USE OF COXCRETE PIPE IX IRRIGATION". 21 



cement. It may not be tamped properly and is nearly always made 

 from a very dry mixture. 



Irrigation pipe if kept full of water free from alkali salts, tends 

 to exclude ground water containing alkali, from the shell of the pipe. 

 If the pipe is under pressure, there seems to be little chance of alkali 

 salts entering the pores of the pipe from the outside. Then too, irri- 

 gation pipe is usually laid on the higher parts of the land that are 

 freest from alkali. Culverts of poor grade of concrete pipe used for 

 road crossings have been observed to be almost eaten away when ex- 

 posed to water strongly saturated with alkali. On the contrary, irri- 

 gation pipe of a good grade of concrete buried in compact soils im- 

 pregnated with alkali adjacent to these culverts has been found to be 

 entirely sound. 



Briefly stated, the sulphates and magnesia salts seem to be the most 

 harmful, but dense concrete made from wet mixtures is usually very 

 little affected. It has been suggested that drain tile laid in alkali 

 soils be made at least of a one to three mixture of cement to aggre- 

 gates. It is probable that a much leaner mixture can be used for low- 

 pressure pipe, if care is taken to grade the sand and rock in the aggre- 

 gate. Rock dust, clay, or even silt may be added to the mixture to 

 make an impervious pipe. In any case the pipe should be made from 

 a wet mixture and thoroughly tamped or compressed. 



Another source of failure in concrete pipe is the rupture of the 

 pipe due to sudden increases of pressure, which may be caused by 

 water hammer. Water hammer is especially troublesome where 

 water is pumped directly into a pipe line and where the pipe is of con- 

 siderable length and runs up grade from the pump. If the pump is 

 started and stopped gradually, the extra pressure due to water ham- 

 mer will be slight, but if the pump is suddenly stopped, pressures 

 will sometimes increase two or even three times above normal. The 

 same thing may be caused if a valve in the discharge main from the 

 pump is suddenly closed. Increased pressures may be recorded in any 

 pipe that is running at full capacity under pressure, if a valve is sud- 

 denly closed. 



The usual method of preventing breaks from water hammer is to 

 provide a standpipe. The standpipe or relief stand should be high 

 enough to allow for grade and friction in the pipe when running at 

 full capacity and should be about the same diameter as the main, 

 although large mains are often protected by stanclpipes of consider- 

 ably less diameter than the main. If pressures are increased sud- 

 denly, water will rise in the standpipe and overflow, thus relieving 

 the pressure in the line. For stands 25 feet in height and under, 

 concrete pipe can be used. It is good practice to reinforce the first 

 two or three joints if high stands are used, or pipe of larger diameter 

 can be used for the first two or three joints. One standpipe near 



