The River Tunnel work, from some points of view, has the most interest.
It is interesting because it is the first main line crossing of the
formidable obstacle of the Hudson River, and also by reason of the long
and anxiously discussed point as to whether, in view of the preceding
experiences and failures to construct tunnels under that river,
foundations were needed under these tunnels to keep them from changing
in elevation under the action of heavy traffic.
The River Tunnels here described start on the east side of the shield
chambers on the New York side and end at the east side of the shield
chambers on the New Jersey side. They thus include the New York and
exclude the New Jersey shield chambers, the reason for such
discrimination being that the New York shield chambers are lined with
cast iron while those on the New Jersey side are of the typical rock
section type, as already described. The design of the tunnels and their
accessories will be first described, then will come the construction of
the tunnels as far as the completion of the metal lining, followed by a
description of the concrete lining and completion of the work.
Design of Metal Lining.
_New York Shield Chambers._--The shield chambers may be seen on Plate
XXXII, previously referred to, which shows the junction of the
iron-lined tunnels and the shield chambers. They consist of two
iron-lined pieces of tunnel placed side by side, with semi-circular
arches and straight side-walls. The segments of the arch are made to
break joint with one another by making the side-wall or column castings
of two different heights, as shown in Fig. 9. The length of each ring is
18 in.
The reason for the adoption of this type of construction was the
necessity for keeping the width of the permanent structure within the
60-ft. width of the street. The length of this twin structure is 28.5
ft., and the weight of the metal in it is as follows:
19 long-column arch rings at 22,802 lb. 433,238 lb.
19 short-column arch rings at 23,028 lb. 437,532 "
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Total weight 870,770 lb.
_General Type of River Tunnel Lining._--The main ruling type adopted for
the tunnels under the Hudson River, and in the soft water-bearing ground
for some distance on the shoreward side of the river lines, consists of
two parallel metal-lined tunnels, circular in cross-section, each tunnel
being 23 ft. outside diameter, and the two tunnels 37 ft. apart from
center to center, as shown on Fig. 10. The metal lining is of cast iron
(except for a few short lengths of cast steel) and of the usual
segmental type, consisting of "Rings" of iron, each ring being 2 ft. 6
in. in length, and divided by radial joints into eleven segments, or
"Plates," with one "Key," or closing segment, having joints not radial
but narrower at the outside circumference of the metal lining than at
the inside. The whole structure is joined, segment to segment, and ring
to ring, by mild-steel bolts passing through bolt holes in flanges of
all four faces of each segment. The joints between the segments are made
water-tight by a caulking of sal-ammoniac and iron borings driven into
grooves formed for the purpose on the inner edges of the flanges. The
clearances between the bolts and the bolt holes are also made
water-tight by using grummets or rings of yarn smeared with red lead,
having a snug fit over the shank of the bolt and placed below the washer
on either end of each bolt. When passing through ground more or less
self-sustaining, the space outside the iron lining (formed by the
excavation being necessarily rather larger than the external diameter of
the lining itself) was filled with grout of 1:1 Portland cement and sand
forced by air pressure through grout holes in each segment. These holes
were tapped, and were closed with a screw plug before and after
grouting.
[Illustration: DETAILS OF MANHATTAN SHIELD CHAMBERS FIG. 9.]
Having thus stated in a general way the main ruling features of the
design, a detailed description of the various modifications of the
ruling type will be given.
[Illustration: TYPICAL CROSS-SECTION OF RULING DESIGN OF METAL-LINED
SHIELD-DRIVEN TUNNELS FIG. 10.]
The two main divisions of the iron lining are the "ordinary" or lighter
type and the heavy type. The details of the ordinary iron are shown in
Fig. 11, which shows all types of lining. It was on this design that the
contract was let, and it was originally intended that this should be the
only type of iron used. The dimensions of the iron are clearly shown on
the drawing, and it will be seen that the external diameter is 23 ft.,
the interior diameter, 21 ft. 2 in., the length of each ring, 2 ft. 6
in., and the thickness of the iron skin or web, 1½ in. The bolt holes
in the circumferential flanges are evenly spaced through the circle, so
that adjacent rings may be bolted together in any relative position as
regards the radial joints, and, as a matter of fact, in the erection of
the tunnel lining, all the rings "break joint," with the exception of
those at the bore segments, as will be described later. This type of
iron, when the original type was modified, came to be known as the
ordinary pocketless iron; that is, the weight is of the ordinary or
lighter type, in contradistinction to the heavier one, which later
supplanted it, and the caulking groove runs along the edges of the
flanges and does not form pockets around the bolt holes, as did the
groove in a later type.
Each ring is made up of eleven segments and a key piece. Of these, nine
have radial joints at both ends, and are called "_A_" segments; two,
called "_B_" segments, have a radial joint at one end and a non-radial
joint at the other. The non-radial joint is placed next to the key,
which is 12.25 in. wide at the outside circumference of the iron and
12.50 in. wide at the inside.
The web is not of uniform thickness. The middle part of each _A_ and _B_
segment is 1½ in. thick; at the distance of 6 in. from the root of
each flange, the thickness of web begins to increase, so that at the
root it is 2-3/8 in. thick. The web of the key plate is 1¾ in. thick.
The bolts are of mild steel, and are 1½ in. in diameter; there are 67
in one circumferential joint and 5 in each radial joint. As there are 12
such radial joints, there are altogether 60 bolts in the cross-joints,
making a total of 127 bolts per ring.
This original type of ordinary iron was modified for a special purpose
as follows: It was known that for some distance on either side of the
river, and especially at Weehawken, the tunnels would pass through a
gravel formation, rather open, and containing a heavy head of water. It
was thought that, by carrying the caulking groove around the bolt holes,
it would be possible to make them more water-proof than by the simple
use of the red-leaded grummets. Hence the "Pocket Iron" was adopted for
this situation, the name being derived from the pocket-like recess which
the caulking groove formed when extended around the bolt hole. The
details of this lining are shown on Fig. 11, and the iron (except for
the pockets) is exactly like the pocketless type.
[Illustration: DETAILS OF ALL TYPES OF METAL LININGS USED IN SUBAQUEOUS
SHIELD-DRIVEN TUNNELS FIG. 11.]
On the New York side, in both North and South Tunnels, two short lengths
were built with cast-steel lining. This was done where unusual stresses
were expected to come on the lining, namely, at the point where the
invert passed from firm ground to soft, and also where the tunnels
passed under the heavy river bulkhead wall.
The design was precisely the same as for the ordinary pocketless iron,
and Fig. 11 shows the details. After the tunnels had entered into the
actual under-river portion, several phenomena (which will be described
later) led to the fear that the tunnels, being lighter than the
semi-liquid mud they displaced, might be subject to a buoyant action,
and therefore a heavier type of lining was designed. The length of ring,
number of bolts, etc., were just the same as for the lighter iron, but
the thickness of the web was increased from 1½ to 2 in., the
thickness of the flanges was proportionately increased, and the diameter
of the bolts was increased from 1½ to 1¾ in. This iron was all of
the pocketless type, shown in Fig. 11. Table 18 gives the weights of the
various types of lining.