Pipe Laving and Pipe Line Failures 109 



to a few hours after sunrise and again after sunset. Customarily the 

 pipe is covered only a couple of inches at first, the better to observe 

 leaks when the pipe line is tested. 



These precautions are not necessary in climates that are cool 

 with cloudy days. But in parts of Arizona and neighboring states 

 the summer temperatures are so high and the humidity is so low 

 that it is doubtful whether cement and concrete work of this char- 

 acter should be attempted in midsummer. Certainly it is preferable 

 to defer such work to the safer part of the year. The first and only 

 recognition of this important limitation, so far as known, is in the 

 contract of Yuma County for road culverts, dated December 15, 

 1915. The clause reads as follows: "It is further agreed that the 

 contractor shall not do any concrete work from June 1st, 1916, to 

 October 1st, 1916, unless with the written permission of the Board 

 of Supervisors." This clause should be used in contracts for many 

 kinds of cement work, possibly with the initial date changed to May 

 I. At Continental very little pipe was laid during the summer of 

 1917; the pipe making was continued, but special care was given to 

 curing the pipe and part of the pipe in the stackyard was covered 

 with arrowweed and brush. 



On a pipe line in British Columbia, where contraction cracks 

 were feared, slip joints were provided every 30 feet. They were 

 made as follows : When the pipe molds were three-fourths filled, a 

 galvanized iron thimble 5 inches wide was inserted and pressed 

 down in the mortar. The thimble was coated with heavy oil and 

 when half buried a layer of oil was poured on the mortar, after 

 which more mortar was put in and the molds were filled. 



EFFECT OF WETTING DRY PIPE 



Another cause of expansion and contraction of cement pipe lines 

 is variation in the degree of dryness. The drying out of mortar or 

 concrete produces contraction, while the saturation of mortar causes 

 it to expand. Running water through a line of pipe which has been 

 thoroughly dried out before it is laid causes the line to expand 

 with great force. This expansion may exert a tremendous pressure 

 against structures such as gate pits, division boxes, and weir boxes. 

 At Continental several of the gate pits as first constructed were de- 

 stroyed in this way. Fig. 20 shows the cracks on four sides of a 

 gate pit due to pressure from the south side. The pipe line at this 

 point runs north with a lateral towards the east. The cracked gate 

 pits were not removed, but the lower portions were enclosed in a 



