4 BULLETIN T2i, U. S. DEPAETMENT OF AGRICULTURE. 



Intercepting ditches also are usually approximately parallel to the 

 center line but are more remote, often being placed outside of grad- 

 ing lines as berm ditches. Not infrequently intercepting ditches, on 

 account of the immediately local conditions, are constructed so close 

 to the roadwa}' that they can be incorporated into the general cross 

 section, but in such cases their design is independent of the other 

 cross-section details, except as limited by them. Extreme cases of 

 this last condition exist when a road is constructed close to a drain- 

 age ditch or irrigation canal, or when, as has occurred not infre- 

 quently, the spoil bank of such a canal is used as a road grade. In 

 such cases the side ditch is omitted sometimes on the side toward 

 the intercepting drainage ditch, or a gutter design is used and the 

 flow in the gutter frequently turned into the larger channel. Lat- 

 eral or cross drains, whether involving the construction of culverts or 

 not, are to divert the flow from any longitudinal drain into a nat- 

 ural drainage channel. If this diversion involves crossing the cen- 

 ter line it usually requires a culvert. If it does not cross the center 

 line open lateral ditches are used. Subdrains are used to collect 

 and carry to a suitable outfall water that exists or may accumulate 

 periodical!}^ in the soil beneath the road surface or grade. Their 

 use, design, and location depend on the presence of ground water and 

 on the nature of the soil. A retentive impervious stratum under- 

 lying a road may prevent the downward percolation of surface 

 water to such an extent as to do injury by saturating the soil, or 

 the presence of springs or other flowing ground water may require 

 their use. In general, subdrains are emploj^ed for three purposes 

 appropriate to the conditions existing. They are used to divert 

 springs or confined streams of underground water; to intercept the 

 flow of underground water in the form of general seepage from 

 sources more or less distant; and to lower the water table in places 

 where the nature of the soil causes such water to rise too close to 

 the surface to permit of securing adequate bearing values in the soil. 



These various forms of drainage structures may be grouped into 

 two classes according as they provide for surface or subsurface drain- 

 age. Obviously, as betAveen the classes, the distinction is reasonably 

 sharp, although, as pointed out, side ditches sometimes serve to lower 

 the water table ; but the distinctions between the various uses of the 

 two types with regard to their respective purposes is often, if not 

 generally, impossible of clear definition. This is true especially of 

 subdrains. No matter what condition has led to the design and use 

 of a subdrain, if occasion arises it may act in all of its common 

 capacities. 



Further, these types of drainage structures must be grouped and 

 interrelated so as to produce the effect desired under the existing 

 conditions. The drainage problem, especially as affecting founda- 

 tions, is essentially a local one. This is apparent in some of the 



