3.5.3
Concrete Encasement
Each single duct shall be completely encased in concrete with a minimum of
75 mm 3 inches of concrete around each duct, except that only 50 mm 2 inches
of concrete are required between adjacent electric power or adjacent
communication ducts, and 100 mm 4 inches of concrete shall be provided
between adjacent electric power and communication ducts. 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
crossings, tops of concrete encasements shall be not less than 450 mm 18
inches below finished grade or paving. At railroad crossings, duct lines
shall be encased with concrete, reinforced as indicated. Tops of concrete
encasements shall be not less than 1.5 m 5 feet below tops of rails, unless
otherwise indicated. Separators or spacing blocks shall be made of steel,
concrete, plastic, or a combination of these materials placed not further
apart than 1.2 m 4 feet on centers. Ducts shall be securely anchored to
prevent movement during the placement of concrete and joints shall be
staggered at least 150 mm 6 inches vertically.
3.5.4
Non-encased Direct-Burial
Top of duct lines shall be below frost line but not less than 600 mm 24
inches below finished grade. Ducts shall be buried below frost line but in
the earth and shall be installed with a minimum of 77 mm 3 inches of earth
around each duct, except that between adjacent electric power and
communication ducts, 300 mm 12 inches of earth is required. Bottoms of
trenches shall be graded toward manholes or handholes and shall be smooth
and free of stones, soft spots, and sharp objects. Where bottoms of
trenches comprise materials other than sand or stone-free earth, 75 mm 3
inch layers of sand or stone-free earth shall be laid first and compacted
to approximate densities of surrounding firm soil before installing ducts
in direct-contact tiered fashion. Joints in adjacent tiers of duct shall
be vertically staggered at least 150 mm 6 inches. The first 100 mm 4 inch
layer of backfill cover shall be sand or stone-free earth compacted as
previously specified. Duct banks may be held in alignment with earth.
However, high-tiered banks shall use a wooden frame or equivalent form to
hold ducts in alignment prior to backfilling. Selected earth at duct banks
shall be thoroughly tamped in 100 to 150 mm 4 to 6 inch layers.
3.5.5
Installation of Couplings
Joints in each type of duct shall be made up in accordance with the
manufacturer's recommendations for the particular type of duct and coupling
selected and as approved. In the absence of specific recommendations,
various types of duct joint couplings shall be made watertight as specified.
3.5.5.1
Asbestos-Cement and Bituminized-Fiber Ducts
To ensure a watertight joint, tapered ends or joints of the same material
as the ducts shall be swabbed with bituminous or joint-sealing compound
before couplings are applied. Plastic or nonmetallic couplings shall be
tightly driven onto unswabbed ducts. Due to the brittleness of plastic
couplings at low temperatures, such couplings shall not be installed when
temperatures are below -18 degrees C 0 degrees F. Couplings shall be
warmed in hot water or by another approved method when installed at
temperatures below 0 degrees C 32 degrees F.
SECTION 16526A
Page 35
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