GROUND-WATER REACHING SEWERS 149 



ciently tight for all practical purposes, provided the joints 

 could be well filled and could remain undisturbed by water 

 or jarring until sufficiently set to resist. Unfortunately, in 

 practical construction joints in a sewer-pipe are never made 

 with the same care as in a laboratory experiment; and further, 

 it seldom happens that the joints are allowed to stand from 

 12 to 48 hours before being covered with water, as was done 

 in these experiments. In a wet trench the cement is not always 

 forced into the joint, and water is admitted to the joint before 

 the cement is thoroughly set, tearing off the coating and leaving 

 an opening into the pipe. 



To approximate actual conditions as nearly as possible, 

 F. S. Senior, as his thesis work, under the direction of the 

 author, made some experiments in which water was admitted 

 to the joint at various intervals from the time of making. 

 His experiments* brought out the following points: First, that 

 there is to be expected a gradual improvement in the tight- 

 ness of cement-joints from the time that they are first laid, 

 amounting to from 40 to 80 per cent, and that the decrease in 

 leakage is greater for Rosendale cement than for Portland. 

 Second, that there is more leakage under high heads, and 

 that the increase with the head is nearly proportional to its 

 square root. Third, that there is a great advantage in using 

 quick-setting cement if there is any probability of having the 

 joints covered with water; and further, that a quick-setting 

 cement will reduce the length of time necessary to pump from 

 a wet trench, since the amount of infiltration after the cement 

 has taken a hard set is inconsiderable. Fourth, that in a wet 

 trench a gasket is of great value; and whereas without it a 

 line on which water has risen before the cement in the joints 

 was hard would admit water to the extent of half filling a 

 6-inch pipe, yet with gaskets the leakage would be no more than 

 if the water had been kept off till the cement had set. This 

 last is in contradiction to Mr. Coffin, who concluded that a 



* Trans. Ass'n Civil Eng'rs, Cornell Univ., 1897, p. 113. 



