PirE Laying and Pipe Line Failures 115 



the joint mortar, to be followed about three days later by a full 

 head. The safest method is to leave the pipe line open at intervals 

 so that a part of the water can rise and flow in the trench, and thus 

 the walls will absorb water from the outside and from the inside 

 simultaneously. 



In the light of these experiments it seems apparent that the 

 longitudinal cracks at Continental were due to progressive satura- 

 tion of the pipe wall from the inside and greater saturation of the 

 under side of the pipe than of the upper side. Of course, other forces 

 were working. The expansion of the invert, together with the ex- 

 ternal load pressure, the internal hydraulic pressure, and longitudi- 

 nal shear on curves, all produce tension at the top of the pipe, that is, 

 these forces are all additive. A break may be the result of a combi- 

 nation of several or all of them. 



It is known that concrete under high stress flows to a consider- 

 able extent and thus tends to relieve itself of stress.* It would ap- 

 pear, however, from the experience had at Continental and else- 

 where that the mortar does not flow fast enough to prevent the de- 

 velopment of very high stresses. 



It is known also that concrete and mortar when wetted, after 

 being cured in dry air, temporarily lose a considerable part of their 

 strength. Van Ornum reports for short concrete cylinders a loss of 

 forty percent.t In the case of rich mortars the percentage of loss 

 may be even greater. 



Failures of pipe lines have occurred in the San Joaquin Valley 

 under similar conditions and for similar reasons. Scores of gate 

 pits have been destroyed, curves have buckled out of line and pipe 

 lines have opened at top and bottom. It appears that the true cause 

 has never been assigned to these failures. In humid regions the 

 pipe does not become dry enough to cause trouble. 



Several important lessons are to be learned from the experiences 

 at Continental. In the first place, it is much safer to do cement- 

 pipe making and laying in the cooler part of the year. In arid cli- 

 mates extreme drying of the pipe in the stackyard should be pre- 

 vented. If pipe is made in the summer time and must be stacked 

 in the open it should be stacked high instead of being spread over 

 a large area. The stacks should be covered with brush or should 

 be under good roofs. The pipe should be wetted occasionally, espe- 

 cially for a few weeks before the pipe is to be laid. If it is neces- 

 sary to lay bone-dry pipe, larger than 14-inch diameter, the pipe 



•-Tour. Amer. Cone. Inst., Feb.. 1917. 



tTrans. Am. Soc. C. E., Vol. LXXVII, p. 438. 1014. 



