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. Wh