3.9
DUCT LINES
3.9.1
Requirements
Numbers and size of ducts shall be as indicated. Duct lines shall be laid
with a minimum slope of 100 mm/30 m 4 inches/100 feet. Depending on the
contour of the finished grade, the high point may be at a terminal, a
manhole, a handhole, or between manholes or handholes. Short radius
manufactured 90 degree duct bends may be used only for pole or equipment
risers, unless specifically indicated as acceptable. The minimum
manufactured bend radius shall be 450 mm 18 inches for ducts of less than
80 mm 3 inches in diameter, and 900 mm 36 inches for duct 80 mm 3 inches or
greater in diameter. Otherwise, long sweep bends having a minimum radius
of 7.6 m 25 feet shall be used for a change of direction of more than 5
degrees, either horizontally or vertically. Both curved and straight
sections may be used to form long sweep bends, but the maximum curve used
shall be 30 degrees and manufactured bends shall be used. Ducts shall be
provided with end bells when duct lines terminate in manholes or handholes.
3.9.2
Treatment
Ducts shall be kept clean of concrete, dirt, or foreign substances during
construction. Field cuts requiring tapers shall be made with proper tools
and shall match factory tapers. A coupling recommended by the duct
manufacturer shall be used when an existing duct is connected to a duct of
different material or shape. Ducts shall be stored to avoid warping and
deterioration with ends sufficiently plugged to prevent entry of any water
or solid substances. Ducts shall be thoroughly cleaned before being laid.
Plastic ducts shall be stored on a flat surface and protected from the
direct rays of the sun.
3.9.3
Concrete Encasement
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NOTE: For crossings of existing railroads and
airfield pavements greater than 15.2 m (50 feet) in
length, the pre-drilling method or the
jack-and-sleeve method will be used.
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Ducts requiring concrete encasements shall comply with NFPA 70 except that
electrical duct bank configurations for ducts 150 mm 6 inches in diameter
shall be determined by calculation and as shown on the drawings. Duct line
encasements shall be monolithic construction. Where a connection is made
to a previously poured encasement, the new encasement shall be well bonded
or doweled to the existing encasement. At any point, except railroad and
airfield crossings, tops of concrete encasements shall not be less than the
cover requirements listed in NFPA 70. At railroad and airfield crossings,
duct lines shall be encased with concrete and reinforced as indicated to
withstand specified surface landings. Tops of concrete encasement shall
not be less than 1.5 m 5 feet below tops of rails or airfield paving unless
otherwise indicated. Where ducts are jacked under existing pavement, rigid
steel conduit shall be installed. To protect the corrosion-resistant
conduit coating, predrilling or installing conduit inside a larger iron
pipe sleeve (jack-and-sleeve) is required. For crossings of existing
railroads and airfield pavements greater than 15 m 50 feet in length, the
predrilling method or the jack-and-sleeve method shall be used. Separators
or spacing blocks shall be made of steel, concrete, plastic, or a
combination of these materials placed not more than 1.2 m 4 foot on
SECTION 16528A
Page 36
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