e always
to be avoided.

Where the side ditch passes from a cut to the berm alongside a fill,
the ditch should be excavated throughout in the undisturbed natural
soil, five feet or more from the toe of the slope of the fill, and
along the filled portion of the road there should be a berm of three
or four feet between the toe of the slope of the fill and the near
edge of the ditch.

=Underground Water.=--In a preceding paragraph, mention was made of
the fact that only a part of the storm water runs off over the surface
of the ground, the larger part being absorbed by the soil. The water
thus absorbed flows downward through the pores in the soil until it is
deflected laterally by some physical characteristic of the soil
structure. The movement of underground water is affected by many
circumstances, but only two conditions need be discussed herein.

Underground water, like surface water, tends to attain a level
surface, but in so doing it may need to flow long distances through
the pores of the soil, and to overcome the resistance incident to so
doing some head will be required. That is to say, the water will be
higher at some places than at others. If a cut is made in grading the
road, the road surface may actually be lower than the ground water
level in the land adjoining the road. As a result, the water will seep
out of the side slopes in the cut and keep the ditches wet, or even
furnish enough water to occasion a flow in the ditch. Similarly, the
higher head of the underground water near the top of a hill may result
in ground water coming quite close to the surface some distance down
the hill. The remedy in both cases is tile underdrains alongside the
road to lower the ground water level so that it cannot affect the road
surface.

Sometimes the ground water encounters an impervious stratum as it
flows downward through the soil, or one that is less pervious than the
surface soil. When such is the case, the water will follow along this
stratum, and should there be an outcrop of the dense stratum, a spring
will be found at that place. This may be on a highway. The impervious
stratum may not actually outcrop but may lie only a few feet under the
surface of the road, in which case, the road surface will be so water
soaked as to be unstable. The so-called "seepy places" so often noted
along a road are generally the result of this condition. This
condition can be corrected by tile laid so as to intercept the flow at
a d