General.
The following is a brief account of the main features of the "Land
Tunnel" work, by which is meant all the part of the structure built
without using tunneling shields.
The Land Tunnels consist of about 977 ft. of double tunnel on the New
York side and 230 ft. on the New Jersey side, or a total of 1,207 lin.
ft. of double tunnel.
The general design of the cross-section consists of a semi-circular
arch, vertical side-walls and a flat invert. The tunnel is adapted for
two lines of track, each being contained in its compartment or tunnel.
The span of the arch is wider than is absolutely necessary to take the
rolling stock, and the extra space is utilized by the provision of a
sidewalk or "bench" forming by its upper surface a gangway, out of the
way of traffic, for persons walking in the tunnels, and embedded in its
mass are a number of vitrified earthenware ducts, for high-and
low-tension electric cables. The provision of this bench enables its
vertical wall to be brought much nearer to the side of the rolling stock
than is usually possible, thus minimizing the effects of a derailment or
other accident. Refuge niches for trackmen, and ladders to the top of
the bench are provided at frequent intervals. In cases where a narrow
street limits the width of the structure, as on the New York side, the
two tunnels are separated by a medial wall of masonry, thus involving
excavation over the entire width of both tunnels, and in such case the
tunnels are spoken of as "Twin Tunnels"; where the exigencies of width
are not so severe, the two tunnels are entirely distinct, and are
separated by a wall of rock. This type is found on the Weehawken side.
The arches are of brick, the remainder of the tunnel lining being of
concrete.
New York Land Tunnels.
The work on the Land Tunnels on the Manhattan side was carried on from
the shaft at 11th Avenue and 32d Street.
The plans and designs for these tunnels are shown on Plate XXXII. In
this short length of about 977 ft. there are no less than nine different
kinds of cross-section. The reason for these changes is the fact that
the two lines of track are here not straight and parallel to the center
line between the tunnels, but are curved, although symmetrical about
this center line. The various changes of section are to enable the
tunnels to be built in straight lengths, thus avoiding the disadvantages
attending the use of curved forms, and at the same time minimizing the
quantity of excavation, an item which accounts for from 60 to 70% of the
total cost of tunnels of this type. Of course, there are corresponding
and obvious disadvantages in the adoption of many short lengths of
different cross-sections, and these disadvantages were well brought out
in the course of the work; on the whole, however, they may be said to
have justified their adoption. These New York Land Tunnels were divided
into three contracts, viz.: From Station 190 + 15 (the Portal to the
open work of the Terminal Station at the east side of Tenth Avenue, New
York City) to Station 197 + 60, called "Section Gy-East." The next
contract, called "Section Gy-West Supplementary," extended from Station
197 + 60 to Station 199 + 20, which is the east side of Eleventh Avenue.
The third contract was called "Section Gy-West," and extended from
Station 199 + 20 to Station 231 + 78 (the dividing line between the
States of New York and New Jersey). Thus, for nearly all its length,
this contract consists of shield-driven tunnel. The portion between
Stations 199 + 20 and 199 + 91.5, however, was of the Land Tunnel type,
and therefore will be included here. A fourth contract extended from
Station 231 + 78 to the Weehawken Shaft at Station 263 + 50, and of this
all but 230 ft. was of the shield-driven type, only the portion next to
the Weehawken Shaft being of the Land Tunnel type.
The four contracts were let to one contractor (The O'Rourke Engineering
Construction Company), and the work was carried on simultaneously in all
four, so that the division into contracts had no bearing on the methods
of work adopted, and these will now be described as a whole and with no
further reference to the different sections.
Excavation.
Work was started on the New York side on April 18th, 1904, the Weehawken
shaft being at that date still under construction. As will have been
noted in the description of the shafts, the contractor found a shaft
already prepared for his use, and cross-drifts at grade and at right
angles to the future tunnels, and extending across their entire width.
The first essential was to get access to the shield chambers, which were
to lie about 330 ft. to the west of the shaft, so that the construction
of these enlargements in which the shields for the subaqueous tunnels
were to be built might be finished as soon as possible and thus allow
the earliest possible start to be made with the shield-driven tunnels.
With this in view, two bottom headings, on the center line of each of
the two tracks, were driven westward from the western cross-heading at
the foot of the shaft. When about 138 ft. had been made in this way,
the two headings were brought together and a break-up was made to the
crown level of the tunnel, as the depth of rock cover was doubtful. From
this break-up a top heading was driven westward to Station 200 + 30.
While widening the heading out at Station 200 + 20 the rock was
penetrated on the south side. The exposed wet sand and gravel started to
run, and, as a consequence, a change in design was made, the shield
chambers (and consequently the start of the shield-driven tunnels) being
moved eastward from their original location 133 ft. to their present
location. A certain amount of time was necessarily spent in making these
changes of design, which involved a rearrangement of the whole layout
from the Terminal Station to the start of the River Tunnels. On July
5th, 1904, however, the new design was formally approved. No sooner had
this been decided than a strike arose on the work, and this was not
settled until August 1st, 1904, but from that time the work progressed
without delay. No further reference will be made to the work in the
shield chambers, as that will come under the heading of "River Tunnels,"
being of the segmental, cast-iron lined type.
A top heading was now driven over the original bottom heading west of
the shaft, and at the same time the original cross-drifts from the shaft
were amalgamated with and broken down by a heading driven at the crown
level of the "Intercepting Arch" which here cuts across the ordinary run
of tunnel at right angles and affords access to the tunnels from the
shafts.
The excavation of the upper portion of the intercepting arch at its
southern end gave some trouble, and caused some anxiety, as the rock
cover was penetrated and the wet sand and gravel were exposed. This made
it necessary to timber all this section heavily, and the tracks of the
New York Central Railroad directly above were successfully supported.
The general way in which this timbering was carried out will be
described under the head of "Timbering."
Meanwhile, the excavation of the tunnels west of the intercepting arch
was continued until the North and South Tunnels were taken out to their
full outlines, leaving a core of rock between them. This core was
gradually removed, and timbering supporting the rock above the middle
wall was put in as excavation went on. The ground, which was entirely of
micaceous schist, typical of this part of Manhattan, seamed with veins
of granite, was rather heavy at the west end, or adjacent to the shield
chambers, and required complete segmental timbering across the whole
span. One heavy fall of rock in the corewall between the North and South
Tunnels took place on November 2d, but fortunately did not extend beyond
the limits of the permanent work. On November 7th, 1904, the excavation
east of the intercepting arch was begun by driving a bottom heading in
the South Tunnel. This was continued to Station 197 + 14 and then was
taken up to the crown level and worked as a top heading with the view of
keeping track, by making exploratory borings upward from the roof at
frequent intervals, of the rock surface, which was here irregular and
not known with any degree of certainty. The work was not pressed with
any vigor, because all efforts were then being bent toward excavating
from the River Tunnels as much rock as possible. In Section Gy-East the
conditions were exceptionally variable, as the rock was subject to
sudden changes from a soft crumbling mica schist to broad bands of hard
granite, and, in addition, the rock surface was very irregular, and, for
a good length of this section, was below the crown of the tunnel, a
condition which led to the adoption of the cut-and-cover method for part
of the work.
The irregularity in conditions called for varying methods of procedure,
but in general the methods were as shown on Plate XXXIII, and described
as follows:
_In Solid Rock._--Where there was plenty of good rock cover, a top
middle heading was driven, which was afterward widened out to the full
cross-section of the twin tunnel arches. If necessary, a few lengths of
segmental timbering were put in before taking down the bench, which was
generally kept some 40 or 50 ft. behind the breast of the heading. After
the bench was down, the middle conduit trench was excavated and the
trimming done.
_In Soft Rock._--Where there was not enough rock cover, or where there
was actual soft ground in the roof, wall-plate headings at the springing
line level were driven ahead of the remainder of the work. The
wall-plates were laid in these, the roof was taken out in short lengths,
and segmental timbering spanning from wall-plate to wall-plate was put
in. The roof being thus held, the bench excavation proceeded without
trouble. Where the rock was penetrated and soft ground showed in the
roof, poling boards were driven ahead over the crown-bars, as shown in
Fig. 4.
_Cut-and-Cover Work._--After some 225 ft. had been driven from the
intercepting arch, it was found that the crown of the tunnel was
continually in soft ground. To ascertain the extent of this condition
the contractor decided to make soundings as far as Tenth Avenue, which
was done by sinking trial pits and making wash-borings in the street.
These soundings showed that there would be soft ground in the crown from
Station 194 + 75 to Station 194 + 25 (at one point to a depth of 12 ft.
below the crown), and on each side of this section the cover was
insufficient from Station 193 + 58 to Station 195 + 30. This condition
being known, it was decided to adopt cut-and-cover work for this length,
the principal reasons being that repairs to sewers, streets, and drains
would be no more, and probably less, expensive than with the tunnel
method; the underpinning of a heavy brick brewery building adjoining the
works on the north side would be facilitated, and the opening in the
street, through which muck and materials could be handled, would relieve
the congested shaft, through which the large volume of muck from the
River Tunnels was then being conveyed. On the other hand, the
cut-and-cover method was adversely affected by the presence of a heavy
timber trestle built down the south side of the street and over which
passed all the excavation from the Terminal Station, amounting to a very
heavy traffic. As this trestle had to be supported, it complicated the
situation considerably. Very little active progress was made between
June, 1905, and April, 1906, as the contractor's energies during that
time were much taken up with the progress of the shield-driven tunnels.
In April, 1906, preparations were made to start a 50-ft. length of open
cut, rangers being fixed and sheathing driven; and the sewer which ran
down the middle of this street was diverted to the outside of the
open-cut length.
April and May were occupied in driving the sheathing down to rock,
supporting the trestle, underpinning the adjoining brewery, and
excavating the soft material above the rock. On June 2d, 1906, rock was
reached, and, by July 31st, the excavation was down to subgrade over
nearly the whole 50 ft. in the first length. In the meantime another
length was opened up, and eventually a third.
The surface of the rock now seemed to be rising, and the heavy buildings
had been passed, so that tunneling was reverted to for the rest of the
work, though many difficulties were caused by soft rock in the roof from
time to time.
[Illustration: METHOD OF DRIVING ROOF LAGGING IN SOFT GROUND. FIG. 4.]
When the excavation for the open-cut work of the Terminal Station had
advanced to the line of Tenth Avenue, the contractor started a heading
from this point and drove westward under Tenth Avenue until the headings
driven eastward from the cut-and-cover portion, were met.
This was done to expedite the work under Tenth Avenue, where the ground
was not very good, where there were several important gas and water
mains in the street, and where, moreover, the tunnels were of
exceptionally large span (24 ft. 6 in.), making a total width of some 60
ft. for the excavation. The excavation for the New York Tunnels was
practically finished in January, 1908.
_Drilling and Blasting._--The foregoing short description will serve to
show in a general way the scheme adopted in making the excavation. A few
details on drilling and blasting methods may not be out of place.
Percussive drills run by air pressure were used. They were
Ingersoll-Sergeant, Nos. 3½, A-86, C-24, and F-24. The air came from
the high-pressure compressor previously described. This compressor,
without assistance, could supply air for nine drills, but, when fed by
compressed air from the lower pressure, its capacity was increased three
or four times.
The air was compressed to 100 lb. per sq. in. in the power-house, and
was delivered at about 80 lb. per sq. in. at the drills. A 3-in. air
line was used. The drill steel was 1-1/8-to 1-3/8-in. octagonal. The
holes were about 3¼ in. in diameter at starting and 2-5/8 in. at the
full depth of 10 ft. The powder used on the New York side was 40%
Forcite, the near presence of heavy buildings and lack of much rock
cover necessitating light charges and many holes spaced close together.
To compensate the contractor for the inevitable excavation done outside
the neat lines of the masonry lining, the excavation was paid for to the
"Standard Section Line" which was 12 in. outside the neat lines on top
and sides and 6 in. outside at the bottom of the cross-section. The
actual amount of excavation done was about 11% greater than that paid
for. The distance excavated beyond the neat line, because of the very
heavy timbering necessary, was about 2.1 ft. instead of the 1 ft.
allowed, and at the bottom about 0.85 ft. instead of the 0.50 ft. paid
for.
For a period of 5 months detailed records were kept of the drilling and
blasting. About 12,900 cu. yd. of excavation are included. A sketch and
table showing the method of driving the heading, the number and location
of the holes drilled, and the amount of powder used, is given in Fig. 5.
From this and similar figures the information in Table 5 is derived.