less than 75 mm 3 inches in diameter and a minimum radius of 915 mm 36
inches for ducts of 75 mm 3 inches in diameter and larger. Excavate
trenches along straight lines from structure to structure before ducts are
laid or structure constructed so the elevation can be adjusted, if
necessary, to avoid unseen obstruction.
3.1.5.3
Conduits
Terminate conduits in end-bells where duct lines enter underground
structures. Stagger conduit joints by rows and layers to strengthen the
duct bank. Provide plastic duct spacers that interlock vertically and
horizontally. Spacer assembly shall consist of base spacers, intermediate
spacers, and top spacers to provide a completely enclosed and locked-in
duct bank. Install spacers per manufacture's instructions, but provide a
minimum of two spacer assemblies per 3050 mm 10 feet of duct bank. Before
pouring concrete, anchor duct bank assemblies to prevent the assemblies
from floating during concrete pouring. Anchoring shall be done by driving
reinforcing rods adjacent to every other duct spacer assembly and attaching
the rod to the spacer assembly.
3.1.5.4
Test Mandrel
As each section of a duct line is completed from structure to structure,
draw a flexible testing mandrel approximately 305 mm 12 inches long with a
diameter less than the diameter of the conduit through each conduit. After
which, draw a stiff bristle brush through the conduit, until conduit is
clear of particles of earth, sand, and gravel; then immediately install end
plugs.
3.1.5.5
Conduit Plugs and Pull Rope
New conduit indicated as being unused or empty shall be provided with plugs
on each end. Plugs shall contain a weephole or screen to allow water
drainage. Provide a plastic pull rope having 915 mm 3 feet of slack at
each end of unused or empty conduits.
3.1.5.6
Connections to Manholes
Duct bank envelopes connecting to underground structures shall be flared to
have enlarged cross-section at the manhole entrance to provide additional
shear strength. Dimensions of the flared cross-section shall be larger
than the corresponding manhole opening dimensions by no less than 300 mm 12
inches in each direction. Perimeter of the duct bank opening in the
underground structure shall be flared toward the inside or keyed to provide
a positive interlock between the duct bank and the wall of the structure.
Use vibrators when this portion of the encasement is poured to assure a
seal between the envelope and the wall of the structure.
3.1.5.7
Connections to Existing [Manholes] [Handholes]
For duct bank connections to existing structures, break the structure wall
out to the dimensions required and preserve steel in the structure wall.
Cut steel and [extend into] [bend out to tie into the reinforcing of] the
duct bank envelope. Chip the perimeter surface of the duct bank opening to
form a key or flared surface, providing a positive connection with the duct
bank envelope.
SECTION 16302N
Page 27
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