USE OF CONCRETE PIPE IN IRRIGATION. 17 



Some soil will settle when first irrigated and this settlement will 

 often allow the pipe line to settle also and cause cracks. If this type 

 of soil is encountered, the bottom of the trench should be thoroughly 

 soaked and settled before laying the pipe. 



Probably the greatest trouble with breaks and leaks in concrete 

 pipe lines is caused by expansion and contraction. It is common 

 knowledge that concrete expands with a rise in temperature, but it 

 is not so generally known that it also expands when saturated and 

 contracts when becoming dry. It may surprise some to learn that a 

 thoroughly air-dried pipe may contract as much in the process as 

 it would under a fall of temperature of 100° F. In such cases 

 cracks 0.18 of an inch every 25 feet or so are liable to appear. These 

 cracks may close up when the pipe is refilled with water, or trash 

 such as small particles of rock, sand or tree roots may enter and pre- 

 vent closure. 



The contraction due to pipe drying out often takes place after the 

 pipe line has been in use for one or more seasons. For this reason 

 it is common for a farmer to have to repair his pipe at the beginning 

 of every irrigation season. The change of temperature in a pipe that 

 is buried deep, especially when under the shade of large fruit trees, 

 is very small. Usually, however, the pipe becomes thoroughly dried 

 during the season of non-use and this drying out process is hastened 

 when orchard distributing stands are left open. 



In this connection it may be stated that a great deal of trouble is 

 experienced where pipe is installed in desert regions. In such places 

 the range of temperature and the drying-out process on the part of the 

 pipe are at a maximum. A sudden contraction of 7 inches when a 

 section of pipe was cut out is recorded. Expansion of pipe due to 

 wetting will often crush diversion boxes or relief stands, and in ex- 

 treme cases the pipe itself has crushed or telescoped. Pipe is also 

 cracked at curves from the same cause. One manufacturer in the 

 San Fernando Valley, Calif., has recorded a case where a 12-inch 

 pipe laid in a straight line for a distance of 4,000 feet, expanded 18 

 inches through a diversion box. There are numerous cases where 

 such stands have been crushed due to expansion of the connecting 

 pipe line. 



If the pipe can be anchored, either by concrete anchors at intervals 

 or by reinforcing the bottoms of relief or diversion stands, failure 

 by expansion may be guarded against, since concrete in compression 

 should have sufficient strength to prevent the crushing of the pipe. 

 On the other hand concrete is relatively weak in tension and the 

 pipe is certain to crack when sufficient contraction occurs. The 

 spacing of contraction cracks will depend upon the strength of the 

 pipe and joints, and the force tending to hold the pipe in place. The 

 3445°— 21 3 



