PIPE LAYING AND PIPE LINE FAILURES 



PIPE LAYING 



The pipe should be laid in the trench in the same manner as 

 sewer pipe is laid, with straig^ht alignment and uniform grade. Many- 

 lines, where the pressure is light, are laid by the contractor without 

 any preliminary surveying. It is better, however, in all cases to 

 have the grades established and stakes set every 50 feet by a sur- 

 veyor. A map and ])rofile showing alignment, elevations, and gates 

 and valves is worth the cost. An undulating grade involves many 

 air pockets, which tend to induce water hammer and to decrease the 

 flow. 



THE TRENCH 



The width of the trench should be 10 inches greater than the 

 external diameter of the pipe. This allows room for the pipe layer 

 and for the bander who must straddle the pipe. 



The bed of the trench should be prepared with care. If the 

 trench is cut too deep and refilled to grade, the refilling should be 

 tamped. There is likely to be a very slight settlement of the entire 

 pipe line, but if it is uniform throughout the length no injury will 

 result. Unequal settlement, however, produces cracks. 



At the University Farm a pipe line was carried across a ravine 

 on an earth fill. In the center was a 5-foot culvert, the top of which 

 was close to the invert of the pipe line. The earth fill settled while 

 the culvert was unyielding, with the result that the pipe line cracked 

 just over the culvert several times until, finally, the settlement 

 ceased. 



The manner of backfilling must depend upon the size of the 

 pipe, the depth of earth to be supported by the pipe, and the nature 

 of the soil. Many expensive failures of pipe lines, usually of large- 

 sized pipe, have been due to insufficient backfilling beneath the in- 

 vert. Soil which breaks up in chunks, such as clay, does not settle 

 back beneath the pipe, while sandy soil or any soil that pulverizes is 

 known to the pipe layer as "a good pipe soil." The best methods of 

 filling the space beneath the pipe are by flooding a small depth of 

 backfill and by tamping. In one district where clay soil predomi- 

 nates, it is customary to backfill only to the top of the pipe and 

 then run a small stream of water down the line. This settles the 

 heavy soil under the pipe, and this supporting soil is allowed to dry 

 out before the trench is filled. Sometimes the flooding is repeated 



