The distribution of materials to be excavated, as previously outlined,
divided the excavation into three distinct classes, for which different
methods had to be developed.
These three classes were:
_First._--All-rock section.
_Second._--Rock in the bottom, earth in the top.
_Third._--All-earth section.
The extent of the second and third classes was much greater than that of
the first, and they, of course, determined the use of the shield.
Shields had not previously been used extensively in rock work, either
where the face was wholly or partly in rock, and it was necessary to
develop the methods by experience. The specifications required that
where rock was present in the bottom, a bed of concrete should be laid
in the form of a cradle on which to advance the shield.
_All Rock._--At different times, three general methods were used for
excavating in all-rock sections. They may be called: The bottom-heading
method; the full-face method; and the center-heading method.
The bottom-heading method was first tried. A heading, about 8 ft. high
and 12 ft. wide, was driven on the center line, with its bottom as
nearly as possible on the grade line of the bottom of the tunnel. It was
drilled in the ordinary manner by four drills mounted on two columns.
The face of the headings varied from 10 to 30 ft. in advance of the
cutting edge. After driving the heading for about 10 ft., the bottom was
cleared out and a concrete cradle was set. The width of the cradles
varied, but was generally from 8 to 10 ft.
The excavation was enlarged to full size as the shield advanced, the
drills being mounted in the forward compartments of the shield, as
shown by Fig. 1, Plate LXVII, which represents the conditions after the
opening had been cut in the bulkhead, but before the new methods,
mentioned later, had been developed.
[Illustration: PLATE LXVIII]
The sides and top were shot downward into the heading. The area of the
face remaining behind the heading was large, and a great number of holes
and several rounds were required to fire the face to advantage. As soon
as firing was started at the face, the heading was completely blocked,
and operations there had to be suspended until the mucking was nearly
completed. The bottom-heading method was probably as good as any that
could be devised for use with the shields as originally installed. All
the muck had to be taken from the face by hand and handled through the
chutes or doors. By drilling from the shield, some muck was blasted on
to the extensions of the floors and could be handled from the upper
compartments. At best, however, the shield with the closed transverse
bulkhead was a serious obstacle to rapid work in rock sections.
The full-face method was only used where the rock was not considered
safe for a heading. A cut was fired at the bottom, together with side
holes, in a manner quite similar to that adopted in the first set of
holes for a bottom heading. The cradle was then placed, in lengths of
either 2.5 or 5 ft., after which the remainder of the face was fired in
the same manner as for the bottom-heading method. The closed transverse
bulkhead with air-locks, as shown in Fig. 1, Plate LXVI, was placed in
the shield in the hope that it would only be necessary to maintain the
full air pressure in the working compartments in front of the bulkhead.
It was also thought that some form of bulkhead which could be closed
quickly and tightly would be necessary to prevent flooding the tunnel in
case of blows. While no attempt was ever made to reduce the pressure
behind the shield bulkhead, it was obvious from the experience with
Tunnels _B_ and _D,_ while working in the sand between Manhattan and the
reef, that the plan was not practicable, and that the closed bulkhead in
the bottom was a hindrance instead of a safeguard. As soon as rock was
encountered in those tunnels at the west edge of the reef, the
contractor cut through the bulkheads and altered them, as shown in Fig.
2, Plate LXVI.
Taking advantage of the experience gained, openings were cut through the
bulkheads in Shields _A_ and _C_, while they were shut down near the
edge of the Manhattan ledge. In erecting the shields at Long Island
City in May and June, 1906, openings were also provided. These shields
had to pass through about 700 ft. of rock at the start, the greater
portion of which was all-rock section. It was at that point that
openings were first used extensively and methods were developed, which
would not have been possible except where ears could be passed through
the shield. The bottom-heading method was first tried, but the working
space in front of the shield was cramped, and but few men could be
employed in loading the cars. To give more room, the heading was
gradually widened. The enlargement at the top, when made from the
shield, blocked all work at the face of the heading while the former
operation was in progress. To reduce the delays, the heading was raised,
thus reducing the quantity of rock left in the top, and the bottom was
taken out as a bench. To avoid blocking the tracks when firing the top,
a heavy timber platform was built out from the floors of the middle
working compartments. Most of the muck from the top was caught on the
platform and dropped into cars below. This method of working is shown by
Fig. 2, Plate LXVII. The platforms were not entirely satisfactory, and,
later, the drills in the heading were turned upward and a top bench was
also drilled and fired, as shown by Fig. 3, Plate LXVII. There was then
so little excavation left in the top that the muck was allowed to fall
on the tracks and was quickly cleared away. The method just outlined is
called the center-heading method, and was the most satisfactory plan
devised for full-rock sections.
_Excavation in Part Rock and Part Earth._--This was probably the most
difficult work encountered, particularly when the rock was covered with
boulders and coarse sharp sand which permitted a free escape of air. It
was necessary, before removing the rock immediately under the soft
ground, to excavate the earth in advance of the shield to a point beyond
where the rock was to be disturbed, and to support, in some way, the
roof, sides, and face of the opening thus made. The hoods were designed
mainly for the purpose of supporting the roof and the sides. With the
fixed hood it was necessary either to excavate for the distance of the
desired shove in front of it or else to force the hood into the
undisturbed material. To avoid this difficulty, the sliding hoods were
tried as an experiment.
In using the sliding hood, which will be described in detail in Mr.
Japp's paper, the segments commencing at the top were forced forward by
the screw rod, one at a time, as far as possible into the undisturbed
material. Just enough material was then removed from underneath and in
front of the section to free it, and it was again forced forward. These
operations were repeated until the section had been extended far enough
for a shove. As soon as two or three sections had been pushed forward in
this way, the face near the advance end of the sliding hood was
protected by a breast board set on edge and braced from the face.
Gradually, all the segments were worked forward, and, at the same time,
the whole soft ground face was sheeted with timber. At times polings
were placed over the extended segments in order to make room for a
second shove, as shown on Plate LXVIII. When the shield was advanced the
nuts on the screw rods were loosened and the sections of the hoods were
telescoped on to the shield. The idea was ingenious, but proved
impracticable, because of the unequal relative movements of the top and
bottom of the shield in shoving, bringing transverse strains on the hood
sections.
[Illustration: PLATE LXIX]
With the fixed hood, poling boards were used to support the roof and
sides, and the face was supported in the manner described for the
sliding hoods. The polings were usually maple or oak planks, 2 in.
thick, about 8 in. wide, and 6-1/2 ft. long. In advancing the face, the
top board of the old breast was first removed, then the material was
carefully worked out for the length of the poling. The latter was then
placed, with the rear end resting over the hood and the forward end
forced as far as possible into the undisturbed material. When two or
three polings had been placed, a breast board was set. After several
polings were in position, their forward ends were supported by some form
a cantilever attached to the hood. Plate LXIX shows one kind of
supports. In this way all the soft material was excavated down to the
rock surface, and the roof, sides, and face were sheeted with timber. In
shoving, the polings in the roof and sides were lost. It was found that
the breast could usually be advanced 5 ft. with safety. The fixed hood
made it possible to set the face about 7 or 8 ft. in front of the
cutting edge without increasing the length of the polings. This distance
was ample for two shoves, and was generally adopted, although a great
many faces were set for one shove only.
Fixed hoods were substituted for those of the sliding type, originally
placed on Shields _B_ and _D_ at Manhattan, at about the time the latter
encountered the rock at the reef.
In placing the polings and breasting, all voids behind them were filled
as far as possible with marsh hay or bags of sawdust or clay. To prevent
loss of air in open material, the joints between the boards were
plastered with clay especially prepared for the purpose in a pug mill.
The sliding extensions to the floors of the working compartments were
often used, in the early part of the work, to support the timber face or
loose rock, as shown in Fig. 1, Plate LXVIII. At such times the front of
the extensions was held tightly against the planking by the pressure of
the floor jacks. While shoving, the pressure on the floor jacks was
gradually released, allowing the floors to slide back into the shield
and still afford support to the face. The extensions also afforded
convenient working platforms. They were subject to severe bending
strains while the shield was being shoved, however, and the cast-iron
rams were frequently broken or jammed. The extensions did not last
beyond the edge of the ledge at Manhattan, nor more than about half
through the rock work at Long Island City. The fixed extensions
originally placed on Shields _A_ and _C_ at Manhattan were not
substantial enough, and lasted only a few days.
Wherever the rock face was sufficiently sound and high, a bottom heading
was driven some 20 or 30 ft. in advance of the shield. The heading was
driven and the cradle placed independently of the face of the soft
ground above, and in the manner described for all-rock sections. The
remainder of the rock face was removed by firing top and side rounds
into the bottom heading after the soft ground had been excavated. Great
care had to be taken in firing in order not to disturb the timber work
or break the rock away from under the breast boards. If either occurred,
a serious run was likely to follow. The bottom-heading method is shown
by Figs. 1, 2, and 3, Plate LXVIII, and the breasting and poling by Fig.
2, Plate LXX.
In the early part of the work, where a bottom heading was impracticable,
the soft ground was first excavated as described above, and the rock was
drilled by machines mounted on tripods, and fired as a bench. By this
plan no drilling could be done until the soft ground was removed. This
is called the rock-bench method.
Later the rock-cut method was devised. Drills were set up on columns in
the bottom compartments of the shield, and the face was drilled while
work was in progress on the soft ground above. The drilling was done
either for a horizontal or vertical cut and side and top rounds. The
drillers were protected while at work by platforms of timber built out
from the floors of the compartments above. This plan, while probably not
quite as economical of explosives, saved nearly all the delay due to
drilling the bench.
[Illustration: PLATE LXX, FIG. 1.--SMALL SHAFT SUNK TO ROCK.]
[Illustration: PLATE LXX, FIG. 2.--BREASTING AND POLING IN FRONT OF
SHIELD.]
[Illustration: PLATE LXX, FIG. 3.--SHUTTERS ON FRONT OF SHIELD.]
[Illustration: PLATE LXX, FIG. 4.--HYDRAULIC ERECTOR PLACING SEGMENT.]
_All-Earth Section._--As described by Messrs. Hay and Fitzmaurice, in a
paper on the Blackwall Tunnel,[C] the contractor had used, with marked
success, shutters in the face of the shield for excavating in loose open
material. He naturally adopted the method for the East River work. When
the shields in Tunnels _B_ and _D_, at Manhattan, the first to be driven
through soft ground, reached a point under the actual bulkhead line,
work was partly suspended and shutters were put in place in the face of
the top and center compartments. The shutters in the center compartments
in Shield _D_ are shown in Fig. 3, Plate LXX, while the method of work
with the shutters is shown by Figs. 4, 5, 6, and 7, Plate LXVIII. Fig. 4
on that plate shows the shield ready for a shove. As the pressure was
applied to the shield jacks, men loosened the nuts on the screws holding
the ends of the shutters, and allowed the latter to slide back into the
working compartments. At the end of the shove, the shutters were in the
position shown in Fig. 5, Plate LXVIII. In preparing for a new shove,
the slides in the shutters were opened, and the material in front was
raked into the shield. At the same time, the shutters were gradually
worked forward. The two upper shutters in a compartment were generally
advanced from 12 to 15 in., after which the muck could be shoveled out
over the bottom shutters, as shown on Fig. 6, Plate LXVIII, and Fig. 3,
Plate LXX. No shutters were placed in the bottom compartments, and as
the air pressure was not generally high enough to keep the face dry at
the bottom, these compartments were pretty well filled with the soft,
wet quicksand. Just before shoving, this material was excavated to a
point where it ran in faster than it could be taken out. Much of the
excavation in the bottom compartment was done by the blow-pipe. During
the shove the material from the bottom compartment often ran back
through the open door in the transverse bulkhead, as shown by Fig. 5,
Plate LXVIII.
In the Blackwall Tunnel the material was reported to have been loose
enough to keep in close contact with the shutters at all times. In the
East River Tunnels this was not the case. The sand at the top was dry
and would often stand with a vertical face for some hours. In advancing
the shutters, it was difficult to bring them into close contact with the
face at the end of the operation. The soft material at the bottom was
constantly running into the lower compartment and undermining the stiff
dry material at the top. The latter gradually broke away, and, at times,
the actual face was some feet in advance of the shutters. Under those
circumstances, the air escaped freely through the unprotected sand face.
The joints of the shutters were plastered with clay, but this did not
keep the air from passing out through the lower compartments. This
condition facilitated the formation of blows, which were of constant
occurrence where shutters were used in the sand. In Tunnels _B_ and _D_,
at Manhattan, the shutters were used in the above manner clear across to
the reef. In Tunnel _C_, which was considerably behind Tunnels _B_ and
_D_, the shutters, although placed, were never used against the face,
and the excavation was carried on by poling the top and breasting the
face. The change resulted in much better progress and fewer blows. The
excavation through the soft material in Tunnel _C_ had just been
completed when Tunnel _A_ was started, and the gangs of workmen were
exchanged.
The work in soft ground in Tunnel _A_ thus gained the benefit of the
experience in Tunnel _C_. Shutters were placed only in the top
compartments in this tunnel, and, as in Tunnel _C_, were never used in
contact with the face. The method of work is shown by Figs. 1, 2, and 3,
Plate LXXI. The result was still more rapid progress in Tunnel _A_, and
although the loss of air was fully as great in this tunnel as in the
other three, there was only one blow which caused any considerable loss
of pressure. In Tunnels _A_ and _C_ the diaphragms in the rear of the
center compartments of the lower tiers of working chambers were removed
before the shields entered the soft ground. The change was not of as
much advantage in soft ground as in rock, but it facilitated the removal
of the soft wet sand in the bottom. In Tunnel _A_, after encountering
gravel, a belt conveyor was suspended from the traveling stage with one
end projecting through the opening into the working compartment. The use
of the conveyor made it possible to continue mucking at the face while
the bottom plates of the iron lining were being put in place, and
resulted in a material increase in the rate of progress.
[Illustration: PLATE LXXI]
The shutters were not placed on the Long Island shields at all. Just
before the shields passed into all soft ground, a fixed hood was
attached to each.
The method of working in soft ground from Long Island City is
illustrated by Plate LXXII. The full lines at the face of the shield
show the position of the earth before a shove of the shield, and the
dotted lines show the same after the shove. The face was mined out to
the front of the hood and breasted down to a little below the floor of
the top pockets of the shield. In the middle pocket the earth was
allowed to take its natural slope back on the floor. Toward the rear of
the bottom pockets it was held by stop-planks. The air pressure was
always about equal to the hydrostatic head at the middle of the shield,
so that the face in the upper and middle pockets was dry. In the lower
pockets it was wet, and flowed under the pressure of shoving the shield.
By this method 4,195 lin. ft. of tunnel was excavated by the four Long
Island shields in 120 days, from November 1st, 1907, to March 1st, 1908.
This was an average of 8.74 ft. per day per shield.
The rate of progress, the nature of the materials, and the methods
adopted are shown in Table 2.
_Preparations for Junction of Shields._--As previously mentioned, the
Manhattan shields were stopped at the edge of the reef. Before making
the final shove of those shields, special polings were placed with
unusual care. The excavation was bell-shaped to receive the Long Island
shields. The arrangement of the polings is shown by Figs. 4 and 5, Plate
LXXI. After the shields were shoved into final position, as shown at the
right in Fig. 5, the rear end of the polings rested over the cutting
edge and allowed room for the removal of the hood. After the latter had
been accomplished, the temporary bulkheads of concrete and clay bags
were built as a precaution against blows when the shields were close
together. An 8-in. pipe was then driven forward through the bulkhead for
distances varying from 30 to 100 ft., in order to check the alignment
and grade between the two workings before the shields were actually
shoved together. The errors in the surveys were negligible, but here, as
elsewhere, the shields were not exactly in the desired position, and it
took careful handling to bring the cutting edges together. The Long
Island shields were driven to meet those from Manhattan.
TABLE 2.--RATE OF PROGRESS, NATURE OF MATERIALS, AND METHODS ADOPTED IN
CONSTRUCTION OF EAST RIVER TUNNELS.