+---------+---------------+--------------+-------------+---------------+
|Reference|Type of Lining.|Weights of |Weights of |Weights of |
|No. | |complete rings|bolts, nuts, |segments and |
| | |(segments |and washers, |bolts in tunnel|
| | |only), in |in pounds. |complete, in |
| | |pounds. | |pounds. |
|---------+---------------+--------------+-------------+---------------|
|1 |Ordinary cast | 9,273.0 | 336.3 | 9,609.6 |
| |iron without | | | |
| |pockets. | | | |
| | | | | |
|2 |Ordinary cast | 9,158.8 | 336.3 | 9,495.2 |
| |iron with | | | |
| |pockets. | | | |
| | | | | |
|3 |Ordinary cast | 10,099.6 | 336.3 | 10,436.0 |
| |steel without | | | |
| |pockets. | | | |
| | | | | |
|4 |Heavy cast iron| 11,594.0 | 533.4 | 12,127.6 |
| |without | | | |
| |pockets. | | | |
+=========+===============+==============+=============+===============+
The weights in Table 18 are calculated by assuming cast iron to weigh
450 lb. per cu. ft., and cast steel 490 lb. In actual practice the
"ordinary" iron was found to weigh a little more than the weights given,
and the "heavy" a little less.
The silt in the sub-river portion averaged about 100 lb. per cu. ft., so
that the weight of the silt displaced by the tunnel was about 41,548 lb.
per lin. ft.
_Taper Rings._--In order to pass around curves (whether horizontal or
vertical), or to correct deviation from line or grade, taper rings were
used; by this is meant rings which when in place in the tunnels were
wider than the standard rings, either at one side (horizontal tapers or
"Liners"), or at the top ("Depressors"), or at the bottom ("Elevators").
In the original design a ½-in. taper was called for, that is, the wide
side of the ring was ½ in. wider than the narrow side, which was of
the standard width of 2 ft. 6 in. As a matter of fact, during
construction, not only ½-in., but ¾-in. and 1-in. tapers were often
used.
These taper rings necessitated each plate having its own unalterable
position in the ring, hence each plate of the taper ring was numbered,
so that no mistake could be made during erection.
The taper rings were made by casting a ring with one circumferential
flange much thicker than usual, and then machining off this flange to
the taper. This was not only much cheaper than making a special pattern
for each plate, but made it possible to see clearly where and what
tapers were used in the tunnel.
Taper rings were provided for all kinds of lining (except the cast
steel), and the lack of taper steel rings was felt when building the
steel-lined parts of the tunnel, as nothing could be done to remedy
deviations from line or grade until the steel section was over and cast
iron could again be used. Table 19 gives the weights of the different
kinds of tapers used.
TABLE 19.--WEIGHTS OF CAST-IRON TAPER RINGS, IN POUNDS PER COMPLETE
RING.
=================================+======================================
Classification. |Weight of cast iron per complete ring,
| in pounds.
---------------------------------+--------------------------------------
Ordinary pocketless ½- in. taper| 23,767.7
" " 1- " " | 24,352.4
" pocket ½- " " | 23,481.7
Heavy pocketless ½- in. taper | 29,564.8
" " ¾- " " | 29,854.7
" " 1- " " | 30,144.6
=================================+=======================================
_Cast-Steel Bore Segments and Accessories._--The following feature of
these tunnels is different from any hitherto built. It was the original
intention to carry the rolling load independent of the tunnel, or to
assist the support of the silt portion of the structure by a single row
of screw-piles, under each tunnel, and extending down to firmer ground
than that through which the tunnels were driven. Therefore, provision
had to be made whereby these piles could be put down through the invert
of the tunnel with no exposure of the ground.
[Illustration: DETAILS OF BORE SEGMENTS AND ACCESSORIES USED IN
SUBAQUEOUS SHIELD DRIVEN TUNNELS FIG. 12.]
This provision was afforded by the "Bore Segments," which are shown in
detail in Fig. 12. There are two segments, called No. 1 and No. 2,
respectively. These two segments are bolted together in the bottom of
two adjacent rings, and thus form a "Pile Bore." As the piles were to be
kept at 15-ft. centers, and as the tunnel rings were 2 ft. 6 in. in
length, it will be seen that, between each pair of bore-segment rings,
there came four "Plain" rings. The plain rings were built up so that the
radial joints broke joint from ring to ring, but with the bore-segment
rings this could not be done, without unnecessarily adding to the types
of segments.
The bore segments were made of cast steel, and were quite complicated
castings, the principle, however, was quite simple. The segments
provided an opening just a little larger than the shaft of the pile, the
orifice being 2 ft. 7 in. in diameter at the smallest (lowest) point,
while the shaft of the pile was to be 2 ft. 5¼ in. In order to allow
of the entry of the screw-blade or helix of the pile, a slot was formed
in the depth of Bore Segment No. 1, so that, when a pile was put in
position above the bore, the blade, when revolved, would enter the slot
and thus pass under the metal lining, although the actual orifice was
only slightly larger than the pile shaft.
The wall of the pile orifice in Segment No. 2 was made lower than that
in No. 1 so as to allow the blade to enter the slot in Segment No. 1.
When the pile is not actually in process of being sunk, this lower
height in No. 2 is made up with the removable "distance piece." This had
a tongue at one end which engaged in a recess cast to take it in Segment
No. 2 and was held in place by a key piece at the other end of the
distance piece. Details of the distance piece and key are shown in Fig.
12.
The flanges around the pile bore were made flat and furnished with
twelve tapped holes, six in Segment No. 1 and six in Segment No. 2, for
the purpose of attaching the permanent arrangements in conjunction with
which the pile was to be attached to the track system, independently of
the tunnel shell, or directly to the tunnel. It was never decided which
of these alternatives would be used, for, before this decision was
reached, it was agreed that, at any rate for the present, it was better
not to put down piles at all.
To close the bore, the "Bore Plug" was used. This is shown on Fig. 12.
It was of cast steel, and was intended to act as a permanent point of
the screw-pile, that is, the blade section was to be attached to the
bore plug, the distance piece and key were to be removed, and the pile
was to be rotated until the blade had cleared the slot; the distance
piece and key were then to be replaced and sinking resumed.
The plug was held in place against the pressure of the silt by the two
"dogs," while the dogs themselves were attached to the tunnel, as shown
in Fig. 12. The ends of the dogs, which rested on the flanges of the
metal lining of the tunnel, were prevented from being knocked off the
flanges (and thus releasing the plug) by steel clips.
It was expected that it might be desirable to keep the lower end of the
piles open during their sinking, so that the bore plugs were not made
permanently closed, but a seating was formed on the inner circumference
of the plug, and on the seating was placed the "Plug Cover," made of
cast iron, 18¾ in. in diameter and 3 in. thick, furnished with a lug
for lifting and a 3-in. tapped hole closed by a screw-plug, through
which any soundings or samples of ground could be taken prior to sinking
the piles. This plug cover was held in place by a heavy steel "Yoke"
under it, which engaged on the under side of the flange, on top of which
the cover was set. The yoke was attached to the cover by a 1¾-in.
tap-bolt, screwed into the yoke and passing through a 2-in. hole bored
in the center of the cover. This rather peculiar mode of attaching the
cover was adopted so that the cover could be removed by taking off the
nut of the yoke, in case it was desired to open the end of the pile
during the process of sinking.
The plug was a fairly close fit at the bottom of the orifice, that is,
at the outside circumference of the tunnel, where the bore was 2 ft. 7
in. in diameter and the plug 2 ft. 6¾ in., but at the top of the
bore-segment there was more clearance, as the plug was cylindrical while
the bore tapered outward. To fill this space, it was intended that steel
wedges should be used while the shield was being driven, so that they
would withstand the crushing action of the thrusting shield, and, when
the shield was far enough ahead, that they should be removed and
replaced by hardwood wedges. This method was only used in the early
weeks of the work; the modification of not using the shield-jacks which
thrust against the bore segments was then introduced, and the wooden
wedges were put in, when the bore plugs were set in place, and driven
down to the stage of splitting.
When it was resolved not to sink the screw-piles, the bores had to be
closed before putting in the concrete lining. This was done by means of
the covers shown in Fig. 13. The bore plug and all its attachments were
removed, and the flat steel cover, 2 in. thick and with stiffening webs
on the under side, was placed over the circular flanges of the pile
bore. The cover was attached to the bore segments by twelve 1½-in.
stud-bolts, 6 in. long, in the bolt holes already mentioned as provided
on these flanges.
When these were in place, with lead grummets under the heads of the
bolts, and the grooves caulked, the bore segments were water-tight,
except in Bore Segment No. 2, at the joint of the distance piece; and,
to keep water from entering here, this segment was filled to the level
of the top of the flanges with 1:1 Portland cement mortar.
[Illustration: SUBAQUEOUS TUNNELS COVER FOR BORE SEGMENTS FIG. 13.]
The weights of the various parts of the bore segments are given in Table
20.