====================================+================+================|
| North | North |
| Manhattan. | Weehawken. |
------------------------------------+----------------+----------------|
Shaft and preliminary headings. | June 10, '03. | June 11, '03. |
Begun. | | |
Shaft and preliminary headings. |December 11, '03|September 1, '04|
Finished. | | |
Excavation of shield chamber. Begun.| May 24, '04. |January 16, '05.|
Excavation of shield chamber. |January 21, '05.| March 25, '05. |
Finished. | | |
Cast-iron lining of shield chambers.|February 4, '05.| None. |
Begun. | | |
Cast-iron lining of shield chambers.| March 13, '05. | None. |
Finished. | | |
Excavation of tunnels begun before |October 17, '04.|January 13, '05.|
installation of shield. | | |
Commenced building falsework for | March 6, '05. | March 23, '05. |
shield. | | |
Shield parts received at shaft. | March 11, '05. | March 20, '05. |
Erection of shield begun. | March 13, '05. | March 27, '05. |
Erection of shield (structural | March 27, '05. | April 12, '05. |
steel). Finished. | | |
Erection of shield (hydraulic | May 11, '05. | May 25, '05. |
fittings). Finished. | | |
First ring of permanent cast-iron | May 12, '05. | May 29, '05. |
lining put in. | | |
First air lock bulkhead wall. Begun.| May 29, '05. | June 15, '05. |
First air lock bulkhead wall. | June 7, '05. | June 23, '05. |
Finished. | | |
Air pressure first put in tunnel. | June 25, '05. | June 29, '05. |
Rock disappeared from invert of |December 1, '05.|October 31, '05.|
tunnel. | | |
First pair of bore segments built in|December 9, '05.|January 12, '06.|
tunnel. | | |
Rip-rap of river bulkhead wall met. |February 8, '06.| None. |
First pile met (in river bulkhead |February 18, '06|January 3, '06. |
wall at Manhattan, and Fowler | | |
warehouse foundation at Weehawken). | | |
Last pile met. | March 2, '06. |February 5, '06.|
First ring erected on river side of | March 3, '06. |February 6, '06.|
shore line. | | |
Removing hood of shield. Begun. | March 27, '06. |February 6, '06.|
Removing hood of shield. Finished. | April 1, '06. |February 8, '06.|
Second air-lock bulkhead wall. | May 12, '06. | March 19, '06. |
Begun. | | |
Second air-lock bulkhead wall. | May 21, '06. | March 24, '06. |
Finished. | | |
------------------------------------+----------------+----------------|
Tunnel holed through with meeting | September 12, 1906. |
tunnel. | |
Last ring of permanent cast-iron | October 9, 1906. |
lining built in. | |
====================================+================+================|
====================================+================+================|
| South | South |
| Manhattan. | Weehawken. |
------------------------------------+----------------+----------------|
Shaft and preliminary headings. |June 10, '03. |June 11, '03. |
Begun. | | |
Shaft and preliminary headings. |December 11, |September 1, 04|
Finished. |'03. | |
Excavation of shield chamber. Begun.|May 24, '04. |January 16, '05.|
Excavation of shield chamber. |May 13, '05. |April 19, '05. |
Finished. | | |
Cast-iron lining of shield chambers.|May 15, '05. |None. |
Begun. | | |
Cast-iron lining of shield chambers.|June 14, '05. |None. |
Finished. | | |
Excavation of tunnels begun before |January 5, '05. |January 25, '05.|
installation of shield. | | |
Commenced building falsework for |June 19, '05. |April 17, '05. |
shield. | | |
Shield parts received at shaft. |June 22, '05. |April 24, '05. |
Erection of shield begun. |June 22, '05. |April 24, '05. |
Erection of shield (structural |June 8, '05. |May 6, '05. |
steel). Finished. | | |
Erection of shield (hydraulic |August 27, '05. |June 13, '05. |
fittings). Finished. | | |
First ring of permanent cast-iron |August 27, '05. |June 14, '05. |
lining put in. | | |
First air lock bulkhead wall. Begun.|September 18, |June 21, '05. |
|'05 | |
First air lock bulkhead wall. |September 23, |July 3, '05. |
Finished. |'05 | |
Air pressure first put in tunnel. |October 6, '05. |July 8, '05. |
Rock disappeared from invert of |February 8, '06.|September 21, 05|
tunnel. | | |
First pair of bore segments built in|February 16, |December 12, '05|
tunnel. |'06. | |
Rip-rap of river bulkhead wall met. |April 11, '06. |None. |
First pile met (in river bulkhead |April 18, '06. |December 4, '06.|
wall at Manhattan, and Fowler | | |
warehouse foundation at Weehawken). | | |
Last pile met. |May 1, '06. |January 9 '06. |
First ring erected on river side of |May 9, '06. |January 19, '06.|
shore line. | | |
Removing hood of shield. Begun. |May 9, '06. |January 19, '06.|
Removing hood of shield. Finished. |May 12, '06. |January 24, '06.|
Second air-lock bulkhead wall. |July 13, '06. |March 11, '06. |
Begun. | | |
Second air-lock bulkhead wall. |July 21, '06. |March 18, '06. |
Finished. | | |
------------------------------------+----------------+----------------|
Tunnel holed through with meeting | October 9, 1906. |
tunnel. | |
Last ring of permanent cast-iron | November 18, 1906. |
lining built in. | |
====================================+================+================+
The excavation was begun on May 24th, 1904, and finished on May 15th,
1905. The segments were placed by an erector consisting of a timber boom
supported by cross-timbers running on car wheels on longitudinal timbers
at each side of the tunnel. Motion was transmitted to the boom by two
sets of tackle, and the heavy (5,000-lb.) segments were easily handled.
The erection of the lining was started on February 4th, 1905, and
finished on June 14th, 1905.
While the shield chambers were being excavated, bottom headings were run
along the lines of the river tunnels and continued until the lack of
rock cover prevented their being driven further. These were afterward
enlarged to the full section as far as possible. The typical working
force in the shield chambers was as follows:
_Ten-hour Shifts._
_Drilling and Blasting._
1 Foreman @ $3.50
6 Drillers " 3.00
6 Drillers' helpers " 2.00
1 Blacksmith " 3.50
1 Blacksmith's helper " 2.25
1 Powderman " 2.00
1 Waterboy " 2.00
1 Nipper " 2.00
1 Machinist " 3.00
1 Machinist's helper " 1.80
_Mucking._
1 or 2 Foremen @ $3.00
16 Muckers " 2.00
[Illustration: PLATE XXXVIII. TRANS. AM. SOC. CIV. ENGRS. VOL. LXVII,
NO. 1155. HEWETT AND BROWN ON PENNSYLVANIA R. R. TUNNELS: NORTH RIVER
TUNNELS. FIG. 1. FIG. 2.]
_Erection of Shields._--The tunneling shields have been described in
some detail in the section of this paper dealing with the contractor's
plant. They consist essentially of two parts, the structural steelwork
and the hydraulic fittings. The former was made by the Riter Conley
Manufacturing Company, of Pittsburg, Pa., and put up by the Terry and
Tench Company, of New York City; the hydraulic fittings were made and
put in by the Watson-Stillman Company, of New York City.
On the New York side, the shields were built inside the iron lining of
the shield chambers, hence no falsework was needed, as the necessary
hoisting tackle could be slung from the iron lining; at Weehawken,
however, the erection was done in the bare rock excavation, so that
timber falsework had to be used. The assembly and riveting took about 2
weeks for each shield; the riveting was done with pneumatic riveters,
using compressed air direct from the tunnel supply.
After the structural steel had been finished, the shields, which had
hitherto been set on the floor of the chambers in order to give room for
working over the top, were jacked up to grade; this involved lifting a
weight of 113 tons. While the hydraulic fittings were being put in, the
shields were moved forward on a cradle, built of concrete with steel
rails embedded, on which the shield was driven for the length in which
the tunnel was in solid rock.
The installation of the hydraulic fittings took from 4 to 6 weeks per
shield. The total weight of each finished shield was about 193 tons. The
completed shield, as it appeared in the tunnel, is shown by Fig. 1,
Plate XXXVIII. The typical force working on shield erection was as
follows:
_Ten-hour Shifts._
_Shield Erection._ (_Terry and Tench._)
1 Superintendent @ $13.00 per day
4 Foremen " 5.50 " "
1 Timekeeper " 2.50 " "
2 Engineers " 4.50 " "
34 Iron workers " 4.50 " "
7 Laborers " 2.25 " "
_Hydraulic Work._ (_Watson-Stillman Company._)
4 Mechanics @ $4.00 per day
_General Labor._ (_O'Rourke Engineering Construction Company._)
1 Inspector @ $4.00 per day
1 Foreman " 4.00 " "
8 Laborers " 2.00 " "
1 Engineer " 2.50 " "
After the shield was finished and in position, the first two rings of
the lining were erected in the tail of the shield. These first rings
were then firmly braced to the rock and the chamber lining; then the
shield was shoved ahead by its own jacks, another ring was built, and so
on.
The description of the actual methods of work in the shield-driven
tunnels can now be given; this will be divided generally into the
different kinds of conditions met at the working face, for example, Full
Face of Rock, Mixed Face, Full Face of Sand and Gravel, Under River
Bulkhead, and Full Face of Silt.
The last heading is the one under which by far the longest length of
tunnel was driven, and, as not much has hitherto appeared descriptive of
the handling of a shield, through this material, considerable space will
be devoted to it.
_Full Face of Rock._--As was described when dealing with the shield
chambers, as much as possible of the rock excavation was done before the
shields were installed. On the New York side, about 146 ft. of tunnel
was completely excavated, with 71 ft. of bottom headings beyond that,
and at Weehawken, 58 and 40 ft. of tunnel and heading beyond,
respectively. This was chiefly done to avoid handling the rock through
the narrow shield doors. Test holes were driven ahead at short intervals
to make sure that the rock cover was not being lost, but, nevertheless,
at Weehawken, on February 14th, 1905, a blast broke through the rock and
let the mud flow in, filling the tunnel for half its height for a
distance of 300 ft. from its face.
Throughout the rock section the shield traveled on a cradle of concrete
in which were embedded either two or three steel rails. In the portion
in which the whole of the excavation had been taken out, it was only
necessary to trim off projecting corners of rock. In the portion in
which only a bottom heading had been driven, the excavation was
completed just in front of the shield, the drilling below axis level
being done from the heading itself, and above that from the front
sliding platforms of the shield. The holes were placed near together and
drilled short, and very light charges of powder were used, so as to
lessen the chance of knocking the shield about too much. In this work
the small shield doors hampered the work greatly, and it might have been
well to have provided a larger bottom opening which could have been
subdivided or partly closed when soft ground was met; on the other hand,
the quantity thus handled was small, owing to the fact that the greater
part of the rock was excavated before the shields were installed.
The space outside the lining was grouted with a 1:1 mixture of Portland
cement and sand. Large voids were hand-packed with stone before
grouting. The details of grouting will be described later.
A typical working gang is given herewith. Two such gangs were worked per
shield per 24 hours, 10 hours per shift. All this work was done under
normal air pressure.
_General:_
½ Tunnel superintendent @ $200.00 per month
1 Assistant tunnel superintendent " 5.00 per day
1 General foreman " 5.00 " "
½ Electrician " 3.50 " "
½ Electrician's helper " 3.00 " "
½ Pipefitter " 3.00 " "
½ Pipefitter's helper " 2.75 " "
_Drilling:_
1 Foreman " 5.00 " "
3 Drillers " 4.00 " "
3 Drillers' helpers " 3.00 " "
1 Nipper " 2.50 " "
½ Waterboy " 2.50 " "
½ Powderboy " 2.75 " "
_Mucking:_
1 Foreman " 3.50 " "
8 Muckers " 2.75 " "
_Erecting Iron and Driving Shield:_
1 Erector runner " 4.00 " "
3 Iron workers " 3.00 " "
The duties of such a gang were as follows: The tunnel superintendent
looked after both shifts of one shield. The assistant or "walking boss"
had charge of all work in the tunnel on one shift. The general foreman
had charge of the labor at the face. The electricians looked after
repairs, extensions of the cables, and lamp renewals. The pipefitters
worked in both tunnels repairing leaks in pipes between the power-house
and the working faces, extending the pipe lines, and attending to shield
repairs, and in the latter work the erector runner helped.
The drillers stuck to their own jobs, which were not subject to
interruption as long as the bottom headings lasted. One waterboy and one
powderboy served two tunnels. The muckers helped the iron men put up the
rings of lining, as well as doing their own work. The iron men tightened
bolts, whenever not actually building up iron. The list does not include
the transportation gang, which will be described under its own heading.
The rate of progress attained was 4.2 ft. per day per shield where most
of the excavation had been done before, and 2.1 ft. where none had been
done before.
When the shields had got far enough away from the shield chamber, and
before rock cover was lost, the first air-lock bulkhead walls were put
in.
_Air-Lock Bulkhead Walls._--The specifications required these walls and
all their fittings to be strong enough to stand a pressure of 50 lb. per
sq. in. Accordingly, all the walls were of concrete, 10 ft. in
thickness, except the first two, which were 8 ft. in thickness, and
grouted up tight.
There were three locks in each bulkhead wall capable of holding men,
namely, the top or emergency lock which is set high in order to afford a
safe means of getting away in case of a flood; this lock was used
continuously for producing the lines and levels into the tunnels. It was
very small and cramped for this purpose, and a larger one would have
been better, both for lines and emergencies. This lock was directly
connected with the overhead platform (also called for in the
specifications) which ran the whole length of the tunnels. Side by side,
on the level of the lower or working platform of the tunnel, were the
man lock and the muck lock. In addition a number of pipes were built in
to give access to the cables and for passing pipes, rails, etc., in and
out.
After each tunnel was about 1,200 ft. ahead of the first walls, a second
wall was built just like the first, and no others were put in, so that
altogether there were eight walls. This second wall not only gave an
added safeguard to the tunnel but enabled the air pressure at the
working face to be divided between the two walls, and this compression
or decompression in stages, separated by a spell of walking exercise,
was found to be very good for the health of those working in the air.
_Mixed Face._--When the rock cover became so thin that it was risky to
go on without the air pressure, the air pressure was turned on, starting
with from 12 to 18 lb., which was enough to stop the water from the
gravel on top of the rock. At first, when the surface of the rock was
penetrated, the soft face was held up by horizontal boards braced from
the shield until the shield was shoved. The braces were then taken out
and, as soon as the shield had been shoved, were replaced by others. As
the amount of soft ground in the face increased, the system of timbering
was gradually changed to one of 2-in. poling boards resting on top of
the shield and supported at the face by vertical breast boards, in turn
held by 6 by 6-in. walings braced both through the upper doors to the
iron lining and from the sliding platforms of the shield. The latter
were in their forward position before the shield was shoved, the
pressure being turned off and the exhaust valves opened just before the
shove began. As the shield went ahead, the platform jacks gradually
exhausted and thus held enough pressure on the face to keep it up. Fig.
17 is a sketch of this method. In driving through mixed ground a typical
working gang was about as follows:
_General:_
1/3 Tunnel superintendent @ $300.00 per month
1 Assistant tunnel superintendent " 5.00 per day
1 General foreman " 5.00 " "
½ Electrician " 3.50 " "
½ Electrician's helper " 3.00 " "
½ Pipefitter " 3.25 " "
½ Pipefitter's helper " 3.00 " "
_Drilling:_
1 Foreman " 5.00 " "
2 Drillers " 3.25 " "
2 Drillers' helpers " 3.00 " "
_Timbering:_
2 Timbermen @ $2.50 per day
2 Timbermen's helpers " 2.00 " "
_Mucking:_
1 Foreman " 3.50 " "
6 Muckers " 2.75 " "
_Erecting Iron and Driving Shield:_
1 Erector runner " 3.25 " "
3 Iron workers " 3.00 " "
The average rate of progress was 2.6 ft. per day.
In this case there were three such gangs, each on an 8-hour shift.
_Full Face of Sand and Gravel._--This condition of affairs was only met
at Weehawken. Two systems of timbering were used. In the first system,
Fig. 17, the ground was excavated 2 ft. 6 in. ahead of the cutting edge,
the roof being held by longitudinal poling boards, resting on the
outside of the skin at their back end and on vertical breast boards at
the forward end. When the upper part of the face was dry, it was held by
vertical breast boards braced from the sliding platform and through the
shield doors to cross-timbers in the tunnel; the lower part, which was
always wet, was held by horizontal breast boards braced through the
lower shield pockets to cross-timbers in the tunnel. This system worked
all right as long as the ground in the top was sandy enough and had
sufficient cohesion to allow the polings to be put in, but, when the
upper part was in gravel, thus making it impossible to put in the
longitudinal polings or the vertical breasting, the second system came
in. Here the excavation was only carried 1 ft. 3 in. (half a shove)
ahead of the cutting edge, and the longitudinal polings were replaced by
transverse boards supported by pipes which were placed in the holes
provided in the shield to accommodate some telescopic poling struts
which had been designed but not made. These pipes acted as cantilevers,
and were in two parts, a 2½-in. pipe wedged tight into the holes and
smaller pipes sliding inside them. After a small section of the ground
had been excavated, a board was placed against it, one of the pipes was
drawn out under it, and wedges were driven between it and the board.
These polings were kept below the level of the hood, so that when the
shield was shoved they would come inside of it; in addition, they were
braced with vertical posts from the sliding platforms. The upper part of
the face was held by longitudinal breast boards braced from the sliding
platform by vertical "soldier" pieces. The lower part of the face was
supported by vertical sheet-piling braced to the tunnel through the
lower doors. Sometimes two rows of piling were used, but generally one,
as shown in Fig. 17. Notwithstanding the fact that the breasting was
only 1 ft. 3 in. ahead of the hood, the shield was moved its full stroke
of 2 ft. 6 in., the ground around the cutting edge of the hood being
scraped away by men working bars in the place from which the temporary
breast boards at the circumference had been removed. The back pressure
on the sliding platform jacks, when the exhaust valves were only partly
open, offered a good deal of resistance, and held the face as long as
the movement of the shield was continuous.
[Illustration: METHOD OF TIMBERING FACE IN MIXED GROUND METHOD OF
TIMBERING FACE IN SAND METHOD OF TIMBERING FACE IN SAND AND GRAVEL FIG.
17.]
On one occasion, when for some reason the shield was stopped with the
shove only partly done, and the exhaust valves had not been shut off,
the platforms continued to slide and allowed the face to collapse; the
shield platforms and doorways, however, caught the falling sand and
gravel and the flow choked itself.
As soon as the rock surface was penetrated and the sand and gravel were
met, which happened almost at the same time in the two Weehawken
Tunnels, the escape of air increased enormously, and it at once became
clear that it was impossible to keep enough air in the two tunnels by
the methods then in use, even when working the three compressors, each
capable of compressing 4,400 cu. ft. of free air per min. at top speed.
When the shields just entered the sand and gravel, the face had been
held by light breasting, without any special effort to prevent the
escape of air, but when it was found impossible to supply enough air, a
large amount of straw and clay was used in front of the boards.
This cut down the escape, but, as much air was escaping through the
joints of the iron lining, these were plastered with Portland cement.
Even then, the loss was too great, therefore one tunnel was shut down
entirely and all the air was sent to the other. This allowed a pressure
of 10 lb. to be kept up in the working tunnel, and this, though less
than the head, was enough to allow progress to be made. In order to use
one tunnel as a drain for the other, the two faces were always kept
within 150 ft. of each other by working them alternately. The timbered
face was never grouted, though this would have reduced the loss of air,
as at the same time it would have decreased the progress very much, and
any one who saw the racing engines in the power-house, and realized
that a breakdown of one of them would mean the loss of the faces, was
ready to admit that the quicker this particular period was cut short,
the better.
Above the sand and gravel lay the silt, and, when it showed in the roof,
the escape of air was immediately reduced and the two faces could be
worked simultaneously. Almost at the same time the piles supporting the
large warehouse, known as the Fowler Building, were met. Although the
face now took much less timber, the same system of breast boards as had
been used in the gravel was kept up, but in skeleton form. They were set
2 ft. 6 in. ahead of the shield, however, instead of 1 ft. 3 in., and
the transverse roof poling boards were replaced by longitudinals resting
on the shield. The more piles in the face the less timbering was done.
The piles were cut into handy lengths with axes and chisels.
All timbering was light compared with the weight of the ground, but, as
the shove took place as soon as the set was made, it served its purpose.
When a face was closed down the whole system was greatly reinforced by
braces from the shield, the face of which was closed by the doors.
In driving through such a face the typical 8-hour shift gang was about
as follows:
_General:_
1/3 Tunnel superintendent @ $300.00 per month.
1 Assistant tunnel superintendent " 5.00 per day.
1 General foreman " 5.00 " "
½ Pipefitter " 3.25 " "
½ Pipefitter's helper " 2.75 " "
½ Electrician " 3.00 " "
½ Electrician's helper " 2.75 " "
_Timbering:_
3 Timbermen " 2.50 " "
3 Timbermen's helpers " 2.00 " "
_Mucking:_
1 Foreman " 3.50 " "
6 Muckers " 2.75 " "
_Erecting Iron and Driving Shield:_
1 Erector runner " 3.25 " "
1 Foreman " 4.00 " "
4 Iron workers " 3.00 " "
The drillers were not kept on after the rock disappeared; a foreman was
added who divided his time between iron erection and mucking.
The average rate of progress in sand and gravel without piles was 5.1
ft. per day per shield. When piles and silt were met in the upper part
of the face, the speed increased to 7.0 ft. per day.
_Passing Under River Bulkhead._--At Weehawken no trouble was found in
passing under the river wall, as the bulkhead consisted of only cribwork
supported on silt, and, though the piles obstructed the motion of the
shield, they were easily cut out, and the cribwork itself was well above
the top of the shield.
On the New York side, however, conditions were not nearly as good. The
heavy masonry bulkhead was supported on piles and rip-rap, as shown in
Fig. 18. The line of the top of the shield was about 6 ft. above the
bottom of the rip-rap, the spaces between the stones of which were quite
open and allowed a free flow of water directly from the river. As soon,
therefore, as the cutting edge of the shield entered the rip-rap there
was a blow, the air escaping freely to the ground surface behind the
bulkhead and to the river in front of it. Clay puddle, or mud made from
the excavated silt, was used in large quantities to plug up the
interstices between the stone in the working face, the air pressure
being slightly greater than that needed to keep out the water holding it
in place. The excavation of the rip-rap was a tedious affair, for it had
to be removed one stone at a time and the spaces between the newly
exposed stones plugged with mud immediately. One man stood ready with
the mud while another loosened the stones with a bar. When the shield
had advanced its own length in the rip-rap, another point for the escape
of the air was exposed at the rear end of the shield. This loss was
closed at the leading end of the last ring with mud and cement sacks.
[Illustration: SKETCH SHOWING RIVER TUNNELS PASSING UNDER RIVER BULKHEAD
WALL AT MANHATTAN CROSS-SECTION OF RIVER BULKHEAD WALL ON AXIS OF NORTH
TUNNEL PLAN SHOWING PILES REMOVED TO ALLOW PASSAGE OF SHIELD FIG. 18.]
As long as the shield was stationary it was possible, by using these
methods and exercising great care and watchfulness, to prevent excessive
loss of air; but, while the shield was being shoved ahead, the
difficulties were much increased, for the movement of the shield
displaced the bags and mud as fast as they were placed, and it was only
by shoving slowly and having a large number of men looking out for leaks
and stopping them up the instant they developed that excessive loss of
air could be prevented. In erecting the iron lining, as each segment was
brought into position, it was necessary to clean off the leading
surface of the previous ring and the adjacent portion of the tail of the
shield; this was always accompanied by a slight "blow," and for some
time the air pressure in the tunnel dropped from 25 to 20 lb., that is,
from greater than the balancing pressure to less, every time a segment
was placed, and on two occasions the "blow" became so great that the
tunnel pressure was reduced considerably further, and in consequence the
water from the river rushed in and was not stopped until it had risen
about 4 ft. in the tunnel invert. On such occasions the surface of the
river was greatly disturbed, rising more than 20 ft. in the air in a
sort of geyser. A large quantity of grout (about 2,500 bbl. of cement
and a similar quantity of sand in the North Tunnel and 1,000 bbl. in the
South Tunnel) was used at this point; it was forced through the tunnel
lining immediately behind the shield, greatly reducing the loss of air
and helping to bind the rip-rap together.
When the shield had traveled 25 ft. through the rip-rap, the piles which
support the bulkhead were met. One hundred of these which were spaced at
3-ft. centers in each direction, were cut out of the path of each shield
in a distance of 35 ft. The presence of the piles caused considerable
extra labor, as each pile had to be cut into several pieces with axes to
enable it to be removed through the shield doors, otherwise they
presented no difficulties. It was not necessary to timber the face, as
the piles supported it most effectively.
When the river line had been passed, the "blow" still continued, and as
there was no heavy ground above the tunnel the light silt was carried
away into the water by the escaping air. At one time the cover over the
crown of the tunnel was reduced to such an extent that for a distance of
30 ft. there was less than 10 ft. of very soft silt, and in some places
none at all. Therefore, the shield was stopped and the air pressure
reduced until it was less than the balancing pressure; the blow then
ceased, and about 28,000 cement bags filled with mud were dumped into
the hole (the location made it impossible to dump them _en masse_ from a
scow). They were then weighted down with rip-rap. This sealed the blow,
and the work was continued without any further disturbance from this
source. Just before the blow reached its maximum it was found that two
of the piles which had been encountered were directly in the path of one
of the proposed screw-piles. It was therefore decided to pull these,
and this was done with two 40-ton hydraulic jacks supported by the upper
sliding platforms and acting on a horizontal timber which was connected
to the piles by tie-rods and chains. The working force here was similar
to that employed in the sand and gravel section previously described.
_In Full Face of Silt._--A full face of silt was first met under the New
York Central Railroad freight yard on the New York side. Up to this
point the ground passed through had been either solid rock or a mixed
face of rock and gravel. In both of these the full excavation had to be
taken out before the shield could be shoved, and the soft ground had
needed timbering. When the rock, gravel, and hardpan gave place to a
full face of silt, the timber was removed, all the shield doors were
opened, and the shield was shoved into the ground without any excavation
being done by hand ahead of the diaphragm. As the shield advanced, the
silt was forced through the open doors into the tunnel. After the work
had gone on in this way for some time, taking in about 90% of the full
volume of the tunnel excavation per foot forward, the air pressure was
raised from 20 to 22 lb. The result was that the silt in the face got
harder and flowed less readily through the shield, and the amount taken
in fell to about 65% of the full volume. This manner of shoving at once
caused a disturbance on the surface and the railroad tracks above the
tunnel were raised, so that the pressure was lowered to 16 lb., then the
muck got softer and the full volume of excavation was taken in; after a
while the pressure was again raised to 20 lb.
The forcing of the shield through the silt resulted in a rising of the
bed of the river, the amount that the bed was raised depending on the
quantity of material brought into the shield.
If the whole volume of excavation was being brought in, the surface of
the bed was not affected; when about 50% was being taken in, the surface
was raised about 3 ft.; if the shield was being driven blind, the bed
was raised about 7 ft.
The number of open doors was regulated so as to take in the minimum
quantity of muck consistent with causing no surface disturbance. On the
average, in the North Manhattan Tunnel, all the doors were open, but in
the South Tunnel there were generally only five or six out of the total
nine.
In front of the bulkhead wall at Manhattan the tunnels were under Pier
No. 72. This structure was supported on wooden piles, some 80 ft. or
more in length, which came down below the tunnel invert. The piles which
lay directly in the path of the tunnels, with a few exceptions, had been
pulled. In driving the tunnels through this section, great care had to
be taken not to disturb the piles on either side of the tunnels, as they
supported a heavy trestle used in disposing of the excavation from the
open cut in the terminal yard. To avoid such disturbance, a large
portion of the total excavation had to be taken through the shields.
The first shield which passed the river bulkhead was the south one at
Weehawken. As soon as this line was crossed the silt was found to be
much softer than behind the wall, in fact it was like a fluid in many of
its properties. The fluidity could be changed by varying the tunnel air
pressure; for example, when the air pressure was made equal to the
weight of the overlying material (water and silt), the silt was quite
stiff, and resembled a rather soft clay; but when the air pressure was
from 10 to 15 lb. per sq. in. lower, it became so liquid that it would
flow through a 1½-in. grout hole in the lining, in a thick stream, at
the rate of from 10 to 50 gal. per min. as soon as the plug was taken
out. This was the point to which the contractor had long looked forward,
as he expected to be able to close all his shield doors and drive the
rest of the way across without taking in a shovelful of muck, as had
just been done under the Hudson River, on the South Tunnel of the Hudson
and Manhattan Railroad Company's Tunnels between Morton Street, New York
City, and Hoboken, N. J. The doors were shut and the shield was shoved;
the tunnel at once began to rise rapidly, notwithstanding that the
heaviest possible downward leads that the clearance between the iron and
the shield would allow were put on. At the same time, the pressures
induced in the silt by the shield shouldering the ground aside caused
the iron lining to rise about 2 in. as soon as the shield left it, and
also distorted it, the horizontal diameter decreasing and the vertical
diameter increasing by about as much as 1¼ in. An anxious discussion
followed these phenomena, as the effects had been so utterly unexpected,
and a good many different theories were advanced as to the probable
cause. It was thought that the hood of the shield might have something
to do with the trouble. The shield was stopped, the hood removed, the
doors were shut, and the driving continued. The same trouble was found,
and it was impossible to keep to grade. Work was stopped, and the
question was thoroughly debated; finally, on January 31st, 1906, the
chief engineer directed that one of the shield doors be opened as an
experiment and 50% of the excavation taken in.
The effect was instantaneous, the shield began to come down to grade at
once, and it soon became necessary to close the door partially and
reduce the quantity of muck taken in in order to prevent the tunnel from
getting below grade. The other troubles from distortion, etc., ceased at
the same time.
It was soon found that a powerful aid in the guidance of the shield was
thus brought to hand, for, if high, the shield could be brought down by
increasing the quantity of muck taken in, if low, by decreasing it. From
this time forward, the quantity of muck taken in at each shove was
carefully regulated according to the position of the tunnel with regard
to grade and the nature of the ground. The quantity varied from nothing
to the full volume displaced by the tunnel, and averaged 33% of the
latter.
To regulate the flow, the bottom middle door was fitted with two steel
angles behind which were placed 6 by 6-in. timbers. In this way the
opening could be entirely closed or one of any size left. The muck
flowed into the tunnel in a thick stream, as shown in Fig. 2, Plate
XXXV, and, by regulating the rate of shove it could be made to flow just
as fast as it could be loaded into cars.
In driving through the silt, the typical gang per shift of 8 hours per
shield was as follows:
_General:_
1/3 Tunnel superintendent @ $300 per month
1 Assistant tunnel superintendent " 6.00 per day
1 General foreman " 5.00 " "
½ Electrician " 3.50 " "
½ Electrician's helper " 3.00 " "
1 Foreman " 4.00 " "
2 Pipefitters " 3.50 " "
2 Pipefitters' helpers " 3.25 " "
_Mucking:_
1 Foreman " 4.00 " "
6 Muckers " 3.00 " "
_Erecting Iron and Driving Shield:_
1 Foreman @ $4.00 per day
1 Erector runner " 3.50 " "
4 Iron workers " 3.00 " "
3 Laborers " 3.00 " "
Three such shifts were worked per day, and the air pressure averaged 25
lb. per sq. in.
The increase in the number of pipefitters was due to the greatly
increased speed, and also the steadily increasing length of completed
tunnel. The three laborers in the erection gang spent their whole time
tightening bolts. The rate of progress in the silt under the river per
ring of 2½ ft. was 3 hours 21 min., exclusive of all time when work
was actually suspended. For a considerable part of the time only two
8-hour shifts were worked, owing to a shortage of iron caused by the
change in the design of the lining, whereby the original lining was
changed to a heavier one, and, as the work was also stopped for
experiments and observations, the average of the actual total time,
including all the time during which work was suspended, was 5 hours 32
min. per ring, or 10.8 ft. per day.
The junction of the shields under the river was made as follows: When
the two shields of one tunnel, which had been driven from opposite sides
of the river approached within 10 ft. of each other, the shields were
stopped, a 10-in. pipe was driven between them, and a final check of
lines and levels was made through the pipe. Incidentally, also, the
first through traffic was established by passing a box of cigars through
the pipe from the Manhattan shield to that from Weehawken. One shield
was then started up with all doors closed while the doors on the
stationary shield were opened so that the muck driven ahead by the
moving shield was taken in through the other one's doors. This was
continued until the cutting edges came together. All doors in both
shields were then opened and the shield mucked out. The cutting edges
were taken off, and the shields moved together again, edge of skin to
edge of skin. The removal of the cutting edge necessitated the raising
of the pressure to 37 lb. As the sections of the cutting edges were
taken off, the space between the skin edges was poled with 3-in. stuff.
Fig. 1, Plate XXXIX, is a view of the shields of the North Tunnel after
being brought together and after parts of the interior frames had been
removed. When everything except the skins had been removed, iron lining
was built up inside the skins, the gap at the junction was filled with
concrete, and long bolts were used from ring to ring on the
circumferential joint. Finally, the rings inside the shield skins were
grouted.
[Illustration: Plate XXXIX. TRANS. AM. SOC. CIV. ENGRS. VOL. LXVIII, No.
1155. HEWETT AND BROWN ON PENNSYLVANIA R. R. TUNNELS: NORTH RIVER
TUNNELS. FIG. 1. FIG. 2.]
In order to make clear the nature of the work done in building these
shield-driven tunnels in silt, a short description will be attempted,
this description falling into three main divisions, namely, Shoving the
Shield, Pushing Back the Jacks, and Erecting the Iron Lining.
_Shoving the Shield._--This part of the work is naturally very
important, as the position of the shield determines within pretty narrow
limits the position of the iron built within it, hence the shield during
its forward movement has to be guided very carefully. On this work
certain instructions were issued for the guidance of the foreman in
charge of the shield. These instructions were based on results of
"checks" of the shield and iron's position by the engineering corps of
the Company, and comprised, in the main, two requirements, namely, the
leads that were to be got, and the quantity of muck to be taken in. The
"lead" is the amount that the shield must be advanced further from the
iron, on one side or the other, or on the top or bottom, as measured
from the front face of the last ring of iron lining to the diaphragm of
the shield. These leads are not necessarily true leads from a line at
right angles to the center line, as the iron may have, and in fact
usually does have, a lead of its own which is known and allowed for when
issuing the requirements for the shove.
The foreman, knowing what was wanted, arranged the combination of shield
jacks which would give the required leads and the amount of opening on
the shield door which would give the required amount of muck. To see how
the shield was going ahead, a man was stationed at each side at axis
level and another in the crown. Each man had a graduated rod on which
the marks were so distinct that they could be read by anyone standing on
the lower platform. These rods were held against the shield diaphragm,
and, as it advanced, its distance from the leading end of the last ring
could be seen by the man in control of the jack valves. If he found that
he was not getting the required leads, he could change the combination
of jacks in action. As the time of a shove was often less than 10 min.,
the man had to be very quick in reading the rods and changing the jacks.
If it was found that extensive change in the jack arrangement was
wanted, the shove could be stopped by a man stationed at the main
hydraulic control valve; but, as any such stoppage affected the quantity
of muck taken in, it was not resorted to unless absolutely necessary.
If the quantity of muck coming in was not as desired, a stop had to be
made to alter the size of the opening, and if, while this was being
done, the exhaust valves were not closed quite tight, the silt pressure
on the face of the shield would force it back against the iron. This
fact was sometimes taken advantage of when a full opening did not let in
the desired quantity, for the shield could be shoved, allowed to return,
and shoved again.
The time taken to shove in silt varied greatly with the quantity of
material taken in; for shoving and mucking combined, it averaged 66
min., with an average of 13 cu. yd. of muck disposed of, or about 5 min.
per cu. yd. of material.
_Pushing Back the Jacks._--This was a simple matter, and merely
consisted in making the loose push-back connection to each jack as it
had to be sent back. Some of the jacks became strained and bent, and had
to be taken out and replaced. Where there was silt pressure against the
face of the shield, the hydraulic pressure had to be kept on until the
ring was erected. In such cases, only two or three jacks could be pushed
back at a time, and only after a segment had been set in position, and
the pressure taken on it, could the next jack be pushed back, and so on
around the ring. The time between the finish of the shove (hydraulic
pressure turned off) and the placing of the first segment, was occupied
in pushing back the bottom jacks and cleaning dirt off the tail of the
shield, and averaged about 14 min.
_Erecting the Iron Lining._--As soon as the shove was over, the whole
force, when in silt, set to work at building up the iron and then
tightening the bolts so that the shield could be shoved again. A section
of the tunnel with bolting and working platform is shown on Plate XL.
In the early part of the work, when the ground was being excavated ahead
of the shield, the whole force, with the exception of those working in
front of the shield, was engaged in erecting the iron, but, as soon as
this was done, most of the men returned to the mucking, and only the
iron workers continued to tighten up bolts. On the other sections, where
the shield was shoved into the silt without excavating ahead, as soon as
the shove was completed, the whole force was engaged in the erection of
the iron and the tightening of the bolts, until they were so tight that
the shield could be shoved again for another ring.
The iron was brought into the tunnel on flat cars, two segments to the
car, and was lifted from the car and lowered into the invert of the
shield by a block and fall and chain sling, as shown in Fig. 2, Plate
XXXIX. The bottom three or four segments were pushed around into
position with the erector, the head simply bearing against the
longitudinal flange without being attached to the segment; the upper
segments, however, were, as shown in Fig. 2, Plate XXXVIII, and Fig. 1,
Plate XLI, attached to the erector, by using the expanding bar and the
erector head designed by Mr. Patrick Fitzgerald, the Tunnel
Superintendent. This was found to be a most convenient arrangement.
The single erector attached to the center of the shield was able to
erect the iron as fast as it could be brought into the tunnel, and even
when the weight of the segments was increased 25% (from 2,060 to 2,580
lb.) it always proved equal to its task, although occasionally one of
the chains in the mechanism broke and delayed the work for an hour or
so; but the sum of all the delays from this cause and from breaks and
leaks in the hydraulic line only averaged 13 min. per ring. The
operating valve which was first used was a four-spindle turning valve,
but this was replaced by a sliding valve which was found to be much more
satisfactory, both in ease of operation and freedom from failure.
As the iron was put into place, two of the middle bolts in each
longitudinal flange and two in each circumferential one were pulled as
tight as possible, and the others put in loosely; then, as soon as the
ring was in position, as large a force as could be conveniently worked
at one time was engaged in tightening the bolts. The shape of the tunnel
depended on the thoroughness of the tightening of the bolts, and the
shield was never shoved until the bolts in all the longitudinal flanges
had been thoroughly tightened. In addition, all the bolts in the
circumferential flanges below the axis were tightened, and at least
three of the six in each segment above. After the shield had been shoved
ahead, the bolts were found to have slackened, and, where the daily
progress was four rings, or more, it was necessary to have a small gang
of men always at this work.
In order to get at the bolts, special platforms were necessary, and
throughout the greater part of the work, a traveling platform was used.
This enabled the men to reach handily all parts of the seven leading
rings. This platform was supported and moved forward on wheels fixed on
brackets to the tunnel, and was pulled forward by connecting chains
every time the shield was shoved. In the early part of the work it was
not possible to use platforms, because, in order to maintain the correct
circular shape of the iron lining, it was necessary to put in temporary
horizontal turnbuckles at axis level. These, however, were very
convenient for supporting the planks which were used as a temporary
bolting platform for the sides of the tunnel, and a temporary platform
resting on 6 by 6-in. timbers across the tunnel enabled the bolts in the
crown of the tunnel to be reached, while the 6 by 6-in. timbers were
left in to support the emergency platform previously described (Plate
XL), which extended the entire length of the tunnel.
The time taken to erect the iron lining became shorter and shorter as
the tunnel organization became more perfect and the force better
trained, so that, whereas, in the early part of the work, it frequently
took 6 hours to erect a ring, in the latter part, when the work was
nearing completion, it was a common occurrence to erect a ring in 30
min. The average time in the "heavy iron" section, which included the
greater part of the work under the river, was 1 hour 4 min. for the
erection of the ring and 40 min. for tightening the bolts after that had
been completed, so that the total time spent by the whole gang on
erection and bolting averaged 1 hour 44 min. per ring, exclusive of the
time spent by the small gang which was always engaged in tightening the
bolts. The average time spent in erecting and bolting, for the whole
length of the tube tunnels, was 2 hours 15 min. per ring.
_Tables of Progress._--Tables 24, 25, 26, and 27 have been prepared to
show the time taken in the various operations at each working face.
[Illustration: PLATE XLI. TRANS. AM. SOC. CIV. ENGRS. VOL. LXVIII, No.
1155. HEWETT AND BROWN ON PENNSYLVANIA R. R. TUNNELS: NORTH RIVER
TUNNELS. FIG. 1.]
[Illustration: PLATE XLI. TRANS. AM. SOC. CIV. ENGRS. VOL. LXVIII, No.
1155. HEWETT AND BROWN ON PENNSYLVANIA R. R. TUNNELS: NORTH RIVER
TUNNELS. FIG. 2.]
In Tables 24, 25, 26, and 27, the following symbols are used:
_A_--Including assistant superintendents, foremen, and
electricians, in driving the shield, erecting iron, mucking,
attending to the electric lights, and repairing the pipe line.
_B_--Drillers, drillers' helpers, drill foremen, and nippers.
_C_--All men grouting.
_D_--Engineers and laborers wholly employed on transport between the
first lock and the face.
_E_--In rock, one car = 0.60 cu. yd.; in sand or silt = 1.20 cu. yd.
in place.
_F_--Time between completion of mucking and putting in first plate,
spent in shoving the jacks back.
_G_--In ordinary iron = the whole time spent on erection and
bolting. In heavy iron = the time between putting in the first
plate and placing the key only.
_H_--Time between placing the key and starting the next shove, spent
by the whole gang in tightening bolts. In addition to this,
there was a small gang which spent its whole time at this work.
_I_--In Table 24 the first pair of bore segments is at ring 207-208.
" " 25 " " " " " " " " " 201-202.
" " 26 " " " " " " " " " 185-186.
" " 27 " " " " " " " " " 171-172.
Outside diameter of tunnel = 23 ft. 0 in.
Inside " " " = 21 ft. 2 in.
Length of ring = 2 ft. 6 in.
In the "Ordinary Iron" section the time is divided between mucking
(which included the shoving and pushing back of the jacks) and the
erection time (which included the time spent by the whole gang in
tightening bolts). In the "Heavy Iron" section these times are all
separated into "Mucking," "Pushing Back Jacks," "Erecting," and
"Bolting," and here the bolting time included only that spent on bolts
by the whole gang; in addition, there was a small gang engaged solely in
tightening bolts. The lost time is the average time lost due to the
break-down of hydraulic pipe lines, damaged jacks, and broken erector
chains. The erection time is separated for the various kinds of rings,
that is, straight ordinary rings, rings containing No. 1 bore segments,
rings containing No. 2 bore segments, and taper rings, and it will be
seen that, on the average, taper rings took 22 min. (or 24%) more time
to erect and to bolt than ordinary ones, and that rings containing No. 2
bore segments took 14 min. (or 15%) more.
TABLE 24.--SHIELD-DRIVEN TUNNEL WORK, MANHATTAN SHAFT, RIVER TUNNEL
NORTH. Table showing the size of the gang, the amount of excavation, and
the time per ring taken for the various operations involved in building
tunnel through the several kinds of ground encountered; also the extent
and nature of all the unavoidable delays.