SEWERS 43 



soil by the dissolving out of soluble matters after the 

 laying of the tile. Furthermore, the land will never be- 

 come as wet as it originally was on account of the fact that 

 the surface water is rapidly disposed of by the surface-water 

 drainage system. 



Secondly, land can be subdrained by the construction of 

 stone or brush ditches. The principle of action is the same 

 as in the case of the tile ditches. Stone or poles are placed 

 in the bottom of the trench to provide open space for the 

 passage of the water. To prevent clogging up with earth, 

 the stone or poles are covered with branches or straw, on 

 which the backfilling is deposited. 



General Remarks. Trenches for underdrainage are 

 usually spaced 50 feet apart. The depth varies from 1| 

 feet to 4 or 5 feet. The slopes should not be less than 

 say 0.25 of a foot in 100 feet except for the mains. 



SEWERS 



Park sewers are of three kinds: 1. Storm- water Sewers; 

 2. Sanitary Sewers; and 3. Combined Sewers. The methods 

 of design and construction are in some respects different 

 from those of municipal sewers. 



Sewers are usually laid along straight lines as in mu- 

 nicipal work, but the great expense of manholes can be 

 saved in whole or in part. Park sewers seldom lie under 

 pavements. If they become stopped, the trench can be 

 reopened and the obstruction removed. It will be cheaper 

 to do this occasionally (although it hardly ever becomes 

 necessary) than to go to the great expense of placing man- 

 holes at every change of line and grade. 



Park sewers are of brick or vitrified pipe. Sewers larger 

 than 36 inches in diameter are built of brick. When the 

 soil is treacherous, steel or spiral-riveted pipe may some- 



