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 
