4 BULLETIN 724, U. S. DEPARTMENT 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 roadway 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 
periodically 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 employed 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 between 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. Xo 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 
