protection, an observer on the surface, and a
safety harness and line may be required for
safe entry. Consult your supervisor before
entering any manhole.
DUCTS AND TRENCHES
A duct line and manhole system provide the best
available underground system. Such a system allows
for growth and permits cost-effective replacement of
existing cables or cable terminations damaged by
faults or made obsolete by aging. Concrete encasement
provides the cables with minimum susceptibility to
damage and optimum safety to personnel. Several
types of underground ducts are in general use, such as
fiber, wood, vitrified tile, iron pipe, asbestos
composition polyvinyl chloride (PVC). and concrete.
The most common type used today by the Navy is
An underground installation usually consists of
several duct lines. Joints between sections should be
staggered, so the joints in several lines do not all occur
at the same place. To ensure staggering, use starting
sections of different lengths at the starting manhole.
For duct set in concrete, there must be at least 3 inches
of concrete around each line of duct. Where concrete
encasement is specified, concrete should have a
strength of not less than 2,500 pounds per square inch
when cured for 28 days. The 3-inch spacing is
accomplished by the use of spacers, like the one shown
in figure 4-42. The upper lines of the duct must be a
minimum of 3 feet below the ground surface.
The location of the trench varies according to
ground condition. The trench should run as straight as
possible from one manhole to the next. To ensure
drainage, slope the line downward about 1 foot but
never less than 3 inches every 100 horizontal feet.
When one manhole cannot be located at a lower level
than the other, the tine must slope downward from
about the midpoint both ways toward the manhole, as
shown in figure 4-43.
Dig the trench to the desired depth and tamp the
bottom hard to ensure a solid bed for the 3-inch bottom
layer of concrete. Spacers can be embedded in the
bottom layer of concrete for a depth of about 1 inch
before the concrete sets to ensure a solid base.
Burying cable directly in the ground is widely
done for installations of single circuits for which the
cost of duct construction would be prohibitive. Some
of the more common applications of direct burial cable
are as follows:
1. Street-lighting circuits, especially on a base
whose outlying sections are without ducts
Figure 4-42.Spacing fiber duct in concrete.
2. Connecting residences to mains
Figure 4-43.Slope for duct run.