were not removed until just before the bench
concrete was placed. This was important, as otherwise the bond between
the old and new concrete would be much impaired by dirt ground into the
surface of the old concrete. The ducts were then laid, as shown in Fig.
2, Plate LXXIV.

The remaining shutters for the face of the bench walls were then placed.
The remainder of the forms for the refuge niches, _RR_, in Fig. 1, Plate
LXXIV, were nailed to the shutters, the steel beam over the niche was
laid in place, the forms for the ladders, _L_ in Fig. 2, Plate LXXIV,
which occur every 25 ft., were tacked to the shutters, the shutters and
forms were given a coat of creosote oil, and then all was ready for
placing the bench concrete.

The specifications required a 2-in. mortar face to be placed on all
exposed surfaces and the remainder to be smoothed with a trowel and
straight-edge. After about 48 hours, the biats were blocked up on the
bench, and all forms between the bench walls below the working floor
were removed.

The centering for the arch concrete consisted of simple 5 by 3-1/2 by
5/16-in. steel-angle arch ribs, curved to the proper radius, spaced at
5-ft. intervals. Each rib was made up of two pieces spliced together at
the top. Two men easily handled one of these pieces. After splicing, the
rib was supported by four hanger-bolts fastened to the iron lining as
shown in Fig. 3, Plate LXXIV.

In the early part of the work, two additional bolts were used about half
way up on the side between the upper and lower hanger-bolts. It was soon
found that by placing the strut between the tunnel lining and the crown
of the rib, these hanger-bolts could be dispensed with. The lagging was
of 3-in. dressed yellow pine, 12 in. wide, and in 15-ft. lengths. Each
piece had three saw cuts on the back, from end to end, allowing it to be
bent to the curve of the arch; it was kept curved by an iron strap
screwed to the back. The arches were put in, either in 15, 30 or 45-ft.
lengths, depending on what was ready for concrete and what could be done
in one continuous working. The rule was that when an arch was begun, the
work must not stop until it was finished. An arch length always ended in
the middle of a ring. The lagging was placed to a height of about 6 ft.
above the bench before any concreting was done. When the concrete had
been brought up to that point, lagging was added, one piece at a time,
just ahead of the concrete, up to the crown,