Details have been omitted from the foregoing description, as they can be
treated better and more fully by the constructing engineers in
succeeding papers. There are, however, some general considerations
involved in the designing of the work, which may, perhaps, be referred
to more conveniently in this introductory paper, and these will now
receive attention.
In all parts of the work problems were encountered requiring for their
solution large expenditures and much engineering skill; but many of
these difficulties had been frequently met in previous engineering
experience, and the methods of overcoming them were well understood.
Thus, in the Meadows Division, a long and heavy embankment, part of
which was on submerged meadow land, and many bridge foundations had to
be constructed; in the Bergen Hill tunnels, very tough trap rock was
encountered; in the tunnels under the city, the work was much
complicated and its cost increased greatly by the necessity of caring
for sewers, water and gas pipes, and the foundations of adjacent
buildings; and many troublesome problems were met in the construction of
the tunnels connecting the East River tunnels with the Sunnyside Yard.
The novel features of the project, however, were the great tunnels
extending the line under the North and East Rivers. Tunnels of the kind
contemplated, to be used for heavy and rapid railroad traffic, had never
been constructed through materials similar to those forming the beds of
the North and East Rivers. Questions arising in connection with the
design and method of construction of the tunnels will be considered
later. Here they are referred to only in their relation to the location
and grades of the line, in which connection the conditions controlling
their establishment were the most important elements.
_Location and Grades._--It was desirable to make the tunnels between
the bulkhead lines of the rivers as straight as possible, and it was
necessary to place them at sufficient depth below the dredging plane of
the War Department (which in the North and East Rivers is 40 and 26 ft.
below mean low water, respectively) to insure them against possible
injury from heavy anchors or sunken vessels. Furthermore, they had to
pass under the piers and bulkheads of Manhattan at a depth sufficient to
make it certain that they would not affect the stability of those
structures. Another consideration influencing the establishment of the
depth of the tunnels below the bottoms of the rivers became important as
soon as the method of construction by shields with compressed air was
adopted, namely, the necessity of providing sufficient cover to guard,
as far as possible, against blow-outs during construction.
The tunnels under the city, connecting the sub-river tunnels with the
Terminal Station, were located so as to give as favorable grades as
possible. The provision of the franchise requiring the tops of the
tunnels to be at least 19 ft. below the Street surface, which had been
suggested by the Company to permit of future subways, had no effect on
their location, as other conditions required them to be at a greater
depth.
The line extending westward from Bergen Hill had to be established so as
to give ample head-room at the numerous bridges over the railroads and
highways which it crosses.
Eastward from the East River tunnels, the grades were established so as
to rise as uniformly as possible to the level of the Sunnyside Yard.
The general features of the line, as finally adopted and constructed,
are as follows:
The maximum grade west of the Terminal Station occurs on the New York
side of the North River, and is 2% in the west-bound and 1.93% in the
east-bound tunnels. The ruling grades (for the ascending traffic) being
1.32% in the west-bound and 1.93% in the east-bound tunnels. In the
tunnels east of the Terminal Station the ruling grade is 1.5% for both
east-bound and west-bound traffic. There is, however, descending with
the traffic, a short section on a grade of 1.9 per cent. These grades
would be objectionable with steam locomotives under a heavy traffic, but
the development of the electric locomotive has rendered possible the
operation of grades which would have formerly been considered
prohibitive.
From the junction with the Pennsylvania Railroad, near Harrison, N. J.,
to Woodside, Long Island, a distance of 13.66 miles, there is an average
of 1.5 curves per mile; the line having a total curvature of 230
degrees. The maximum curvature is 2 degrees.
[Illustration: PLATE III.--P. T. & T. R. R. East River Division.
Sunnyside Yard]
_Method of Construction of Sub-River Tunnels._--The character of the
material through which the tunnels were to be constructed differed
greatly in the two rivers. The bed of the North River, at the level of
the tunnels, consists of silt composed principally of clay, sand, and
water, while that of the East River is formed of a great variety of
materials, such as quicksand, sand, boulders, gravel, clay, and
bed-rock. When the method of construction had to be decided there were
no thoroughly satisfactory precedents to follow in the case of either
river, although the Gas Tunnel under the East River, the partly
constructed Hudson Tunnels under the North River, the St. Clair Tunnel
under the St. Clair River, the Blackwall and several other tunnels under
the Thames River at London, supplied much useful information. The
smaller tunnels for a lighter traffic, since so successfully constructed
under the North and East Rivers, had not then been completed. Under
these circumstances, it was the desire of the Management that the Board
should receive and consider proposed methods of construction from all
available sources; and during the first year of its labors much of its
time was devoted to the examination and discussion of projects submitted
for its consideration by engineers and practical builders, some of these
projects having decided merit. Most of the methods proposed involved
temporary structures, or the use of floating plant, in the navigable
channels of the river. This was objectionable in view of the resulting
obstruction to the enormous river traffic. After full consideration of
the subject, it was decided to adopt the shield method with compressed
air for the construction of the tunnels under both rivers, this being
the only method recommended by the Chief Engineers, and having the great
advantage of conducting all operations below the bottom of the river,
thus avoiding obstruction of the channel.
Experience has shown, as was anticipated, that it is much more difficult
to construct tunnels in such material as occurs in the East River and on
the New Jersey side of the North River, than in more homogeneous
material such as is found in the greater part of the North River. During
the progress of construction under the East River, there were frequent
blow-outs through fissures opened in the river-bed, and the bottom of
the river over the tunnel had to be blanketed continuously with clay, to
check the flow of the escaping air.
In view of the serious difficulties which it was thought might be
encountered in the application of the shield method to the East River
work, other methods for the execution of this part of the project
received special consideration, one of the methods considered being the
freezing process. It was proposed to drive a small pilot tunnel and
freeze the ground for a sufficient distance around it by circulating
brine through a system of pipes established in the tunnel. The pilot
tunnel was then to be removed and the full-sized tunnel was to be
excavated in the frozen material and its lining placed in position. By
this means, it was intended to avoid the danger incident to the use of
compressed air in material of greatly varying character. This method
contained too many elements of uncertainty to justify its adoption; but
as the Management considered it desirable to have, if possible, an
alternative method, an extended experiment was made with the freezing
process. A pilot tunnel, 7 ft. 6 in. in diameter, was driven in the bed
of the East River for a distance of 160 ft., circulating pipes were
established in it, and brine at a very low temperature was passed
through the pipes until the ground was frozen for a distance of about
11.5 ft. around the tunnel. Observations to determine the rate of
cooling and other important points connected with the process were
carefully made. When it was found that the construction of the tunnels
was progressing satisfactorily by the shield method, and that so much
time was required to freeze the material that the freezing process could
not be used to advantage in this particular case, the experiment was
discontinued.
_Design of the Sub-River Tunnels._--The sub-river tunnels consist of a
circular cast-iron shell, of the segmental, bolted type, having an
outside diameter of 23 ft., lined with concrete having a normal
thickness of 2 ft. from the outside of the shell. Through each plate of
the shell there is a small hole, closed with a screw plug, through which
grout may be forced into the surrounding material. Each tunnel contains
a single track. A concrete bench, the upper surface of which is 1 ft.
below the axis of the tunnel, is placed on each side of the track, the
distance between benches being 11 ft. 8 in. These benches contain ducts
for carrying electric cables. The main reason for adopting single-track
tunnels instead of a larger tunnel containing two tracks was to avoid
the danger of accidents due to the obstruction of both tracks by
derailment or otherwise. The tunnels are made just large enough to allow
the passage of a train with perfect safety, as it was believed that with
such an arrangement thorough ventilation would be secured by the motion
of the trains. Experience seems to justify this assumption, but, in
order to assure thorough ventilation under unusual conditions, such as
the stoppage of trains in the tunnels, a complete ventilating plant will
be provided for each tunnel. The rapidity and safety of construction
were increased by making the tunnel as small as possible, one of the
difficulties in the shield method of construction being the difference
in hydrostatic pressure between the top and bottom of the shield, which
increases with the diameter of the tunnel.
The concrete lining was introduced to insure the permanency of the
structure, strengthen it from outward pressure and guard it against
injury from accidents which might occur in the tunnel. The side concrete
benches were suggested by Mr. Cassatt, President, to confine the trains
to the center of the tunnels in case of derailment, and to furnish
sidewalks on each side of the trains so as to obviate the necessity of
walking on the track.
Refuge niches are constructed in the side benches of the tunnels.
Manholes, splicing chambers, pump chambers, and other features for the
handling of the electric cables and drainage, are established at
intervals.
At points where unusual stresses were anticipated, as for instance where
the tunnels pass from rock to soft ground, the shell was composed of
steel instead of cast-iron plates. In the North River tunnels the
concrete lining in the invert and in the arch was reinforced by
longitudinal steel bars, but these were not introduced in the East River
tunnels.
Other details connected with the structures, including the drainage,
lighting, ventilation, signaling, and electrification systems, will be
given in succeeding papers.
_Stability of the Sub-River Tunnels._--One of the most important
questions connected with the design of these tunnels was their probable
stability under the long-continued action of a heavy and rapid railroad
traffic. The tunnels are lighter than the materials which they displace
even when the weight of the heavy live load is included. In the East
River the character of the material seemed to justify the conclusion
that the tunnels would not be displaced even under the action of the
live load. In the North River, however, the tunnels are enveloped by a
soft silt and it was at first apprehended that some system of supports
would be advisable to carry the heavy traffic and insure the tunnels
against displacement under its action. To meet this contingency, which
was then believed to be a very serious one, it was proposed to sink
cast-iron screw-piles through the bottom of each tunnel into and through
the underlying silt until satisfactory bearing material was reached. The
pile supports were worked out in sufficient detail to be embraced in the
contract for the construction of these tunnels, with provision, however,
for omitting them should it be determined subsequently that their use
was undesirable. The contract plans contained provisions for sliding
joints where the piles pass through the tunnel floor, so that the live
load might be carried directly to the pile heads by a system of girders,
and also for attaching the piles directly to the tunnel, the two plans
being alternatives.
Investigations, made during the progress of the work to determine the
physical character of the silt and its action on the tunnels, suggested
the possibility that the use of pile supports might be inadvisable. This
view was confirmed by actual experience in the operation of the tunnels
of the Hudson Companies between Hoboken, N. J., and Morton Street,
Manhattan, which were opened to traffic in February, 1908. The stability
of these tunnels under traffic gave further assurance that supports were
unnecessary under the North River tunnels of the Pennsylvania Railroad
Company, and they were therefore dispensed with.
_Cross-Passages Between the Tunnels._--The Bergen Hill tunnels, the land
portions of the North River tunnels and the tunnels under Manhattan are
connected by cross-passages at intervals varying from 50 to 300 ft. As
it was the desire of the Management to provide every arrangement
possible to insure the safety of its passengers and employees and also
to provide for the convenience of inspection, the question of
establishing cross-passages between the tunnels under the rivers was
given most careful consideration. The conclusion was finally reached
that such passages as it was possible to construct between these tunnels
might increase instead of diminish the danger in case of accident. No
more cross-passages have therefore been constructed in the sub-river
sections, except in the East River, where there is a cross-passage and
pump chamber combined between each pair of tunnels about 750 ft. from
the Manhattan bulkhead line.