from curbs, 3 feet from fire hydrants, 12 feet from the
nearest track of a railroad track, and 7 feet from
railway sidings.
When you are staking pole locations, the center of
each pole is marked with a hub on the line; the hub
may be offset. On the guard stake, you put the pole
number, the line elevation, and the distance from the
top of the hub to the top of the pole obtained from the
profile.
TOWER LINE SURVEYS.— High-voltage
lines are often supported by broad-based steel towers.
For a tower line, construction economy requires that
changes in direction be kept at a minimum. That is
because a tower located where a line changes direction
must withstand a higher stress than one located in a
straight direction part of the line. In general, tower
construction is cheaper in level country than in broken
country; however, the line may be run over broken
country to minimize changes in direction, to make the
distance shorter, or to follow a line where the cost of
obtaining right-of-way is inexpensive. Lines should
be located adjacent to existing roads, whenever
practical, to provide easier access for construction and
future maintenance. When a change in direction in a
tower line is unavoidable, it should be made gradually
in as small-angular increments as possible. Suppose,
for example, a change in direction of 90° is required.
Instead of an abrupt change in direction of 90°, towers
should be set so as to cause the line to follow a gradual
curve in a succession of chords around an arc of 90°.
Route Surveys for Drainage
When man-made structures are erected in a certain
area, it is necessary to plan, design, and construct an
adequate drainage system. Generally, an underground
drainage system is the most desirable way to remove
surface water effectively from operating areas. An
open drainage system, like a ditch, is economical;
however, when not properly maintained, it is unsightly
and unsafe. Sometimes, an open drainage system also
causes erosion, thus resulting in failures to nearby
structures. Flooding caused by an inadequate drainage
system is the most prevalent cause leading to the rapid
deterioration of roads and airfields. The construction
and installation of drainage structures will be
discussed later in this chapter. At this point we are
mainly interested in drainage systems and types of
drainage.
DRAINAGE SYSTEM.—Sanitary sewers carry
waste from buildings to points of disposal; storm
sewers carry surface runoff water to natural water
courses or basins. In either case the utility line must
have a gradient; that is, a downward slope toward the
disposal point, just steep enough to ensure a gravity
flow of waste and water through the pipes. This
gradient is supplied by the designing engineer.
Natural Drainage.—To understand the con-
trolling considerations affecting the location and other
design features of a storm sewer, you must know
something about the mechanics of water drainage
from the earth’s surface.
When rainwater falls on the earth’s surface, some
of the water is absorbed into the ground. The amount
absorbed will vary, of course, according to the
physical characteristics of the surface. In sandy soil,
for instance, a large amount will be absorbed; on a
concrete surface, absorption will be negligible.
Of the water not absorbed into the ground, some
evaporates, and some, absorbed through the roots and
exuded onto the leaves of plants, dissipates through a
process called transpiration.
The water that remains after absorption, evapora-
tion, and transpiration is technically known as runoff.
This term relates to the fact that this water, under the
influence of gravity, makes its way (that is, runs off)
through natural channels to the lowest point it can
attain. To put this in terms of a general scientific
principle, water, whenever it can, seeks its own level.
The general, final level that unimpeded water on the
earth’s surface seeks is sea level; and the rivers of the
earth, most of which empty into the sea, are the earth’s
principal drainage channels. However, not all of the
earth’s runoff reaches the great oceans; some of it is
caught in landlocked lakes, ponds, and other non-
flowing inland bodies of water.
Let’s consider, now, a point high in the mountains
somewhere. As rain falls in the area around this point,
the runoff runs down the slopes of a small gully and
forms a small stream, which finds a channel down-
ward through the ravine between two ridges. As the
stream proceeds on its course, it picks up more and
more water draining in similar fashion from high
points in the area through which the stream is passing.
As a result of this continuing accumulation of runoff,
the stream becomes larger until eventually it either
becomes or joins a large river making its way to the
sea—or it may finally empty into a lake or some other
inland body of water.
In normal weather conditions, the natural
channels through which this runoff passes can
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