CONSTRUCTION NOTES 245 



of course be simplified in practice by the use of engineers' tables for the flow 

 of water in pipes. 



(b) Ground water. Ground water finds its way to drainage outlets 

 much more slowly and more constantly than surface water. Therefore the 

 element of time required to remove surplus soil-moisture is more favorable, 

 and smaller pipe may be used. Drainage lines to meet the requirements of 

 surface water and ground water may often be combined for the reason that 

 in practically every instance the surface water is removed so promptly, 

 following a storm, that the bulk of the flow in the drainage lines is com- 

 pleted at the time when the ground water flow reaches the pipes. In other 

 words the same pipe line may serve both requirements, bearing in mind 

 that it is not necessary to adopt a pipe of much additional capacity to 

 care for surface water and ground water near the immediate points where 

 this type of drainage originates. If this double use is made of the pipes, 

 however, there should be proper catch basins to prevent silting up the lines 

 with surface dirt. Also there is danger that the open joint pipe, if more than 

 full of surface water, will irrigate, not drain, for a time at least. Practice 

 has shown the four-inch tile to be the best minimum size for general ground 

 water drainage and the six-inch tile to be the best minimum size for drainage 

 from inlets and catch basins. 



The question of determining the required sizes of pipes in different 

 portions of a drainage system is an important one, and the factor of safety as 

 stated above should always be allowed, to meet the requirement of abnormal 

 conditions. The installation of a slightly larger pipe when the trench is 

 once open is small additional cost, while the expense of reexcavating and 

 installing a larger pipe is a tremendous relative cost and inconvenience, as 

 well as possibly a source of temporary damage to finished surface conditions. 



Effect of soil types on spacing and depth of pipes. This refers primarily to 

 ground water drainage. Gravel and sandy soils need very little or no under- 

 draining. Stiff clay soils on recreation areas may require laterals at inter- 

 vals as frequent as 10 feet, while normal farm practice is to place laterals 

 approximately 40 feet apart. Under garden areas and average lawn areas 

 the spacing of laterals at 2O-foot intervals is reasonable unless extremely 

 stiff clay conditions are common, in which case 12 to 15 feet is the 

 average spacing. Laterals placed under recreational areas such as tennis 

 courts, bowling greens, turf terraces, will average from 10 to 15 feet 

 apart depending upon the condition of the soil (whether heavy clay or light 

 clay loam). All laterals and mains for drainage areas should be laid, in 

 general, to conform with figure 8 and figure 9 on Plate 108. It should be 

 remembered, in determining the spacing of laterals, that the hydraulic 

 gradient for ground water in a heavy clay soil is much greater than the 



