The placing of concrete inside the iron tube was done by an organization
entirely separate from the tunneling force. A mixing plant was placed in
each of the five shafts. The stone and sand bins discharged directly
into mixers below, which, in turn, discharged into steel side-dump
concrete cars. All concrete was placed in normal air.
The first step, after the iron lining was scraped clean and washed down
and all leaks were stopped, was the placing of biats, marked _B_ on
Plate LXXIV. These were made up of a 6 by 12-in. yellow pine timber, 17
ft. long, with two short lengths of the same size spliced to its ends by
pieces of 12-in. channels, 3 ft. 9 in. long, clamped upon the sides.
These biats were placed every 5 ft. along the tunnel in rings having
side keys. Next, a floor, 13 ft. wide, was laid on the biats and two
tracks, of 30-in. gauge and 6-1/2-ft. centers, were laid upon the floor.
There were three stages in the concreting. Fig. 2, Plate LXXIV, shows
the concrete in place at the end of the first, and Fig. 3, Plate LXXIV,
at the end of the second stage. The complete arch above the bench walls
was done in the last operation.
Two 3 by 10-in. soldiers (_SS_ in Figs. 1 and 2, Plate LXXIV) were
fastened to each biat and braced across by two horizontal and two
diagonal braces. To each pair of soldiers a floor template, _T_, was
then nailed. The form for the center drain was then suspended as shown
in Fig. 1, Plate LXXIV. Three pieces of shuttering, _FFF_, 20 ft. long,
were then nailed to the bottom of the soldiers. One is all that would
have been needed for the first concrete placed, but it was easier to
place them at this stage than later, when there was less room. Three
rough shutters were also nailed to the curved portion for the floor
template. Opposite each biat, a bracket, _bb_, was then nailed, which
carries a set of rough boards which formed the risers for the duct
steps. Everything was then ready for concreting except that, where
refuge niches occurred, a form for the portion of the niche below the
seat was nailed to the shuttering. This form is shown at _R_ in Fig. 1,
Plate LXXIV.
[Illustration: PLATE LXXIV]
The concrete was dumped down on each side from side-dump cars standing
on the track, and, falling between the risers for the duct steps, ran or
was shoveled under the forms and down into the bottom. The horizontal
surface on each side the center drain was smoothed off with a shovel.
The workmen became very skillful at this, and got a fairly smooth
surface. This concrete was usually placed in lengths of 45 or 60 ft.
After setting for about 24 hours, the brackets, _bb_, were removed,
together with the shuttering on the steps. The triangular pieces, _t_ in
Fig. 1, Plate LXXIV, were not removed until later. Instead, a board was
laid upon this lower step on which the duct layers could work. This and
the triangular piece 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, where a space of about 18
in. was left. When the lagging had reached the upper hanger-bolts, they
were removed, which left only the two bottom bolts fixed in the
concrete. Most of these were unscrewed from the eye and saved, as tin
sleeves were placed around them before concreting. Two cast-iron eyes
were lost for every 5 ft. of tunnel. To place the key concrete, a stage
was set up in the middle of the floor, and, beginning at one end, about
2 ft. of block lagging was placed. Over this, concrete was packed,
filling the key as completely as possible. This was done partly by
shoveling and using a short rammer, and partly by packing with the hands
by the workmen, who wore rubber gloves for the purpose. Another 2 ft. of
lagging was then placed, and the operation was repeated, and thus
working backward, foot by foot, the key was completed. This is the usual
way of keying a concrete arch, but in this case the difficulty was
increased by the flanges of the iron lining. It was practically
impossible to fill all parts of the pockets formed by these flanges. To
meet this difficulty, provision was made for grouting any unfilled
space. As the concrete was being put in, tin pipes were placed with
their tops nearly touching the iron lining, and their bottoms resting on
the lagging. Each pocket was intended to have two of these pipes, one to
grout through and the other to act as a vent for the escape of air. Each
center key ring had six pipes, and each side key had eight. The bottoms
of the pipes were held by a single nail driven half way into the
lagging. This served to keep the pipes in position and to locate them
after the lagging was taken down.
The cost of labor in the tunnels directly chargeable to concrete was
$1.80 per cu. yd. The top charges, exclusive of the cost of materials
(cement, sand, and stone), amounted to $3.92.