Transactions of the American Society of Civil Engineers, Vol. LXVIII, Sept. 1910 | Page 5

drift was excavated in three
stages, a top heading and a bench in two lifts. While blasting the cut in
the top heading, there was enough concussion to break glass in the
neighboring buildings. The use of a radialax machine reduced the

concussion somewhat, but it was very quickly abandoned on account of
the length of time required for the drilling.
The construction of the 32d Street Shaft was quite similar to the one on
33d Street. It was 31.5 ft. long, 20.5 ft. wide, and 71 ft. deep. The depth
of earth excavation averaged 19.5 ft. The rock in this shaft was seamy
and not quite as hard or dry as that in 33d Street, and timbering was
required for practically the full depth to the crown of the drift. Sinking
was started on May 15th, 1905, and was completed on October 26th,
1905. The daily average rate was 0.30 ft. in earth and 0.52 ft. in rock.
The drift was excavated in much the same manner as the one in 33d
Street, but the rock being softer the radialax machine was not used.
TUNNEL EXCAVATION.
During the early part of the work, the contractor devoted his entire
attention to the work of excavation. Nearly all the excavation east of
Fifth Avenue was done before any of the lining was placed. At a
number of points west of Fifth Avenue and at a few points to the east
the nature of the rock was such that the two operations had to be done
simultaneously.
Single-Tunnel Method.--For an average distance of 350 ft. west from
the First Avenue Shafts there were four single tunnels. The rock was
sound and comparatively dry. A top heading of the full size of the
tunnel and about 8 ft. high was first driven. It was drilled by four drills
mounted on two columns, and was blasted in the ordinary way. The
bench was about 13 ft. high. Tripod drills, standing on the bench,
drilled the usual holes, but, owing to the lack of head-room, steels long
enough to reach the bottom of the bench could not be used. Tripod
drills were set as low as possible at the foot of the bench and drilled
lifting holes. These holes were inclined downward from 10° to 15° to
the horizontal, and were spaced to converge at the location of the
drainage ditches. The heading was usually driven from 10 to 20 ft. in
advance of the bench. At this distance a large part of the muck from the
heading was shot backward over the bench. In the single tunnels the
muck was loaded by hand.

Twin-Tunnel Methods.--From the end of the single-track tunnel
westward to Fifth Avenue on 33d Street, and to Madison Avenue on
32d Street, with some exceptions, each pair of tunnels was excavated
for the entire width at one operation. Three different methods of work
were extensively used. They were the double-heading method, the
center-heading method, and the full-sized-heading method, and these
differed only in the manner of drilling and blasting. The bench was
usually within 10 or 15 ft. of the face of the heading, and was drilled
and fired in the same way as in the single tunnels. After the installation
of the permanent plant, most of the muck was handled by steam
shovels.
In the double-heading method, shown on Plate LVII, the top headings
for each tunnel of the pair were driven separately, leaving a short rock
core-wall between them. The headings were drilled from columns in
the manner described for the single tunnels. The temporary rock
dividing wall between the headings was drilled by a tripod drill on the
bench of one of the headings, and was fired with the bench.
In the center-heading method, also shown on Plate LVII, only one
heading was driven. It was rectangular in shape, about 8 ft. high and 14
ft. wide. It was located on the center line between the tunnels. In
general, the face was from 6 to 12 ft., or the length of one or two
rounds, in advance of the remainder of the face at the top. The center
heading was drilled by four drills mounted on two columns. By turning
these drills to the side, they were used for holes at right angles to the
line of the tunnels, by which the remainder of the face of the heading
was blasted. By turning the drills downward, the bench holes under the
center heading were also drilled. The center heading explored the rock
in advance of the full-width heading, and gave a good idea as to the
care needed in firing.
For the full-width-heading method, Fig. 2, ten drills were mounted on
five columns set abreast across the face. Holes were drilled to form a
cut near the center line between