Transactions of the American Society of Civil Engineers, Vol. LXVIII, Sept. 1910 | Page 7

cover of fair quality to enable the Twin
Tunnel to be driven without great risk. The new plan (continuing the
Twin Tunnel westward at a lower grade) was adopted in March, 1907,
and work was immediately resumed at Fifth Avenue.
The relation between the cross-sections under the old and new plans at
that point is shown by Fig. 3. Before the new section was excavated it
was necessary to support the timber work in the old headings. The plan
adopted is also shown by Fig. 3. The rock was excavated under the
center heading, as shown in cross-section, for a length of about 3 ft. A
girder composed of two 18-in. I-beams was then put in position over
each line and supported on the sides by posts. The ends at the center
lines between the tunnels were supported on short posts bearing on the
rock bench. The support of the timbering in the headings was then
transferred to the girders by additional posts. Blocking was also
inserted between the tops of the beams and the rock walls between the
headings. Fig. 2, Plate LIX, gives a good idea of the timber work in the
top headings above the I-beams. When the roof had been made secure,
the removal of the bench was begun. As the work advanced it was
necessary to replace the short posts at the center of the tunnel by others
of full height, and there was considerable settlement in the I-beams
during this operation. When the bench had been removed to a point 61
ft. west of Fifth Avenue, settlement was detected in the street surface
above. Bench excavation was suspended and a section of the permanent
lining, 35 ft. long, was placed. The space between the lining and the
beams and between the beams and the roof was filled with rubble
masonry. Grout pipes were built into the masonry and later all voids
were filled with grout. Fig. 3, Plate LIX, shows the first section of the
concrete lining completed and part of the rubble in place; and Fig. 4,
Plate LIX, shows details of the work above the tunnels. A second
section of bench was next removed and more lining was placed. Work
was continued in this way until all the roof at the old three-track
headings had been secured. In this portion of the work the posts were

embedded in the concrete.
Between Fifth and Sixth Avenues there were two more sections of bad
rock where it was necessary to support the roof with steel beams. At
these latter points there were no complications with the excavation for
the Three-Track Tunnel, and the work was much simpler. To avoid
leaving the center posts in the permanent work, two rows of temporary
posts were placed, as shown by Fig. 1, Plate LX, the center wall and
skewback were built, and the posts were removed, as shown by Fig. 2,
Plate LX, before placing the remainder of the lining.
In 32d Street the normal progress of the excavation was frequently
interrupted by encountering soft and unsound rock. In the excavation
between the East River and the Intermediate Shafts it was possible to
overcome these conditions by temporarily narrowing the excavation on
one side and supporting the roof on 16 by 16-in. transverse timbers
caught in niches in the rock at the sides, leaving sufficient room for the
steam shovel to work through. In order to save time, the height of the
excavation was not increased before placing these timbers, so that,
previous to the concreting, they all required to be raised to clear the
masonry lining and were then supported on posts on the center line
between the tunnels. This permitted the remainder of the excavation to
be made, and such additional timbering as was required was placed. At
most of these sections a brick arch and water-proofing were used, on
account of the presence of water. In certain places the center line posts
were buried in the core-wall, and, in order to permit the placing of the
water-proofing, were then cut off one by one flush with its top as the
load was transferred to the completed masonry. In other cases the load
was transferred to posts clear of the masonry and the center line posts
were entirely removed. Under such conditions the normal concrete
methods, to be described later, could not be used, and special forms
were substituted.
[Illustration: FIG. 3. CONSTRUCTION OF TWIN TUNNELS,
THROUGH EXCAVATION STARTED FOR THREE-TRACK
TUNNEL IN 33D STREET NEAR 5TH AVENUE]
In this section of the work the most serious difficulties were

encountered near Fourth Avenue a short distance east of the
Intermediate Shaft, and beneath the site of the old pond shown on
General Viele's map. The rock cover was known from the boring to be
very thin, and