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, 



