GRADING, DRAINAGE AND LAWNS 15 



to prevent backwashing or undermining. This type of outlet should be 

 covered with a hinged screen or grating that will exclude animals, but 

 allow the escape of water. 



Subsurface drainage (the removal of ground water) is accomplished 

 by laying open-jointed drains of porous or vitrified pipe. The vitrified 

 pipe is seldom used except where a much stronger pipe is required. 

 In many cases the subsurface drainage is all that 1 is required, as water 

 on the surface of the soil will quickly disappear if the ground beneath 

 is drained. 



In small areas there is need, usually, only of a few lines of tile 

 connected with a suitable outlet, but in large areas a regular system 

 of mains and laterals has to be devised and laid out according to the 

 lay of the ground. The laterals are usually parallel approaching the 

 mains obliquely or at right angles. 



The average depth of tile line is 2^4 to 3 feet, although the 

 possible range is from 1 J/2 to 4J/2 feet. Shallow-laid tile is of less 

 benefit for growing purposes and more liable to harm from frost action. 

 The distance apart of tile lines is determined by the existing conditions 

 and the purpose served. In draining lawn areas the tile lines may be as 

 close as 1 to 15 feet apart this, as in all subdrainage, is to be 

 understood to exclude all light porous soils where the ground water 

 level is not near the surface. In fields used for agricultural purposes 

 the average distance apart for tile lines in heavy clay soils is 30 feet; 

 in muck soils, 60 to 80 feet, in ordinary loams, 70 to 100 feet. Fre- 

 quently, as in gardens, the placing of the tile lines is determined by the 

 location of the planting beds or walks which have to be drained. (See 

 chapter on "Walks and Driveways"). 



The amount of water to be carried, the size of the area drained 

 and the gradient, all have to be considered in determining the size of 

 the tile. In large systems it is a matter of economy to u:e tiles, of the 

 proper sizes. In small areas when the difference in cost of the differ- 

 ent sizes does not make much difference in the expense, the average 

 sizes can be used. The average size tile is 3 to 5 inches in diameter; 

 it is advisable not to use anything smaller than the 3 inch size in 

 most cases. 



