The standard cast-iron tunnel lining was of the usual tube type, 23 ft.
in outside diameter. The rings were 30 in. wide, and were composed of
eleven segments and a key. The webs of the segments were 1-1/2 in. thick
in the central portion, increasing to 2-3/8 in. at the roots of the
flanges, which were 11 in. deep, 2-1/4 in. thick at the root, and 1-1/2
in. at the edge, and were machined on all contact faces. Recesses were
cast in the edge of the flanges, forming a groove, when the lining was
in place, 1-1/2 in. deep and about 3/8 in. wide, to receive the
caulking. The bolt holes were cored in the flanges, and the bosses
facing the holes were not machined. The customary grout hole was tapped
in the center of each plate for a standard 1-1/4-in. pipe. In this work,
experience indicated that the standard pipe thread was too fine, and
that the taper was objectionable. Each segment weighed, approximately,
2,020 lb., and the key weighed 520 lb., the total weight being 9,102 lb.
per lin. ft. of tunnel. Fig. 1 shows the details of the standard heavy
lining.
In addition to the standard cast-iron lining, cast-steel rings of the
same dimensions were provided for use in a short stretch of the tunnel,
when passing from a rock to a soft ground foundation, where it was
anticipated that unequal settlement and consequent distortion and
increase in stress might occur, but, aside from the small regular drop
of the lining as it passed out of the tail of the shield, no such
settlement was observed.
Two classes of lighter iron, one with 1-in. web and 8-in. flanges and
the other with 1-1/4-in. web and 9-in. flanges--the former weighing
5,166 lb. per lin. ft. of tunnel and the latter, 6,776 lb.--were
provided for use in the land sections between East Avenue and the Long
Island City shafts. Two weights of extra heavy segments for use at the
bottom of the rings were also furnished. The so-called _XX_ plates had
webs and flanges 1/4 in. thicker than the standard segment and the _YY_
plates were similarly 1/2 in. heavier. The conditions under which they
were used will be referred to later. All the castings were of the same
general type as shown by Fig. 1.
Rings tapering 3/4 in. and 1-1/2 in. in width were used for changes in
alignment and grade, the former being used approximately at every fourth
ring on the 1 deg. 30' curves. The 1-1/2-in. tapers were largely used for
changes in grade where it was desired to free the iron from binding on
the tail of the shield. Still wider tapers would have been advantageous
for quick results in this respect.
No lug was cast on the segments for attachment to the erector, but in
its place the gadget shown on Fig. 4, Plate LXX, was inserted in one of
the pairs of bolt holes near the center of the plate, and was held in
position by the running nut at one end.
In the beginning it was expected that the natural shape of the rings
would not show more than 1 in. of shortening of the vertical diameter;
this was slightly exceeded, however, the average distortion throughout
the tunnels being 1-7/16 in. The erectors were attached to the shield
and in such a position that they were in the plane of the center of the
ring to be erected when the shove was made without lead and just far
enough to permit placing the segments. If the shield were shoved too
far, a rare occurrence, the erection was inconvenienced. In driving with
high vertical leads, which occurred more frequently, the disadvantage
of placing the erector on the shield was more apparent. Under such
conditions the plane of the erector's motion was acutely inclined to the
plane of the ring, and, after placing the lower portion of the ring, it
was usually necessary to shove the shield a few inches farther in order
to place the upper plates. The practical effect of this action is
referred to later.
[Illustration: FIG. 1.]
At first the erection of the iron in the river tunnels interfered
somewhat with the mucking operations, but the length of time required to
complete the latter was ample for the completion of the former; and the
starting of a shove was seldom postponed by reason of the non-completion
of a ring. After the removal of the bottom of the diaphragms, permitting
the muck cars to be run into the shield and beyond, the two operations
were carried on simultaneously without serious interference. The
installation of the belt conveyor for handling the soft ground spoil in
Tunnel _A_ was of special benefit in this respect.
Preparatory to the final bolt tightening of each ring as erected, a
15-ton draw-jack, consisting of a small pulling-jack inserted in a light
eye-bar chain, was placed on the horizontal diameter, and frequently the
erectors were also used to boost the crown of the iron, the object being
to erect the ring truly circular. Before shoving, a 1-1/4-in.
turn-buckle was also placed on the horizontal diameter in order to
prevent the spreading of the iron, previous to filling the void outside
with grout. The approach of the supports for the upper floor of the
trailing platform necessitated the removal of these turnbuckles from all
but the three leading rings, but if the iron showed a tendency to
continue distortion, they were re-inserted after the passage of the
trailing platform and remained until the arch of the concrete lining was
placed.
The cost of handling and erecting the iron varied greatly at different
times, averaging, for the river tunnels, $3.32 per ton for the directly
chargeable labor of handling and erecting, to which must be added $7.54
for "top charges." The cost of repairing broken plates is included in
this figure.
_Broken Plates._--During the construction of the river section of the
tunnels, a number of segments were found to have been broken while
shoving the shield. The breaks, which with few exceptions were confined
to the three or four bottom plates, almost invariably occurred on the
advanced face of the ring, and rarely extended beyond the bottom of the
flange. A careful study of the breaks and of the shoving records
disclosed several distinct types of fracture and three principal known
causes of breakage by the shield.
In the first case, the accidental intrusion of foreign material between
the jack head and the iron caused the jack to take its bearings on the
flange above its normal position opposite the web of the ring, and
resulted usually in the breaking out of a piece of the flange or in
several radiating cracks with or without a depression of the flange.
These breaks were very characteristic, and the cause was readily
recognizable, even though the intruding substance was not actually
observed.
In the second case, the working of a hard piece of metal, such as a
small tool, into the annular space between the iron and the tail of the
shield, where it was caught on the bead and dragged along as the shield
advanced, was the known cause of a number of broken segments. Such
breaks had no particular characteristic, but were usually close above
the line of travel of the lost tool or metal. Their cause was determined
by the finding of a heavy score on the underside of the segment or the
discovery of the tool wedged in the tail of the shield or lying under
the broken plate when it was removed. It is probable that a number of
breaks ascribed to unknown causes should be placed in this class.
The third cause includes the largest number of breaks, and, while
difficult to define closely, is the most interesting. Broadly speaking,
the breaks resulted from the movements of the shield in relation to the
position of the tunnel lining. While shoving through soft ground, it was
frequently difficult to apply sufficient power to the lower jacks to
complete the full shove of 30 in. on the desired alignment. The shield,
therefore, was driven upward at the beginning of the shove, and, as the
sand packed in front of the shield and more power was required, it was
furnished by applying the upper jacks. The top of the shield was slowly
pushed over, and, at the close of the shove, the desired position had
been obtained; but the shield had been given a rocking motion with a
decided lifting of the tail toward the close of the shove. A similar
lifting of the tail occurred when, with high vertical leads, the top of
the shield was pushed over in order to place the upper plates of the
ring. Again, when the shield was driven above grade and it was desired
to descend, the passage of the shield over the summit produced a like
effect. In all these movements, with the space between the tail of the
shield and the iron packed tight with pugging, the upward thrust of the
shield tended to flatten the iron in the bottom and occasional broken
plates were the result. The free use of the taper rings, placed so as to
relieve the binding of the lining on the tail of the shield, forces the
tunnel to follow the variations in the grade of the shield, but reduces
greatly the injuries to the rings from this action.
In Tunnel _D_, where very high vertical leads were required through the
soft sand, combined with a marked tendency of the shield to settle, the
shield was badly cramped on the iron and dragged along it at the top.
The bearing of the iron on its soft foundation tended to thrust up the
bottom in this case also, as shown by the opening of the bottom
cross-joints when the bolts were slackened to relieve the strain during
a shove. The anticipated cracks in the crown plates, which have been
more frequently observed in other tunnels, did not occur here, and were
not found elsewhere except in one place in Tunnel _B_ where they were
traced to a similar action of the shield. The cracks resulting from the
movements of the shield, as briefly described above, in this third case
were not confined to any particular type, but occurred more frequently
at the extreme end of the circumferential flange than at any other
point.
The number of broken plates occurring in the river tunnels was 319, or
0.42% of the total number erected. Of these, 52 were found and removed,
either before or immediately after a shove, by far the greater number
being broken in handling before or during erection. The remaining 267
are considered below.
_Repair of Broken Plates._--On the completion of a shove, the tail of
the shield lacked about 5 in. of covering the full width of the last
ring, and the removal of a plate broken during the shove, therefore,
would have exposed the ground at the tail of the shield. With a firm
material in the bottom, this introduced no particular difficulties, and,
under such conditions, a broken plate was usually removed at once. In
the sand, however, and especially on the Manhattan side where it was
quick and flowing, the removal of a plate was attended with some danger,
and such plates were usually left to be removed on the completion of the
tunnel. Many of these had been reinforced by the use of _XX_, _YY_, and
steel segments placed adjacent to the break in the following rings.
After the meeting of the shields, the postponed replacement of the
broken segments was taken up. The pressure was raised sufficiently to
dry thoroughly the sand outside the segments, which were drilled and
broken out usually in quarters as shown on Fig. 1, Plate LXXIII. A steel
segment was then inserted in the ring and drawn into place by
turnbuckles. The application of the draw-jack, with a pull of about 30
tons to each end successively, brought the plate to a firm bearing on
the radial joints at the ends.
Where the broken plate was isolated and was reinforced by steel or extra
heavy segments in the adjacent ring, the crack, if slight, was simply
caulked to insure water-tightness. If, however, the crack was opened or
extended to the web of the plate, the cross-flanges were tied together
by a 1-1/2-in. by 7-ft. bolt, inserted through the bolt holes nearest
the broken flange. The long bolt acted in the nature of a bow string,
and was provided at its ends with two nuts set on opposite sides of the
cross-joints to replace the standard bolts removed for its insertion.
Fig. 4, Plate LXXIII shows one of these bolts in place. In addition, all
broken plates remaining in the tunnel were reinforced with 1-in.
twisted-steel rods in the concrete lining, also shown in Fig. 4, Plate
LXXIII.
_Special Construction at River Shield Junctions._--Dismantling the
shields was started as soon as they came to rest in their final position
with the cutting edges together. The plans contemplated their entire
removal, with the exception of the cylindrical skins and cast-steel
cutting edges. Inside the former the standard tunnel lining was erected
to within 4 ft. of the heels of the cutting edges. Spanning the latter,
and forming the continuous metal tunnel lining, the special construction
shown by Fig. 2 was built. This consisted of a 1-1/4 in. rolled-steel
ring, 7 ft. long, erected inside the cutting edges, with an annular
clearance of 1 in., and two special cast-iron rings shaped to connect
the rolled-steel ring with the normal lining. One flange of the special
cast-iron rings was of the standard type, the other was returned 9 in.
in the form of a ring, the inside diameter of which was the same as the
outside diameter of the rolled-steel ring to which it was bolted.
The space between the standard and special construction was of varying
width at the various shields, and was filled with a closure ring cast to
the lengths determined in the field. Fig. 2 shows the completed
construction.
Hook-bolts, screwed through threaded holes and buried in 1 to 1 Portland
cement grout ejected through similar holes, reinforced the rolled-steel
ring against external water pressure. In two of the tunnels the concrete
lining was carried completely through the junction, and covered the
whole construction, while in the remaining two tunnels it was omitted at
the rolled-steel ring, leaving the latter exposed and set back about 3
in. from the face of the concrete.
[Illustration: FIG. 2.]