Transactions of the American Society of Civil Engineers, Vol. LXVIII, Sept. 1910 | Page 6

the tunnels. The remainder of the holes
were located so that they would draw into the cut. The bench was
frequently drilled from the same set-up of columns by turning the drills

downward. In sound rock this method proved to be the most rapid of
any.
Practically all trimming was left until immediately before the
concreting. It was then taken up as a separate operation, but proved to
be costly and tedious, and a hindrance to the placing of the lining.
Materials Encountered.--All the rock encountered was the familiar
Hudson schist, but it varied widely in its mineral constituents and in its
physical characteristics. In many places where the rock surface was
penetrated, a fine sand was found that was probably quicksand. The
material above the rock in the open-cut sections was mostly sand.
[Illustration: FIG. 2. METHOD OF EXCAVATING WITH
FULL-WIDTH HEADING CROSS-TOWN TUNNELS,
MANHATTAN SIDE ELEVATION FRONT ELEVATION PLAN
SHOWING POSITION OF COLUMNS FOR DRILLING FACE]
The concurrence of the watercourse, shown on General Viele's map of
Manhattan Island (Plate IX[D]), with the points where difficulties in
the construction of the tunnels were encountered has been noted in a
previous paper.
In all cases where the course of this ancient stream was crossed (except
at its final intersection of 33d Street), the rock was found to be very
soft and disintegrated, a large quantity of water was encountered, and
heavy timbering was required. The construction at these localities will
be taken up later. In addition, disintegrated rock, but of a less
troublesome character, was invariably met under the depressions in the
rock surface developed by the borings from the streets and test holes
from the tunnels. Many of these places required timbering, and no
timbering was elsewhere necessary except at the portals. These
coincident conditions were especially marked in 32d Street, which for a
long distance closely adjoins the course of the former creek.
Disposal of Spoil.--The materials excavated from the tunnels were
dumped at the 35th Street pier on barges furnished by the Railroad
Company under another contract, and were towed to points near the

Bayonne peninsula where the spoil was used principally in the
construction of the Greenville Freight Yards and the line across the
Hackensack Meadows to the tunnels. Details of this work will be given
in a subsequent paper. After December, 1907, when the excavation was
about 85% completed, the contractor furnished the barges and effected
the complete disposal of the spoil.
Difficulties of Excavation.--As stated in a previous paper, the
excavation of the Twin Tunnel in 33d Street was continued westward
to the west line of Fifth Avenue on the original grade. At that point the
contractor started three drifts in the three-track section. The relation of
the drifts to each other and to the cross-section are shown by Fig. 3.
The center heading was driven a little in advance of those on the sides.
At a distance of 65 ft. west of Fifth Avenue the rock surface was
broken through in the top of the heading, and a very fine sand was
encountered. For some distance east of this point the rock was badly
disintegrated, and the heading required timbering. Through the soft
material, tight lagging was placed on the sides and roof of the heading,
and the face was protected by breast boards. There was a moderate flow
of water through the cracks, and, in spite of every effort, some of the
fine sand was constantly carried into the heading.
In one or two instances considerable ground was lost at the face. On the
evening of December 14th, 1906, as a heavy coal wagon was passing
along 33d Street above the heading, the rear wheels dropped through
the asphalt pavement. An examination disclosed a cavity under the
pavement about 14 ft. long, 12 ft. wide and 14 ft. deep. Evidently, the
fine sand had gradually settled into the voids caused by the loss of
material at the face, and the settlement broke the brick sewer over the
heading. The sewer was temporarily repaired, and the hole in the street
was filled before morning. A tight bulkhead was built across the
heading, and work was abandoned at that point. The north drift was
advanced to a point 108 ft. west of Fifth Avenue where sand was also
encountered and a considerable run occurred. After that time all work
on the three-track section was discontinued.
The Company then took up the consideration of changes in plan. To

determine the difficulties of driving a Twin Tunnel at a lower elevation,
an exploration drift, 8 ft. high and 12 ft. wide, was driven on the center
line of the street as a top heading on the proposed new grade. Test holes
were drilled above this heading and to the sides. The results indicated
that there was sufficient rock