GROUND-WATER REACHING SEWERS 151 



circumference or directly as the diameter, except that there 

 is an abrupt change between the 10- and 1 2-inch pipes. 



As an example, a 1 2-inch pipe 2 miles long, laid in water, 

 might be reasonably expected to carry as a minimum amount 

 of subsoil infiltration 4000x2x2 or 16,000 gallons of water, 

 while the capacity of the pipe flowing half-full at a velocity of 

 2 feet per second is 650,000 gallons per day, or the leakage 

 is about i\ per cent of the capacity of the pipe. This might 

 be somewhat reduced by allowing 12 hours for the cement 

 to set before the water is turned on, but it takes into account 

 not a single bad -joint nor one which is not fully filled, of which 

 there are always many in actual construction. If the leak- 

 age were at the rate of 10,000 gallons for a 6-inch, or 20,000 

 gallons for the 1 2-inch pipe, it would be less than most of the 

 examples cited and would be only 6 per cent for the 2 miles. 



In Vol. XIX of the Journal of the Association of Engi- 

 neering Societies is a valuable paper by Mr. F. A. B arbour 

 of Brockton, Mass., on the strength of sewer-pipe, and inci- 

 dentally on some tests of the tightness of pipe-joints. He gives, 

 however, no conclusions as to amounts, saying that the results 

 have been decidedly unsatisfactory from the standpoint of a 

 written report, and no tabulations of the figures will be given. 



The evident lesson so far as ground-water is concerned is 

 that, instead of adding a certain percentage to the desired 

 capacity of the sewers, a more rational method is to consider 

 in detail the lengths of pipe to be laid under a head of ground- 

 water, and to increase those lines and the mains lower down by 

 a certain amount of leakage per mile, the amounts to be 

 arrived at by actual experience and by the experiments quoted. 



Mr. Alexander Potter, Engineer of the Joint trunk sewer 

 of New Jersey, concludes* that even with rigid inspection it is 

 not safe to count on less than about 25,000 gallons per mile 

 per day leakage with cement joints, but with sulphur-sand 

 joints it may be reduced to about 5000 gallons per day. These 



* Engineering Record, Vol. LX, p. 377. 



