smaller sizes 

 of pipe have ample strength, the 20-inch pipe, when buried over 10 

 feet, has a very small margin of safety. It is recommended that 

 designers should use a factor of safety of 1.25 to 1.5, depending on 

 how much care is likely to be taken in laying the pipe. It is not 

 practicable ordinarily to demand pipe of varying wall thicknesses 

 because pipe makers cannot afford to carry more than one set of 

 pallets for each size ; but the requisite strength can be secured in 

 other w-ays, especially by the control of the curing of the pipe, and 

 in important cases it may be necessary to require extra thick pipe 

 walls. 



Although the external pressure tests exhibit approximately equal 

 supporting strength for the various sizes of pipe, this is not a 

 rational relation ; the larger pipe should have greater strength. The 

 present-dav practice with respect to thickness of pipe is exhibited 

 in Table II. The practice should be changed by using slightly thin- 

 ner walls for small pipe and increasing the wall thickness of the 

 larger sizes. A great deal of 36-inch pipe is made with 3-inch walls. 

 It is said in defense that more gravel can be used in the concrete in 

 large pipe and that it can be better tamped than can small pipe. 

 But the percentage of failures of 36-inch pipe is greater than for any 

 other size. Thirty-six-inch pipe should have walls 3^ inches in 

 thickness. 



From the few tests made on hand-tamped pipe, it is seen that 

 16-inch pipe of that character is in danger of crushing if buried 10 

 feet in depth. 



Unlike drain tile and sewer pipe, both of w^hich are laid on even 

 gradients, irrigation pipe can follow the surface undulations, and 

 hence it is seldom laid over six feet in depth. Usually the internal 

 pressure head is the limiting factor for which the strength of the 

 pipe must be designed. Occasionally, however, a pipe line is placed 

 under a deep fill, and in those cases the considerations relating to 

 external pressure cannot be overlooked. 



