In one bench wall of each tunnel there were fifteen openings for power
cables and in the other, between the river shafts, there were forty
openings for telephone, telegraph, and signal cables. East of the Long
Island shaft, the number of the latter was reduced to twenty-four. The
telephone ducts were all of the four-way type. The specifications
required that the power ducts should have an opening of not less than
3-1/2 in., nor more than 3-7/8 in., and that after laying they should
pass a 4-ft. mandrel, 3-3/8 in. at the leading end and 2-5/8 in. at the
other. The outside dimension was limited between 5 and 5-3/8 in. The
openings of the four-way ducts were required to be not less than 3-3/8
in., nor more than 3-5/8 in., and after laying to pass a 5-ft. mandrel,
3-1/4 in. at the leading end and 2-1/2 in. at the other. The outside
dimensions were limited between 9 and 9-1/2 in. All were to be laid in
1/4-in. beds of mortar. The specifications were not definite as to the
shape of the opening, but those used were square with corners rounded to
a radius of 3/8 in. The four-ways were 3 ft. long, and the singles, 18
in.
A study of the foregoing dimensions will show that the working limits
were narrow. Such narrow limits would not pay for the ordinary conduit
line in a street, where there is more room. In the tunnel greater
liberality meant either reducing the number of conduits or encroaching
on the strength of the concrete tunnel lining. The small difference of
only 1/8 in. in the size of the mandrel, or a clearance of only 1/16 in.
on each side, no doubt did increase the cost of laying somewhat, though
not as much as might at first be supposed. All bottom courses were laid
to a string, in practically perfect line and grade, and all joints were
tested with mandrels which were in all openings, and pulled forward as
each piece of conduit was laid. As the workmen became skillful, the
progress was excellent.
All costs of labor in the tunnel chargeable to duct laying amounted to
$0.039 per ft. of duct; top charges brought this up to $0.083.
The serious problem was to guard against grout and mortar running into
the duct opening through the joints from the concrete, which was a
rather wet mixture. Each joint was wrapped, when laid, with canvas,
weighing 10 oz. per sq. yd., dipped in cement grout immediately before
using. These wraps were 6 in. wide, and were cut long enough to go
around the lap about the middle of the duct. As soon as all the ducts
were laid, the entire bank was plastered over with fairly stiff mortar,
which, when properly done, closed all openings. The plastering was not
required by the specifications, but was found by the contractor to
result in a saving in ultimate cost.
The concrete on the two sides of the bank of ducts was bonded together
by 2 by 1/8-in. steel bonds between the ducts, laid across in horizontal
joints. Both ends were split into two pieces, 1 in. long, one of which
was turned up and the other down. These bonds projected 1-1/2 in. into
the concrete on either side. Where the bond came opposite the risers of
the duct step, against which the ducts were laid, recesses were provided
for the projecting bond. This was done by nailing to the rough shutters
for the steps a form which when removed left a dove-tailed vertical
groove. This form was made in two pieces, one tapering inward and the
other with more taper outward. As the bonds were placed, these grooves
were filled with mortar.
The ducts usually received their final rodding with the specification
mandrel a month or more after they were laid, after which all openings
into splicing chambers were stopped by wooden plugs, 8 in. long tapering
from 3-3/4 in. at one end to 2-3/4 in. at the other end, and shaped to
fit the opening tightly. At first the plugs were paraffined, to keep
them from swelling and breaking the ducts, but were not successful, as
the paraffin lubricated them so that they would not stay in place. They
were expensive, and there was some swelling in the best that were
obtained. A better plug was made by using no paraffin, but by making six
saw cuts, three horizontal and three vertical, in the larger end,
cutting to within about 2 in. of the smaller end. The swelling of the
wood was then taken up by the saw cuts and the spring of the wood.
The splicing chambers are at 400-ft. intervals. They are 6 ft. long, 4
ft. 9 in. high, with a width varying from 3 ft. 2 in. at the top to 1
ft. 2 in. at the bottom.