Transactions of the American Society of Civil Engineers, Vol. LXVIII, Sept. 1910 | Page 8

the presence of the subway overhead caused some
anxiety. The excavation was at first taken out to practically full width
and timbered, but the rock became so treacherous that the heading was
narrowed to a width sufficient for one tunnel only. With this span the
rock in the roof held without timbering. As the masonry lining
approached, sufficient trimming was done to permit the placing of the
core-wall and one arch. Above the completed core-wall and brick arch
the voids were filled solid with rubble masonry to give an unyielding
support to the roof. The excavation of the remaining width of tunnel
was then undertaken. Near the west side of Fourth Avenue, the
excavation broke out of rock at the top, and fine sand and gravel with a
large quantity of water were encountered. The work of excavation was
arduous, and proceeded very slowly, on account of the care with which
it was executed. Only a small amount of sand entered the tunnel, but
the lining was placed as soon as the excavation was completed. Rubble
masonry packing and grout ejected through pipes built into the arch
were used to fill the voids above the roof. As a further precaution
against the settlement of the subway, 2-in. pipes were washed down
from the street above the point where soft ground was exposed in the
roof of the tunnel, and through them grout was forced into the ground
at various depths. Careful levels show that no settlement of the subway
has taken place.
West of the Intermediate Shaft the tunnel was excavated for full width
until bad rock was encountered about 60 ft. west of Madison Avenue.
(See General Viele's map, Plate IX.) Timbering was used for a short
distance, and then the heading and bench were narrowed to 18 ft., and
steam-shovel excavation was abandoned. As the heading advanced the
rock grew steadily softer, the difficult conditions in this locality
culminating when a slushy disintegrated feldspar was met, requiring
poling and breasting. Thereafter the rock improved markedly, but near
the east side of Fifth Avenue its thickness above the roof was found to
be only 1-1/2 ft., and the advance was stopped, pending a decision as to
a change of plan.

[Illustration: PLATE LX, FIG. 1.--DOUBLE ROW OF POSTS
UNDER I-BEAMS, SUPPORTING ROOF IN BAD ROCK
SECTION.]
[Illustration: PLATE LX, FIG. 2.--CENTER WALL AND
SKEWBACK UNDER I-BEAMS, AFTER REMOVAL OF DOUBLE
ROW OF POSTS.]
[Illustration: PLATE LX, FIG. 3.--TIMBERING IN FULL-WIDTH
HEADING OF THREE-TRACK TUNNEL.]
[Illustration: PLATE LX, FIG. 4.--UNDERPINNING WALLS IN
OPEN-CUT SECTION.]
After some delay, an exploration drift, similar to the one already
described, was driven through to Sixth Avenue, and a change in plan
was made, substantially the same as for the 33d Street tunnels.
Enlargement to full size was at once started, but, for 400 ft. the rock
was very soft and poor, and required extremely careful handling. The
exploration drift was widened out to the full Twin-Tunnel width, and
I-beams were placed and supported, in much the same manner as in 33d
Street. The rock was so soft that it was frequently necessary to drive
poling boards ahead as the face was mined out with picks and shovels.
The load was very heavy, and the work the most difficult encountered
in the tunnels.
After this stage of the enlargement was reached, the excavation of the
bench and the placing of the lining proceeded alternately, with the
I-beams temporarily supported on long posts while the concrete
core-wall was being built. Considerable settlement took place while
shifting the posts, and eventually showed on the street surface and in
the adjacent sidewalk vaults, but no damage was done to the structural
portions of the buildings.
While the above work had been going on westward from Fifth Avenue,
the excavation of the Twin Tunnel eastward from the end of the
open-cut section at Sixth Avenue had been proceeding rapidly, and,
toward the end of the difficult Fifth Avenue work, it was being attacked

from both directions.
PROGRESS OF EXCAVATION.
Owing to the numerous sections of poor rock, interspersed throughout
the work with stretches of sound rock, the progress of the excavation
was very irregular, especially in 32d Street. The rate of excavation in
good ground is shown in Table 1. In the sections of bad ground, the
operations of excavation, timbering, and lining were often carried on
alternately, and it is impracticable to include them in the table.
TABLE 1.--PROGRESS AND METHODS OF EXCAVATION IN
GOOD GROUND.
THIRTY-THIRD STREET.
===================================================
========= 1 | 2 | 3 |
-----------------------------+--------+--------------------+ | | | | | | Type of
excavation. |Tunnels.| Worked from: | | | | | | |
-----------------------------+--------+--------------------+ Full-sized single
tunnel | B | 1st Ave. shaft. | | | | Full-sized single tunnel | A | 1st Ave.
shaft. | |