When it became evident that the work through the Fifth Avenue section
would be extremely slow, shafts were sunk in each street between Sixth
and Seventh Avenues. The shafts, as shown on Plate XIV, were located in
the streets, but in such a way as to block only half of the roadway. At
the same time it was decided to construct in open cut about 200 ft. of
the Three-Track Tunnel at the west end of the contract in 32d Street,
where the rock surface was below the top of the tunnel. It was hoped
that the remainder of the work could be built without opening the
street, but further investigation showed that this was impracticable,
and eventually all the Three-Track Tunnel in 32d Street, except 120 ft.
east of the shaft, was built in open cut.
_Thirty-second Street Work in Tunnel._--Following the sinking of the
shaft, a drift was driven across the street at the crown of the tunnel,
and a top heading on the south side was excavated in both directions.
Frequent cross-drifts to the north side showed that the rock was nowhere
very sound and that, except for a short distance east of the shaft, it
was distinctly unfavorable for the wide Three-Track excavation. In this
stretch the north ends of these cross-cuts were connected by a second
heading, and wall-plates and sets of three-segment arch timbering were
set up to support the roof of the drifts. The cross-cuttings were
gradually widened and timbered until the entire excavation had been made
down to the level of the wall-plates, as shown in Fig. 3, Plate LX. The
bench was then excavated in two lifts, leaving the wall-plates supported
on narrow longitudinal berms, which were removed in short sections to
permit the placing of posts under the wall-plates.
_Thirty-second Street Open-Cut Work._--Before actual open-cut excavation
was started, all buildings facing it were underpinned to rock. For this
purpose, a trench was dug along the face of the buildings and of the
same depth as their cellars. Holes were cut in the front foundation
walls through which long needle-beams (Fig. 4, Plate LX) were inserted
and jacked up on blocking placed on the cellar floor and in the trench,
until the weight of the building had been taken off its foundations. A
close-sheeted trench was then sunk to rock under the front building
walls, and a light rubble masonry retaining wall was built in it to
support the building permanently. Frequently, the excavation for the
underpinning wall, which was taken out in sections from 30 to 40 ft.
long, and in places was carried to a depth of 40 ft., was very
troublesome on account of the large quantity of water encountered and
the fineness of the sand, which exhibited a tendency to flow when
saturated.
The Elevated Railroad columns in Sixth Avenue, near the north and south
lines of 32d Street, were underpinned in a manner similar to the
building foundations, while those on the center line of the street were
supported by girders riveted to them close under the track level. The
girders in turn were supported on posts footed on the new underpinning
of the adjacent columns. On the completion of the tunnels, concrete
piers were built up from the roof of the tunnel to form a permanent
foundation for the center-line columns. The area to be excavated under
Sixth Avenue was enclosed by a rubble masonry retaining wall constructed
in a trench.
Open-cut excavation was started by planking over the street on stringers
resting on transverse 12 by 12-in. caps. The caps were gradually
undermined and supported on temporary posts which were then replaced by
short posts resting on 12 by 12-in. sills about 7 ft. below the cap. The
operation was then repeated and the sill was supported on another set of
short posts resting on a second sill. When the excavation had been
carried down in this manner to the level of the top of the tunnel,
diagonal 3 by 10-in. timbers were cut in between the posts and sills to
form a species of double A-frame, the legs of which rested in niches cut
in the rock and on posts carried up the face of the underpinning wall,
and the whole was stiffened with vertical tie-rods. This construction is
shown by Fig. 3, Plate LXII. The brick sewer was replaced temporarily by
one of riveted steel pipe. This pipe and the water and gas pipes and
electric conduits were suspended from the timbers as the pipes were
uncovered.
Excavation in rock was made by sinking a pit to sub-grade for the full
width of the tunnel and advancing the face of the pit in several lifts,
the muck being blown over the slope and loaded into buckets at its foot.
The work was attacked at several places simultaneously, and the spoil
was hoisted by derricks located at convenient points along the side of
the cut.
_Thirty-third Street Work in Tunnel and Open Cut._--The West 33d Street
Shaft was similar to the one in 32d Street, and was sunk during
February, March, and April, 1907, through 10 ft. of earth, 21 ft. of
soft rock, and 29 ft. of fairly hard rock. It was necessary to timber
heavily the upper 30 ft. of the shaft. The timber later showed evidences
of severe strain, and had to be reinforced.
[Illustration: Plate LXI.--EXCAVATION AND TIMBERING IN HEAVY GROUND
OF THREE-TRACK TUNNEL OF 33D ST.]
As soon as the shaft excavation was deep enough, a drift was driven
part way across the tunnels, and top headings were started both east and
west to explore the rock. The heading to the west was divided into two
drifts, as shown on Plate LXI. These two drifts were continued to the
west end of the contract, and were then enlarged to a full-sized heading
and timbered, as shown on Plate LXI and Fig. 3, Plate LX. The rock near
the shaft contained many wet rusty seams, and settlement was detected in
the segmental tunnel timbering soon after the widening of the heading
was completed. Short props were placed under the timbers, and the street
surface was opened with a view of stripping the earth down to the rock
and thus lightening the load on the timbering. Street traffic was
maintained on a timber structure with posts eventually carried down to
the rock surface, and the walls of the buildings on the north side of
the street were underpinned to rock. The settlement of the tunnel
timbering was checked for a time, and the bench was excavated as shown
on Plate LXI. In this work the cut in the center was first made, and the
short props were replaced by struts, as shown; after this the berms were
removed and the side posts were placed. While building the brick arches,
holes were left in the masonry around the struts. After the masonry had
hardened, piers were built on the arches to support the segmental
timbers. The struts were then removed and the openings filled with
masonry. The voids above the arch were packed with rock and afterward
thoroughly grouted.
The timbers near the shaft continued to settle, and, although they had
been placed from 9 to 12 in. above the level of the top of the masonry,
by October 1st, they encroached 9 in. within the line of masonry. It was
then decided to remove the rock for a distance of 48 ft. west of the
shaft, and build this portion of the tunnel in open cut. The posts
supporting the deck forming the street surface were replaced by an
A-frame structure similar to that developed for the 32d Street open cut,
without interruption of the street traffic.
After making the open cut to the westward of the shaft, there was a slip
in the rock north of and adjoining the shaft. Fortunately, the timbers
did not give way entirely, and no damage was done. The open cut was
extended eastward for a distance of 46 ft., making the total length of
tunnel built in open cut on this street 94 ft.
East of the shaft, for a distance of about 125 ft., the rock was broken
and could not be excavated to full size without timbering the roof, but
between this section of poor rock and those already mentioned in
connection with the work at Fifth Avenue, there was a stretch of 600 ft.
of good rock where all the spoil was handled with a steam shovel.