The masonry lining for the tunnels was not started until the late fall
of 1906, after excavation had been in progress for a year and a half. At
that time concreting was started in the single tunnels westward from the
First Avenue Shafts, and by spring was in full swing in the Twin
Tunnels.
The plans contemplated the use of a complete concrete lining except
where large quantities of water were encountered; in which case the
arches, beginning at a point 15° above the springing line, were to be
built of vitrified paving brick. By reference to Plate XII it will be
seen that the water-proofing, which in the concrete-roof tunnels
extended the full height of the sides to the 15° line, was carried in
the brick-roof tunnels completely around the extrados of the arch. The
cross-sections also show the location of the electric conduits which
were buried in the mass of the side and core-walls and which limited the
height to which the concrete could be carried in one operation.
The same general scheme of operations was used wherever possible
throughout the Twin-Tunnel work, but was subject to minor modifications
as circumstances dictated. Concrete was first deposited in the bottom,
to the grade of the flow line of the drains; after it had set,
collapsible box forms, of 2-in. plank with 3-in. plank tops, were laid
on it to form the ditch and the shoulders for the flagstone covers. The
track, which had previously been blocked up on the rock between the
ditches, was raised and supported on the ditch boxes above the finished
floor level. At the same time, light forms were braced from the ditch
boxes to the grade of the base of the low-tension and telephone-duct
bank. After depositing the concrete to this level, the telephone ducts
were laid.
The forms for the water-proofing or sand-wall up to the 15° line and for
the main side-walls and core-walls were built in 30-ft. panels and were
supported on carriages, which, traveling on a broad-gauge track above
the ditches, moved along the tunnel, section by section, as the work
advanced. The panels were hung loosely from joists carrying a platform
on the top chord of the carriage trusses, and were adjusted
transversely by bracing and wedging them out from the carriage. The
small forms for the refuge niches, ladders, etc., were collapsible, and
were spiked to the main panel forms just previous to the deposition of
the concrete. The concrete was deposited from the platform on top of the
carriage, to which the cars were elevated in various ways. Plate LXI
shows the details of the carriages, and is self-explanatory.
The concrete for the sand-walls and the core-wall, to the level of the
sidewalk, was deposited at the same time; two carriages in each tunnel,
placed opposite each other, forming a 60-ft. length, were used at each
setting. The floor section of the 4-in. tile drains had been laid with
the floor concrete, and, as the sand-wall concrete was deposited, the
drains were brought up simultaneously, broken stone being deposited
between the tile and the rock to form a blind drain and afford access to
the open joints of the tile for the water entering the tunnel through
seams in the rock. The drains were spaced at intervals not exceeding 25
ft., depending on the wetness of the rock, and were placed at similar
intervals in the core-wall under the lowest projecting points of the
rock on the center line between the tunnels. A small ditch lined with
loose 6-in. vitrified half pipe was provided in the top of the sand-wall
to collect the water from the extrados of the arch and lead it to the
top of the drains. Great difficulty was experienced in maintaining these
drains clear, and, on completion of the work, a large amount of labor
was expended in removing obstructions from the floor sections, the only
portion then accessible.
After water-proofing the sand-walls and laying the low-tension ducts, a
second pair of carriages, with panels on one side only, for 60 ft. of
side-wall and skewback to the 15° line, were set and braced against the
core-wall. These forms are shown in connection with the carriage on
Plate LXI. They were concreted to the base of the high-tension duct
bank, and, after the concrete had hardened and the bank of ducts had
been laid, the concreting was completed in a second operation.
In places where the roof was supported temporarily by posts and heavy
timbering, such as at Fifth Avenue, the form carriages could not be
used, and special methods were devised to suit the local conditions.
Usually, the panels were stripped from the carriages and moved from
section to section by hand, and, when in position, were braced to the
timbering.
The arch centers were built up of two 5 by 3 by 3/8-in. steel angles,
and, when set, were blocked up on the sidewalks opposite each other in
the two tunnels. A temporary platform was laid on the bottom chord
angles of the ribs, on which the concrete was dumped, the same as on the
form carriages. The lagging used was 3 by 3-in. dressed pine or spruce
16 ft. long, and was placed as the concreting of the arch proceeded
above the 15° line on the side-wall and above the sidewalk on the
core-wall. After the arch had reached such a height that the concrete
could not be passed over the lagging directly from the main platform, it
was cast on a small platform on the upper horizontal bracing of the
centers, shown in Fig. 3, Plate LIX, and was thence shoveled into the
work. In the upper part of the arch the face of the concrete was kept on
a radial plane, and, when only 3 ft. remained to be placed, it was keyed
in from one end, the key lagging being set in about 5-ft. lengths. The
arches were concreted usually in 60-ft. lengths.
Where brick arches were used, the core-wall skewback was concreted
behind special forms set up on the sidewalks, or the arch ribs and
lagging were used for forms, and the brick arch was not started until
after the concrete had set. In laying the brick in the arch, the five
courses of the ring were carried up as high as the void between the
extrados and the rock would permit and still leave a working space in
which to place the water-proofing. This was usually not more than 3 ft.,
except on the core-wall side. The felt and pitch water-proofing was then
laid for that height, joined to the previous water-proofing on the
side-walls, and was followed by the brick armor course over the
water-proofing and by the rock packing, after which another lift of
brick was laid and the operations were repeated. The large void (Fig. 1,
Plate LXII) above the core-wall gave convenient access for working on
top of the adjacent sides of the roof, and the keying of the arches and
the water-proofing and rock packing above the core-wall were usually
carried on from that point, the work progressing from one end.
The concrete for all work above the floor was dumped on the platform of
the carriages, to which it was transported in the early part of the work
in cars running on a high-level track laid on long ties, resting on the
finished sidewalks. This arrangement, although requiring a large amount
of timber for the track, permitted the muck to be carried out on the
low-level track without interference. Later, when the advance of the
heading had ceased and the heavy mucking was over, all concrete was
transported on the floor level, and the cars were lifted to the carriage
platforms by elevators and were hauled by hoisting engines up a movable
incline. The latter method is shown by Fig. 3, Plate LIX.
_Water-Proofing._--The water-proofing referred to above was in all cases
felt and pitch laid with six thicknesses of felt and seven of pitch. The
sub-contractor for the work was the Sicilian Asphalt Paving Company. All
joints were lapped at least 1 ft., and, where work was suspended for a
time and a bevel lap could not be made, the edges of the felt were left
unpitched for 1 ft. and the newer work was interlaced with the old. This
method was not always successful, however, on account of the softening
of the unpitched felt on long-continued exposure to the water. The felt
used was mainly "Tunaloid," together with some "Hydrex." It weighed
about 12 lb. per 100 sq. ft. when saturated and coated on one side only,
and contained about 25% of wool. The coal-tar pitch used had a melting
point of 100° Fahr.
After the completion of the tunnel, the concrete arch showed some
leakage and in places unsightly lime deposits. It was determined to
attempt to stop these leaks by the application of a water-proof cement
coating on the intrados of the arch. Extended experimental application
of two varieties of materials used for this purpose--"Hydrolithic"
cement and the U. S. Water-proofing Company's compound--have been made
with apparent success up to the present time, and the results after the
lapse of a considerable period are awaited with interest.
_Duct Laying._--The position of the electric conduits, buried in the
heart of the concrete walls, interfered greatly with the economical and
speedy placing of the lining, and their laying proved to be one of the
most troublesome features of the work. The power conduits were
single-way, with the bank for high-tension cables separated in the
side-walls from the low-tension bank, as shown on Plate XII. The
conduits for telephone and telegraph service were four-way, and were
located in the core-wall. All ducts had 3/4-in. walls and a minimum
clear opening of 3-3/8 in. square, with corners rounded. They were laid
with joints broken in all directions, and in about 1/4-in. beds of
1:2-1/2 mortar. Flat steel bond-irons, 2 by 1/8 in., with split and bent
ends, were placed in the joints at intervals of 3 ft. and projected into
the concrete 3 in. on each side, tying together the concrete on opposite
sides of the ducts. The joints were wrapped with a 6-in. strip of
10-oz. duck saturated with neat-cement grout, and, in addition, the
power conduits were completely covered with a 1/2-in. coat of mortar to
prevent the intrusion of cement and sand from the fluid concrete. The
four-way conduits were plastered only over the wraps. Splicing chambers
were provided at intervals of 400 ft.
[Illustration: PLATE LXII, FIG. 1.--WATER-PROOFING OVER BRICK
ARCHES.]
[Illustration: PLATE LXII, FIG. 2.--TRESTLE USED IN CONCRETING IN
THREE-TRACK TUNNEL.]
[Illustration: PLATE LXII, FIG. 3.--METHOD OF STREET SUPPORT OVER
OPEN-CUT EXCAVATION.]
[Illustration: PLATE LXII, FIG. 4.--JUNCTION OF TWIN AND THREE-TRACK
TUNNELS.]