18 BULLETIN 906, U. S. DEPARTMENT OF AGRICULTURE. 
weight of the pipe and earth pressure over it are usually the forces 
tending to prevent the pipe moving, therefore contraction cracks will 
occur when the strength of the pipe in tension is not sufficient to pull 
a certain length of pipe. There are a number of formulae for 
estimating the distance apart that contraction cracks will appear, 
but no records have been found where experiments were carried on 
in the field. It is probably safe to expect pipe of small diameters to 
crack about every 25 to 40 feet when laid under unfavorable conditions 
and subjected to sufficient contractive power. Contraction cracks 
usually appear at the joints, although at times the pipe is broken. 
There are two methods of reducing to a minimum the trouble with 
expansion or contraction. The first is to lay the pipe under favorable 
conditions, the second is to provide expansion joints at frequent 
intervals. 
Favorable conditions for laying concrete pipe depend upon tem- 
perature, moisture in the pipe when laid, and moisture in the ground. 
Pipe laid in winter, when the ground is moist from rains, seldom 
gives trouble, especially if the pipe is kept continually full of water 
after being laid. Pipe laid in sandy soil, when the ground is hot and 
dry, is almost certain to develop contraction cracks unless water is 
turned in promptly. If it is necessary to lay pipe in hot, sandy soil, 
the trench should be deep and the backfill kept moist until water can 
be turned in. As bands and joints set very rapidly, pipe can often be 
filled with water 24 hours after laying. Heavier soil containing 
moisture will protect pipe from drying out. If the pipe is dry when 
laid, and the soil is dry and warm, there will probably be little change 
in the length of the pipe until it is filled with water, but when the 
pipe is thoroughly soaked there is liable to be considerable expansion. 
This accounts for expansion troubles that are so common under 
desert conditions. If the movement of pipe due to expansion is pre- 
vented, no trouble may be experienced thereafter, as the elasticity 
of the concrete may prevent contraction when the pipe is dried out 
again after the first irrigation. Some engineers have advocated that 
the pipe be wetted before laying in desert soil, but this would not 
seem to be good practice because if the soil is dry the pipe would 
soon become dry too, causing contraction cracks, before the water 
was turned in. It would seem a good plan to have the pipe as cool 
as practicable before laying under the above conditions, as the pipe 
that is placed along the trench in the hot sun before being laid 
cools off when covered with the backfill, and this will cause con- 
traction. It should be possible in many cases to lay pipe so that 
expansion due to wetting the pipe when first laid in the trench will 
counteract the contraction due to cooling off when covered with soil. 
The safest procedure to be followed in laying pipe in the desert in 
the summer is to do the work at night, when the temperature is low, 
