SEWERAGE 391 



either of which may add 1 T. loading to the pipe. A 12-in. 

 pipe in an 8-ft. trench will have a mass of 1X8X1 = 8 cu. ft. 

 of earth, or about 1,000 Ib. with 2,000 Ib. pressure on the top 

 resting on it. Only a fraction of this loading, however, is 

 transmitted to the pipe, the rest being supported by the sides 

 of the trench. A factor of safety of 3 should be employed. 

 It is safe practice to use double-strength pipe when the pipe 

 is in a trench less than 6 ft. deep, and heavy surface loads 

 may be expected. Under other conditions,, standard pipe 

 may be used, though double-strength pipe is always safer. 



Depth of Socket. There are two types of socket, the stand- 

 ard and the deep-and-wide, or deep, socket. The depths of 

 socket, in inches, are shown in the preceding table. The advan- 

 tage of the deep-and-wide socket lies in the fact that the 

 jointing material can be rammed into the sockets to a greater 

 depth, and there is therefore less leakage through the joints. 



BRICK AND CONCRETE SEWERS 



Brick Sewers. Sewers of a larger diameter than 24 in. are 

 generally built of brick or concrete and can be made in any 

 desired form. 



For ordinary conditions, the following empirical formula will 

 generally be found satisfactory for indicating the number of 

 rings required: 



in which R is the number of 4-in. rings or courses; D, the inter- 

 nal diameter of a circular sewef , or horizontal diameter of an 

 egg-shaped sewer; and H, the total depth of the trench all in 

 feet. 



Any fraction greater than .25 in the value of R should be 

 considered as 1. 



Concrete Sewers. The concrete used for sewers should be 

 of first-class quality, carefully proportioned to have as small 

 a percentage of voids as possible. The concrete must be 

 strong, to take up the tensile stresses in the arch; and imper- 

 vious, to keep ground water out of the sewers. A mixture 



