Other Usf.s of Cement Pipe 155 



failures of sewer pipe are due to crushing in by the overlying load. 

 The specifications require that the pipe should withstand an internal 

 pressure of 25 pounds per square inch, and also the equivalent of a 

 20-foot backfill. Our tests show that with few exceptions the pipe 

 will stand much higher internal pressures than 25 pounds. The im- 

 plication, therefore, is that the sewer pipe is of exceptionally strong 

 quality. 



However, inasmuch as the relationship between the hydrostatic 

 pressures and the equivalent external pressures has never been de- 

 termined, we recommend that the pipe shall be tested also to deter- 

 mine its resistance to external loads, according to any one of several 

 methods that are in use in other places. It is not essential that the 

 pipe laying shall be held up while the apparatus for making these 

 tests is being prepared. 



We find a small percentage of underburned, cracked, and warped 

 tiles, but these are discovered and thrown out by the inspector, ac- 

 cording to the system universally in use. 



In conclusion, we would say that in our judgment the pipe being 

 laid is the equal of the average of such pipe; that from the stand- 

 point of durability, sanitation and general fitness for the service to 

 which it will be subjected it will be satisfactory. But we believe 

 that in general the pipe is somewhat overburned and would advise 

 that the pipe to be delivered for the balance of the contract shall be 

 somewhat less vitrified, so as to have a breaking strength of from 

 25 to 40 pounds and so as to make a better showing in the perco- 

 lation test. 



Respectfully submitted, 



G. E. P. Smith, 



J. C. McClure, 

 I. McAvoY. 



In general, it may be said that the sewer pipe is as good as can 

 be obtained from California factories, presumably as good as can 

 be made from California clays. It is unfortunate that the specifica- 

 tions were drawn so rigidly as otherwise lower bids might have 

 been received. 



In making the tests it was noticeable that there were two kinds 

 of vitrification exhibited. In one case the surface was of a purple 

 black color, excessively fused and glazed, and the interior was 

 burned black. This pipe was found to be very strong, but also 

 pervious. In the other type the surface color was a dark neutral 

 tint, the glazing smooth and pimply, apparently less fused, and the 

 color of the fracture was slightly yellow instead of solid black. This 

 pipe was less strong than the other type, but much more imper- 

 vious. It can be concluded that excessive burning in the kiln hard- 

 ens the pipe, increasing the strength but making it porous. A small 



