3.6.6 Spiral Welded Steel Conduits. T h i s is a smooth steel conduit with
s p i r a l joints of the lockseam welded type. T h e steel sheets used for this
t y p e of conduit are heavier than those used for corrugated steel casings. The
end connections of the conduit can be made with or without sleeves. In the
l a t t e r case, the ends are reinforced to an adequate thickness and beveled for
f i e l d w e l d i n g . When welding sleeves are used, they are of a thickness at
l e a s t equal to that of the conduit.
3 . 7 Class B Conduits. Class B conduits are used in Class B sites. They must
fulfill the following requirements.
( a ) They must be watertight as determined by a laboratory water-
t i g h t n e s s test applied to a test length of conduit of the same construction as
u s e d in the field (refer to section 4).
( b ) They must have a free air passage suitable for drying out the
i n s u l a t i o n if it becomes wet.
( c ) They must be capable of being drained in place, if required.
3 . 7 . 1 Type B-1, Tile Conduit. T i l e conduits consist of a continuous
r e i n f o r c e d concrete slab furnished with an internal base drain and provided
w i t h v i t r i f i e d - c l a y - t i l e s i d e w a l l b l o c k s , f i t t i n g s , a n d a r c h e s . The pipes
e n c l o s e d within the conduit are supported by rigid metal frames securely
a n c h o r e d , equipped with steel bars and cast iron rollers. An annular airspace
( l - i n c h minimum) is provided between the outer surface of the pipe insulation
a n d the inner surface of the tile for the entire length of the system.
3 . 7 . 2 Type B-2, Concrete Trench. C o n c r e t e trenches usually consist of
c a s t - i n - p l a c e bottom and sides covered with a precast top slab in lengths not
exceeding 3 feet. T h e concrete is generally reinforced with 10-gauge welded
w i r e f a b r i c . The conduits may be square or rectangular in cross section with
v a r y i n g dimensions according to its use. P i p e supports, i n s u l a t i o n , a i r -
s p a c e s , and expansion joints are as described for tile conduits (paragraph
3 . 7 . 1 ) . Trenches permit complete drainage of the system and free expansion
a n d contraction of the pipes without damage to the insulation (refer to
c h a p t e r 7 for details). A l l joints are sealed with portland cement and an
a p p l i c a t i o n of creosote primer and coal tar enamel. T h e coal tar is of a type
which neither softens nor flows at temperatures below 220F.
3 . 7 . 3 Piping Expansion and Contraction. Expansion and contraction of the
p i p i n g is provided by expansion loops or offsets installed within the conduit
between anchor points. I n restricted spaces mechanical expansion joints are
o f t e n provided (refer to chapter 7 for details).
3 . 8 Walking Tunnel. A walking tunnel is a conduit sufficiently large to
p e r m i t personnel walking through it for inspection or repairs. They are
r e l a t i v e l y high in construction cost when compared with smaller conduits. In
congested areas or where additional facilities must be accommodated (such as
water systems, e l e c t r i c l i n e s , o r t e l e p h o n e l i n e s ) , t h e y a r e o f t e n j u s t i f i e d .
P i p i n g is stacked vertically on one side of the tunnel. The means for
The tunnel is
s u p p o r t i n g the pipes are similar to those described previously.
waterproofed by adequate sealing.