94 BULLETIN 852, U. S. DEPARTMENT OF AGRICULTURE. 



The writer's experience has been that curves should have a radius of 

 not less than six times the diameter of the pipe. 



If these precautions are taken there is no reason why concrete pipes 

 can not be so produced as to have a value of n = 0.011, or even 0.0105 

 in Kutter's formula. It is therefore principally a question of engi- 

 neering, which I am sorry to say has been greviously neglected in 

 connection with concrete pipe work in the past. 



Little light can be thrown on the possible carrying capacity of 

 concrete pipes by studying most of the old pipes of this class found in 

 southern California, as they were nearly all laid out without any 

 engineering supervision, or a proper survey to give uniform grade 

 and proper alignment. A review of the author's experiments on 

 these old pipes clearly prove this point. 



In practice, the carrying capacity of concrete pipes, in common with 

 other kinds, is often greatly reduced by the entrance of air at the 

 intake. It is often customary to take the water into a pipe line 

 from the overfall of a Francis weir for measuring it. The effect of 

 this is to cause a large volume of air to be carried into the pipe with 

 the water. The writer has often found this condition in connection 

 with concrete pipes in southern California and elsewhere, and, after 

 correcting it, has increased the carrying capacity of the line from 

 10 to 20 per cent. 



The examples of concrete pipe given by the author, with which the 

 writer was connected, are the 36-inch Boulder Creek concrete pipe 

 line in Colorado, designated No. 52, experiment S 41, for which 

 work he prepared the specifications and acted as consulting engineer. 

 The experiment from which the author obtains a value of n = 0.012 

 was near the upper end of this pipe. If the test had been made at a 

 point farther down the line it is probable that a value of n= 0.0116, 

 or better, would have been obtained, the same as for the Mill Creek 

 No. 2 pipe (No. 50, experiment FF 1), because the two pipes were 

 manufactured and laid in the same summer. 



It has been well demonstrated as a hydraulic principle that there 

 is a slight but constant acceleration of the velocity in long gravity- 

 flow pipe lines of this kind, and the writer has found a difference in 

 the value of n near the upper and lower ends of such pipes, where all 

 the conditions were exactly alike. This phenomenon has been referred 

 to by some authorities and called by the name "Constant acceleration 

 in gravity conduits." 



Both the Boulder pipe (No. 52, experiment S 41) and the Mill 

 Creek No. 2 pipe (No. 50, experiment FF 1) were made and laid by 

 day work under careful engineering supervision, which explains the 

 results obtained as to carrying capacity, although these pipes were 

 both made in 2-foot sections by the dry-mix process. It is the opinion 

 of the writer that, had these pipes been made with wet mix, they would 

 have showed a value of n = 0.0105, or probably even a little better. 



As to the other two pipes, for which the writer was engineer, 

 namely Mill Creek No. 3 (No. 51, experiment FF 2) and Lytle Creek 

 (No. 53, experiment FF 3), it must be remembered that both of these 

 were installed by contract and without proper precautions in regard 

 to making the joints, which is wholly responsible for the higher value 

 of n and less carrying capacity of these pipes. 



From the above and from all other experiences of the writer, his 

 conclusion is that the values of n given by the author, as applicable 

 to the best constructed pipes, which can be produced by the dry-mix 



