TYPES AND METHODS OF CONSTRUCTION
Five types of construction have been employed in building the road:
(1) the typical subway near the surface with flat roof and "I" beams
for the roof and sides, supported between tracks with steel bulb-angle
columns used on about 10.6 miles or 52.2 per cent. of the road; (2)
flat roof typical subway of reënforced concrete construction supported
between the tracks by steel bulb-angle columns, used for a short
distance on Lenox Avenue and on the Brooklyn portion of the Brooklyn
Extension, also on the Battery Park loop; (3) concrete lined tunnel
used on about 4.6 miles or 23 per cent. of the road, of which 4.2 per
cent. was concrete lined open cut work, and the remainder was rock
tunnel work; (4) elevated road on steel viaduct used on about 5 miles
or 24.6 per cent. of the road; (5) cast-iron tubes used under the
Harlem and East Rivers.
[Sidenote: _Typical
Subway_]
The general character of the flat roof "I" beam construction is shown
in photograph on page 28 and drawing on this page. The bottom
is of concrete. The side walls have "I" beam columns five feet apart,
between which are vertical concrete arches, the steel acting as a
support for the masonry and allowing the thickness of the walls to be
materially reduced from that necessary were nothing but concrete used.
The tops of the wall columns are connected by roof beams which are
supported by rows of steel columns between the tracks, built on
concrete and cut stone bases forming part of the floor system.
Concrete arches between the roof beams complete the top of the subway.
Such a structure is not impervious, and hence, there has been laid
behind the side walls, under the floor and over the roof a course of
two to eight thicknesses of felt, each washed with hot asphalt as
laid. In addition to this precaution against dampness, in three
sections of the subway (viz.: on Elm Street between Pearl and Grand
Streets, and on the approaches to the Harlem River tunnel, and on the
Battery Park Loop) the felt waterproofing has been made more effective
by one or two courses of hard-burned brick laid in hot asphalt, after
the manner sometimes employed in constructing the linings of
reservoirs of waterworks.
[Illustration: TYPICAL SECTION OF FOUR TRACK SUBWAY]
[Illustration: FOUR-TRACK SUBWAY--SHOWING CROSS-OVER SOUTH OF 18TH
STREET STATION]
In front of the waterproofing, immediately behind the steel columns,
are the systems of terra-cotta ducts in which the electric cables are
placed. The cables can be reached by means of manholes every 200 to
450 feet, which open into the subway and also into the street. The
number of these ducts ranges from 128 down to 32, and they are
connected with the main power station at 58th and 59th Streets and the
Hudson River by a 128-duct subway under the former street.
[Sidenote: _Reinforced
Concrete
Construction_]
The reinforced concrete construction substitutes for the steel roof
beams, steel rods, approximating 1-1/4 inches square, laid in varying
distances according to the different roof loads, from six to ten
inches apart. Rods 1-1/8 inches in diameter tie the side walls,
passing through angle columns in the walls and the bulb-angle columns
in the center. Layers of concrete are laid over the roof rods to a
thickness of from eighteen to thirty inches, and carried two inches
below the rods, imbedding them. For the sides similar square rods and
concrete are used and angle columns five feet apart. The concrete of
the side walls is from fifteen to eighteen inches thick. This type is
shown by photographs on page 41. The rods used are of both square
and twisted form.
[Illustration: LAYING SHEET WATERPROOFING IN BOTTOM]
[Illustration: SPECIAL BRICK AND ASPHALT WATERPROOFING]
[Sidenote: _Methods of
Construction
Typical
Subway_]
The construction of the typical subway has been carried on by a great
variety of methods, partly adopted on account of the conditions under
which the work had to be prosecuted and partly due to the personal
views of the different sub-contractors. The work was all done by open
excavation, the so-called "cut and cover" system, but the conditions
varied widely along different parts of the line, and different means
were adopted to overcome local difficulties. The distance of the rock
surface below the street level had a marked influence on the manner in
which the excavation of the open trenches could be made. In some
places this rock rose nearly to the pavement, as between 14th and 18th
Streets. At other places the subway is located in water-bearing loam
and sand, as in the stretch between Pearl and Grand Streets, where it
was necessary to employ a special design for the bottom, which is
illustrated by drawing on page 42.
This part of the route includes the former site of the ancient Collect
Pond, familiar in the early history of New York, and the excavation
was through made ground, the pond having been filled in for building
purposes after it was abandoned for supplying water to the city. The
excavations through Canal Street, adjacent, were also through made
ground, that street having been at one time, as its name implies, a
canal.
From the City Hall to 9th Street was sand, presenting no particular
difficulties except through the territory just described.
At Union Square rock was encountered on the west side of Fourth Avenue
from the surface down. On the east side of the street, however, at the
surface was sand, which extended 15 feet down to a sloping rock
surface. The tendency of the sand to a slide off into the rock
excavation required great care. The work was done, however, without
interference with the street traffic, which is particularly heavy at
that point.
[Illustration: DUCTS IN SIDE WALLS--EIGHT ONLY OF THE SIXTEEN LAYERS
ARE SHOWN]
[Illustration: REINFORCED CONCRETE CONSTRUCTION]
[Illustration: ROOF SHOWING CONCRETE-STEEL CONSTRUCTION--LENOX AVENUE
AND 140TH-141ST STREETS]
[Illustration: SECTION OF SUBWAY AT PEARL STREET
This construction was made necessary by encountering a layer of Peat
resting on Clay]
[Illustration: SURFACE RAILWAY TRACKS SUPPORTED OVER EXCAVATION ON
UPPER BROADWAY]
[Illustration: SUBDIVISION OF 36" AND 30" GAS MAINS OVER ROOF OF
SUBWAY--66TH STREET AND BROADWAY]
The natural difficulties of the route were increased by the network of
sewers, water and gas mains, steam pipes, pneumatic tubes, electric
conduits and their accessories, which filled the streets; and by the
surface railways and their conduits. In some places the columns of the
elevated railway had to be shored up temporarily, and in other places
the subway passes close to the foundations of lofty buildings, where
the construction needed to insure the safety of both subway and
buildings was quite intricate. As the subway is close to the surface
along a considerable part of its route, its construction involved the
reconstruction of all the underground pipes and ducts in many places,
as well as the removal of projecting vaults and buildings, and, in
some cases, the underpinning of their walls. A description in detail
of the methods of construction followed all along the line would make
an interesting book of itself. Space will only permit, however, an
account of how some of the more serious difficulties were overcome.
On Fourth Avenue, north of Union Square to 33d Street, there were two
electric conduit railway tracks in the center of the roadway and a
horse car track near each curb part of the distance. The two electric
car tracks were used for traffic which could not be interrupted,
although the horse car tracks could be removed without inconvenience.
These conditions rendered it impracticable to disturb the center of
the roadway, while permitting excavation near the curb. Well-timbered
shafts about 8 x 10 feet, in plan, were sunk along one curb line and
tunnels driven from them toward the other side of the street, stopping
about 3-1/2 feet beyond its center line. A bed of concrete was laid on
the bottom of each tunnel, and, when it had set, a heavy vertical
trestle was built on it. In this way trestles were built half across
the street, strong enough to carry all the street cars and traffic on
that half of the roadway. Cableways to handle the dirt were erected
near the curb line, spanning a number of these trestles, and then the
earth between them was excavated from the curb to within a few feet of
the nearest electric car track. The horse car tracks were removed.
Between the electric tracks a trench was dug until its bottom was
level with the tops of the trestles, about three feet below the
surface as a rule. A pair of heavy steel beams was then laid in this
trench on the trestles. Between these beams and the curb line a second
pair of beams were placed. In this way the equivalent of a bridge was
put up, the trestles acting as piers and the beams as girders. The
central portion of the roadway was then undermined and supported by
timbering suspended from the steel beams. The various gas and water
pipes were hung from timbers at the surface of the ground. About four
sections, or 150 feet, of the subway were built at a time in this
manner. When the work was completed along one side of the street it
was repeated in the same manner on the other side. This method of
construction was subsequently modified so as to permit work on both
sides of the street simultaneously. The manner in which the central
part of the roadway was supported remained the same and all of the
traffic was diverted to this strip.
[Illustration: SUPPORT OF ELEVATED RAILWAY STATION AT 42D STREET AND
SIXTH AVENUE]
Between 14th and 17th Streets, because of the proximity of the rock to
the surface, it was necessary to move the tracks of the electric
surface railway from the center of the street some twenty feet to the
east curb, without interrupting traffic, which was very heavy at all
times, the line being one of the main arteries of the Metropolitan
system. Four 12 x 12-inch timbers were laid upon the surface. Standard
cast-iron yokes were placed upon the timbers at the usual distance
apart. Upon this structure the regular track and slot rails were
placed. The space between the rails was floored over. Wooden boxes
were temporarily laid for the electric cables. The usual hand holes
and other accessories were built and the road operated on this timber
roadbed. The removal of the tracks was made necessary because the rock
beneath them and the concrete around the yokes was so closely united
as to be practically monolithic, precluding the use of explosives.
Attempts to remove the rock from under the track demonstrated that it
could not be done without destroying the yokes of the surface railway.
[Illustration: SUPPORTING ELEVATED RAILROAD BY EXTENSION GIRDER--64TH
STREET AND BROADWAY]
The method of undermining the tracks on Broadway from 60th to 104th
Streets was entirely different, for the conditions were not the same.
The street is a wide one with a 22-foot parkway in the center, an
electric conduit railway on either side, and outside each track a wide
roadway. The subway excavation extended about 10 feet outside each
track, leaving between it and the curb ample room for vehicles. The
construction problem, therefore, was to care for the car tracks with a
minimum interference with the excavation. This was accomplished by
temporary bridges for each track, each bridge consisting of a pair of
timber trusses about 55 feet long, braced together overhead high
enough to let a car pass below the bracing. These trusses were set up
on crib-work supports at each end, and the track hung from the lower
chords. (See photograph on page 42.) The excavation then proceeded
until the trench was finished and posts could be put into place
between its bottom and the track. When the track was securely
supported in this way, the trusses were lifted on flat cars and moved
ahead 50 feet.
At 66th Street station the subway roof was about 2 feet from the
electric railway yokes and structures of the street surface line. In
order to build at this point it was necessary to remove two large gas
mains, one 30 inches and the other 36 inches in diameter, and
substitute for them, in troughs built between the roof beams of the
subway, five smaller gas mains, each 24 inches in diameter. This was
done without interrupting the use of the mains.
[Illustration: MOVING BRICK AND CONCRETE RETAINING WALL TO MAKE ROOM
FOR THIRD TRACK--BROADWAY AND 134TH STREET]
At the station on 42d Street, between Park and Madison Avenues, where
there are five subway tracks, and along 42d Street to Broadway, a
special method of construction was employed which was not followed
elsewhere. The excavation here was about 35 feet deep and extended 10
to 15 feet into rock. A trench 30 feet wide was first sunk on the
south side of the street and the subway built in it for a width of two
tracks. Then, at intervals of 50 feet, tunnels were driven toward the
north side of the street. Their tops were about 4 feet above the roof
of the subway and their bottoms were on the roof. When they had been
driven just beyond the line of the fourth track, their ends were
connected by a tunnel parallel with the axis of the subway. The rock
in the bottom of all these tunnels was then excavated to its final
depth. In the small tunnel parallel with the subway axis, a bed of
concrete was placed and the third row of steel columns was erected
ready to carry the steel and concrete roof. When this work was
completed, the earth between the traverse tunnels was excavated, the
material above being supported on poling boards and struts. The roof
of the subway was then extended sidewise over the rock below from the
second to the third row of columns, and it was not until the roof was
finished that the rock beneath was excavated. In this way the subway
was finished for a width of four tracks. For the fifth track the earth
was removed by tunneling to the limits of the subway, and then the
rock below was blasted out.
[Illustration: MOVING WEST SIDE WALL TO WIDEN SUBWAY FOR THIRD
TRACK--135TH STREET AND BROADWAY]
[Illustration: SUBWAY THROUGH NEW "TIMES" BUILDING, SHOWING
INDEPENDENT CONSTRUCTION--THE WORKMEN STAND ON FLOOR GIRDERS OF
SUBWAY]
[Illustration: COLUMNS OF HOTEL BELMONT, PASSING THROUGH SUBWAY AT 42D
STREET AND PARK AVENUE]
In a number of places it was necessary to underpin the columns of the
elevated railways, and a variety of methods were adopted for the work.
A typical example of the difficulties involved was afforded at the
Manhattan Railway Elevated Station at Sixth Avenue and 42d Street. The
stairways of this station were directly over the open excavation for
the subway in the latter thoroughfare and were used by a large number
of people. The work was done in the same manner at each of the four
corners. Two narrow pits about 40 feet apart, were first sunk and
their bottoms covered with concrete at the elevation of the floor of
the subway. A trestle was built in each pit, and on these were placed
a pair of 3-foot plate girders, one on each side of the elevated
column, which was midway between the trestles. The column was then
riveted to the girders and was thus held independent of its original
foundations. Other pits were then sunk under the stairway and trestles
built in them to support it. When this work was completed it was
possible to carry out the remaining excavation without interfering
with the elevated railway traffic.
At 64th Street and Broadway, also, the whole elevated railway had to
be supported during construction. A temporary wooden bent was used to
carry the elevated structure. The elevated columns were removed until
the subway structure was completed at that point. (See photograph on
page 45.)
[Illustration: SMALL WATER MAINS BETWEEN STREET SURFACE AND SUBWAY
ROOF, SUBSTITUTED FOR ONE LARGE MAIN--125TH STREET AND LENOX AVE.]
[Illustration: SPECIAL CONSTRUCTION OF 6-1/2-FOOT SEWER, UNDER CHATHAM
SQUARE]
A feature of the construction which attracted considerable public
attention while it was in progress, was the underpinning of a part of
the Columbus Monument near the southwest entrance to Central Park.
This handsome memorial column has a stone shaft rising about 75 feet
above the street level and weighs about 700 tons. The rubble masonry
foundation is 45 feet square and rests on a 2-foot course of concrete.
The subway passes under its east side within 3 feet of its center,
thus cutting out about three-tenths of the original support. At this
place the footing was on dry sand of considerable depth, but on the
other side of the monument rock rose within 3 feet of the surface. The
steep slope of the rock surface toward the subway necessitated
particular care in underpinning the footings. The work was done by
first driving a tunnel 6 feet wide and 7 feet high under the monument
just outside the wall line of the subway. The tunnel was given a
2-foot bottom of concrete as a support for a row of wood posts a foot
square, which were put in every 5 feet to carry the footing above.
When these posts were securely wedged in place the tunnel was filled
with rubble masonry. This wall was strong enough to carry the weight
of the portion of the monument over the subway, but the monument had
to be supported to prevent its breaking off when undermined. To
support it thus a small tunnel was driven through the rubble masonry
foundation just below the street level and a pair of plate girders run
through it. A trestle bent was then built under each end of the
girders in the finished excavation for the subway. The girders were
wedged up against the top of the tunnel in the masonry and the
excavation was carried out under the monument without any injury to
that structure.
[Illustration: THREE PIPES SUBSTITUTED FOR LARGE BRICK SEWER AT 110TH
STREET AND LENOX AVENUE]
[Illustration: SEWER SIPHON AT 149TH STREET AND RAILROAD AVENUE]
[Illustration: CONCRETE SEWER BACK OF ELECTRIC DUCT MANHOLE--BROADWAY
AND 58TH STREET]
At 134th Street and Broadway a two-track structure of the steel beam
type about 200 feet long was completed. Approaching it from the south,
leading from Manhattan Valley Viaduct, was an open cut with retaining
walls 300 feet long and from 3 to 13 feet in height. After all this
work was finished (and it happened to be the first finished on the
subway), it was decided to widen the road to three tracks, and a
unique piece of work was successfully accomplished. The retaining
walls were moved bodily on slides, by means of jacks, to a line 6-1/4
feet on each side, widening the roadbed 12-1/2 feet, without a break
in either wall. The method of widening the steel-beam typical subway
portion was equally novel. The west wall was moved bodily by jacks
the necessary distance to bring it in line with the new position of
the west retaining wall. The remainder of the structure was then moved
bodily, also by jacks, 6-1/4 feet to the east. The new roof of the
usual type was then added over 12-1/2 feet of additional opening. (See
photographs on pages 46 and 47.)
[Illustration: CONCRETE SEWER BACK OF SIDE WALL, BROADWAY AND 56TH
STREET]
[Illustration: LARGE GAS AND WATER PIPES, RELAID BEHIND EACH SIDE WALL
ON ELM STREET]
Provision had to be made, not only for buildings along the route that
towered far above the street surface, but also for some which
burrowed far below the subway. Photograph on page 47 shows an
interesting example at 42d Street and Broadway, where the pressroom of
the new building of the "New York Times" is beneath the subway, the
first floor is above it, and the first basement is alongside of it.
Incidentally it should be noted that the steel structure of the
building and the subway are independent, the columns of the building
passing through the subway station.
[Illustration: DIFFICULT PIPE WORK--BROADWAY AND 70TH STREET]
At 42d Street and Park Avenue the road passes under the Hotel Belmont,
which necessitated the use of extra heavy steel girders and
foundations for the support of the hotel and reinforced subway
station. (See photograph on page 48.)
Along the east side of Park Row the ascending line of the "loop" was
built through the pressroom of the "New York Times" (the older
downtown building), and as the excavation was considerably below the
bottom of the foundation of the building, great care was necessary to
avoid any settlement. Instead of wood sheathing, steel channels were
driven and thoroughly braced, and construction proceeded without
disturbance of the building, which is very tall.
At 125th Street and Lenox Avenue one of the most complicated network
of subsurface structures was encountered. Street surface electric
lines with their conduits intersect. On the south side of 125th Street
were a 48-inch water main and a 6-inch water main, a 12-inch and two
10-inch gas pipes and a bank of electric light and power ducts. On the
north side were a 20-inch water main, one 6-inch, one 10-inch, and one
12-inch gas pipe and two banks of electric ducts. The headroom between
the subway roof and the surface of the street was 4.75 feet. It was
necessary to relocate the yokes of the street railway tracks on Lenox
Avenue so as to bring them directly over the tunnel roof-beams.
Between the lower flanges of the roof-beams, for four bents, were laid
heavy steel plates well stiffened, and in these troughs were laid four
20-inch pipes, which carried the water of the 48-inch main. (See
photograph on page 49.) Special castings were necessary to make
the connections at each end. The smaller pipes and ducts were
rearranged and carried over the roof or laid in troughs composed of
3-inch I-beams laid on the lower flanges of the roof-beams. In
addition to all the transverse pipes, there were numerous pipes and
duct lines to be relaid and rebuilt parallel to the subway and around
the station. The change was accomplished without stopping or delaying
the street cars. The water mains were shut off for only a few hours.
[Illustration: SPECIAL RIVETED RECTANGULAR WATER PIPE, OVER ROOF OF
SUBWAY AT 126TH STREET AND LENOX AVENUE]
As has been said, the typical subway near the surface was used for
about one-half of the road. Since the sewers were at such a depth as
to interfere with the construction of the subway, it meant that the
sewers along that half had to be reconstructed. This indicates but
very partially the magnitude of the sewer work, however, because
nearly as many main sewers had to be reconstructed off the route of
the subway as on the route; 7.21 miles of main sewers along the route
were reconstructed and 5.13 miles of main sewers off the route. The
reason why so many main sewers on streets away from the subway had to
be rebuilt, was that, from 42d Street, south, there is a natural
ridge, and before the construction of the subway sewers drained to the
East River and to the North River from the ridge. The route of the
subway was so near to the dividing line that the only way to care for
the sewers was, in many instances, to build entirely new outfall
sewers.
[Illustration: THREE-TRACK CONCRETE ARCH--117TH STREET AND BROADWAY]
A notable example of sewer diversion was at Canal Street, where the
flow of the sewer was carried into the East River instead of into the
Hudson River, permitting the sewer to be bulkheaded on the west side
and continued in use. On the east side a new main sewer was
constructed to empty into the East River. The new east-side sewer was
built off the route of the subway for over a mile. An interesting
feature in the construction was the work at Chatham Square, where a
6-1/2-foot circular brick conduit was built. The conjunction at this
point of numerous electric surface car lines, elevated railroad
pillars, and enormous vehicular street traffic, made it imperative
that the surface of the street should not be disturbed, and the sewer
was built by tunneling. This tunneling was through very fine running
sand and the section to be excavated was small. To meet these
conditions a novel method of construction was used. Interlocked
poling boards were employed to support the roof and were driven by
lever jacks, somewhat as a shield is driven in the shield system of
tunneling. The forward ends of the poling boards were supported by a
cantilever beam. The sides and front of the excavation were supported
by lagging boards laid flat against and over strips of canvas, which
were rolled down as the excavation progressed. The sewer was completed
and lined in lengths of from 1 foot to 4-1/2 feet, and at the maximum
rate of work about 12 feet of sewer were finished per week.
[Illustration: CONSTRUCTION OF FORT GEORGE TUNNEL]
At 110th Street and Lenox Avenue a 6-1/2-foot circular brick sewer
intersected the line of the subway at a level which necessitated its
removal or subdivision. The latter expedient was adopted, and three
42-inch cast-iron pipes were passed under the subway. (See photograph
on page 50.) At 149th Street and Railroad Avenue a sewer had to be
lowered below tide level in order to cross under the subway. To do
this two permanent inverted siphons were built of 48-inch cast-iron
pipe. Two were built in order that one might be used, while the other
could be shut off for cleaning, and they have proved very
satisfactory. This was the only instance where siphons were used. In
this connection it is worthy of note that the general changes referred
to gave to the city much better sewers as substitutes for the old
ones.
A number of interesting methods of providing for subsurface structures
are shown in photographs pages 51 to 54. From the General
Post-office at Park Row to 28th Street, just below the surface, there
is a system of pneumatic mail tubes for postal delivery. Of course,
absolutely no change in alignment could be permitted while these tubes
were in use carrying mail. It was necessary, therefore, to support
them very carefully. The slightest deviation in alignment would have
stopped the service.
[Illustration: TWO COLUMN BENT VIADUCT]
[Illustration: TRAVELER FOR ERECTING FORMS, CENTRAL PARK TUNNEL--(IN
THIS TUNNEL DUCTS ARE BUILT IN THE SIDEWALLS)]
[Sidenote: _Concrete-lined
Tunnel_]
Between 33d Street and 42d Street under Park Avenue, between 116th
Street and 120th Street under Broadway, between 157th Street and Fort
George under Broadway and Eleventh Avenue (the second longest
double-track rock tunnel in the United States, the Hoosac tunnel being
the only one of greater length), and between 104th Street and Broadway
under Central Park to Lenox Avenue, the road is in rock tunnel lined
with concrete. From 116th Street to 120th Street the tunnel is 37-1/2
feet wide, one of the widest concrete arches in the world. On the
section from Broadway and 103d Street to Lenox Avenue and 110th Street
under Central Park, a two-track subway was driven through micaceous
rock by taking out top headings and then two full-width benches. The
work was done from two shafts and one portal. All drilling for the
headings was done by an eight-hour night shift, using percussion
drills. The blasting was done early in the morning and the day gang
removed the spoil, which was hauled to the shafts and the portal in
cars drawn by mules. A large part of the rock was crushed for
concrete. The concrete floor was the first part of the lining to be
put in place. Rails were laid on it for a traveler having moulds
attached to its sides, against which the walls were built. A similar
traveler followed with the centering for the arch roof, a length of
about 50 feet being completed at one operation.
[Illustration: FOUR COLUMN (TOWER) VIADUCT CONSTRUCTION]
[Illustration: MANHATTAN VALLEY VIADUCT, LOOKING NORTH]
[Illustration: ERECTION OF ARCH, MANHATTAN VALLEY VIADUCT]
On the Park Avenue section from 34th Street to 41st Street two
separate double-track tunnels were driven below a double-track
electric railway tunnel, one on each side. The work was done from four
shafts, one at each end of each tunnel. At first, top headings were
employed at the north ends of both tunnels and at the south end of the
west tunnel; at the south end of the east tunnel a bottom heading was
used. Later, a bottom heading was also used at the south end of the
west tunnel. The rock was very irregular and treacherous in character,
and the strata inclined so as to make the danger of slips a serious
one. The two headings of the west tunnel met in February and those of
the east tunnel in March, 1902, and the widening of the tunnels to the
full section was immediately begun. Despite the adoption of every
precaution suggested by experience in such work, some disturbance of
the surface above the east tunnel resulted, and several house fronts
were damaged. The portion of the tunnel affected was bulkheaded at
each end, packed with rubble and grouted with Portland cement mortar
injected under pressure through pipes sunk from the street surface
above. When the interior was firm, the tunnel was redriven, using much
the same methods that are employed for tunnels through earth when the
arch lining is built before the central core, or dumpling of earth, is
removed. The work had to be done very slowly to prevent any further
settlement of the ground, and the completion of the widening of the
other parts of the tunnels also proceeded very slowly, because as soon
as the slip occurred a large amount of timbering was introduced, which
interfered seriously with the operations. After the lining was
completed, Portland cement grout was again injected under pressure,
through holes left in the roof, until further movement of the fill
overhead was absolutely prevented.
[Illustration: COMPLETED ARCH AT MANHATTAN STREET]
As has been said, the tunnel between 157th Street and Fort George is
the second longest two-track tunnel in the United States. It was built
in a remarkably short time, considering the fact that the work was
prosecuted from two portal headings and from two shafts. One shaft was
at 168th Street and the other at 181st Street, the work proceeding
both north and south from each shaft. The method employed for the work
(Photograph on page 56) was similar to that used under Central
Park. The shafts at 168th Street and at 181st Street were located at
those points so that they might be used for the permanent elevator
equipment for the stations at these streets. These stations each have
an arch span of about 50 feet, lined with brick.
[Sidenote: _Steel Viaduct_]
The elevated viaduct construction extends from 125th Street to 133d
Street and from Dyckman Street to Bailey Avenue on the western branch,
and from Brook and Westchester Avenues to Bronx Park on the eastern, a
total distance of about 5 miles. The three-track viaducts are carried
on two column bents where the rail is not more than 29 feet above the
ground level, and on four-column towers for higher structures. In the
latter case, the posts of a tower are 29 feet apart transversely and
20 or 25 feet longitudinally, as a rule, and the towers are from 70 to
90 feet apart on centers. The tops of the towers have X-bracing and
the connecting spans have two panels of intermediate vertical sway
bracing between the three pairs of longitudinal girders. In the low
viaducts, where there are no towers, every fourth panel has zigzag
lateral bracing in the two panels between the pairs of longitudinal
girders.
[Illustration: PROFILE OF HARLEM RIVER TUNNEL AND APPROACHES]
[Illustration: SECTION OF HARLEM RIVER TUNNEL DURING CONSTRUCTION]
[Illustration: ASSEMBLING IRON WORK ON PONTOON--HARLEM RIVER TUNNEL]
The towers have columns consisting as a rule of a 16 x 7/16-inch web
plate and four 6 x 4 x 5/8-inch bulb angles. The horizontal struts in
their cross-bracing are made of four 4 x 3-inch angles, latticed to
form an I-shaped cross-section. The X-bracing consists of single 5 x
3-1/2-inch angles. The tops of the columns have horizontal cap angles
on which are riveted the lower flanges of the transverse girders; the
end angles of the girder and the top of the column are also connected
by a riveted splice plate. The six longitudinal girders are
web-riveted to the transverse girders. The outside longitudinal girder
on each side of the viaduct has the same depth across the tower as in
the connecting span, but the four intermediate lines are not so deep
across the towers. In the single trestle bents the columns are the
same as those just described, but the diagonal bracing is replaced by
plate knee-braces.
The Manhattan Valley Viaduct on the West Side line, has a total length
of 2,174 feet. Its most important feature is a two-hinged arch of
168-1/2 feet span, which carries platforms shaded by canopies, but no
station buildings. The station is on the ground between the surface
railway tracks. Access to the platforms is obtained by means of
escalators. It has three lattice-girder two-hinge ribs 24-1/2 feet
apart on centers, the center line of each rib being a parabola. Each
half rib supports six spandrel posts carrying the roadway, the posts
being seated directly over vertical web members of the rib. The chords
of the ribs are 6 feet apart and of an H-section, having four 6 x
6-inch angles and six 15-inch flange and web plates for the center rib
and lighter sections for the outside ribs. The arch was erected
without false work.
[Illustration: SHOWING CONCRETE OVER IRON WORK--HARLEM RIVER TUNNEL]
The viaduct spans of either approach to the arch are 46 to 72 feet
long. All transverse girders are 31 feet 4 inches long, and have a 70
x 3/8-inch web plate and four 6 x 4-inch angles. The two outside
longitudinal girders of deck spans are 72 inches deep and the other 36
inches. All are 3/8-inch thick and their four flange angles vary in
size from 5 x 3-1/2 to 6 x 6 inches, and on the longest spans there
are flange plates. At each end of the viaduct there is a through span
with 90-inch web longitudinal girders.
Each track was proportioned for a dead load of 330 pounds per lineal
foot and a live load of 25,000 pounds per axle. The axle spacing in
the truck was 5 feet and the pairs of axles were alternately 27 and 9
feet apart. The traction load was taken at 20 per cent. of the live
load, and a wind pressure of 500 pounds per lineal foot was assumed
over the whole structure.
[Sidenote: _Tubes under
Harlem River_]
One of the most interesting sections of the work is that which
approaches and passes under the Harlem River, carrying the two tracks
of the East Side line. The War Department required a minimum depth of
20 feet in the river at low tide, which fixed the elevation of the
roof of the submerged part of the tunnel. This part of the line, 641
feet long, consists of twin single-track cast-iron cylinders 16 feet
in diameter enveloped in a large mass of concrete and lined with the
same material. The approach on either side is a double-track concrete
arched structure. The total length of the section is 1,500 feet.
The methods of construction employed were novel in subaqueous
tunneling and are partly shown on photographs on pages 62 and 63.
The bed of the Harlem River at the point of tunneling consists of mud,
silt, and sand, much of which was so nearly in a fluid condition that
it was removed by means of a jet. The maximum depth of excavation was
about 50 feet. Instead of employing the usual method of a shield and
compressed air at high pressure, a much speedier device was contrived.
The river crossing has been built in two sections. The west section
was first built, the War Department having forbidden the closing of
more than half the river at one time. A trench was dredged over the
line of the tunnel about 50 feet wide and 39 feet below low water.
This depth was about 10 feet above the sub-grade of the tunnel. Three
rows of piles were next driven on each side of the trench from the
west bank to the middle of the river and on them working platforms
were built, forming two wharves 38 feet apart in the clear. Piles were
then driven over the area to be covered by the subway, 6 feet 4 inches
apart laterally and 8 feet longitudinally. They were cut off about 11
feet above the center line of each tube and capped with timbers 12
inches square. A thoroughly-trussed framework was then floated over
the piles and sunk on them. The trusses were spaced so as to come
between each transverse row of piles and were connected by eight
longitudinal sticks or stringers, two at the top and two at the bottom
on each side. The four at each side were just far enough apart to
allow a special tongue and grooved 12-inch sheet piling to be driven
between them. This sheathing was driven to a depth of 10 to 15 feet
below the bottom of the finished tunnel.
A well-calked roof of three courses of 12-inch timbers, separated by
2-inch plank, was then floated over the piles and sunk. It had three
timber shafts 7 x 17 feet in plan, and when it was in place and
covered with earth it formed the top of a caisson with the sheet
piling on the sides and ends, the latter being driven after the roof
was in place. The excavation below this caisson was made under air
pressure, part of the material being blown out by water jets and the
remainder removed through the airlocks in the shafts. When the
excavation was completed, the piles were temporarily braced and the
concrete and cast-iron lining put in place, the piles being cut off as
the concrete bed was laid up to them.
The second or eastern section of this crossing was carried on by a
modification of the plan just mentioned. Instead of using a temporary
timber roof on the side walls, the permanent iron and concrete upper
half of the tunnels was employed as a roof for the caisson. The trench
was dredged nearly to sub-grade and its sides provided with wharves as
before, running out to the completed half of the work. The permanent
foundation piles were then driven and a timber frame sunk over them to
serve as a guide for the 12-inch sheet piling around the site. Steel
pilot piles with water jets were driven in advance of the wood-sheet
piles, and if they struck any boulders the latter were drilled and
blasted. The steel piles were withdrawn by a six-part tackle and
hoisting engine, and then the wooden piles driven in their place.
When the piling was finished, a pontoon 35 feet wide, 106 feet long,
and 12 feet deep was built between the wharves, and upon a separate
platform or deck on it the upper half of the cast-iron shells were
assembled, their ends closed by steel-plate diaphragms and the whole
covered with concrete. The pontoon was then submerged several feet,
parted at its center, and each half drawn out endwise from beneath the
floating top of the tunnel. The latter was then loaded and carefully
sunk into place, the connection with the shore section being made by
a diver, who entered the roof through a special opening. When it was
finally in place, men entered through the shore section and cut away
the wood bottom, thus completing the caisson so that work could
proceed below it as before. Three of these caissons were required to
complete the east end of the crossing.
[Illustration: LOOKING UP BROADWAY FROM TRINITY CHURCH--SHOWING
WORKING PLATFORM AND GAS MAINS TEMPORARILY SUPPORTED OVERHEAD]
The construction of the approaches to the tunnel was carried out
between heavy sheet piling. The excavation was over 40 feet deep in
places and very wet, and the success of the work was largely due to
the care taken in driving the 12-inch sheet piling.
[Sidenote: _Methods of
Construction
Brooklyn
Extension_]
A number of interesting features should be noted in the methods of
construction adopted on the Brooklyn Extension.
The types of construction on the Brooklyn Extension have already been
spoken of. They are (1) typical flat-roof steel beam subway from the
Post-office, Manhattan, to Bowling Green; (2) reinforced concrete
typical subway in Battery Park, Manhattan, and from Clinton Street to
the terminus, in Brooklyn; (3) two single track cast-iron-lined
tubular tunnels from Battery Park, under the East River, and under
Joralemon Street to Clinton Street, Brooklyn.
Under Broadway, Manhattan, the work is through sand, the vehicular
and electric street car traffic, the network of subsurface structures,
and the high buildings making this one of the most difficult portions
of the road to build. The street traffic is so great that it was
decided that during the daytime the surface of the street should be
maintained in a condition suitable for ordinary traffic. This was
accomplished by making openings in the sidewalk near the curb, at two
points, and erecting temporary working platforms over the street 16
feet from the surface. The excavations are made by the ordinary drift
and tunnel method. The excavated material is hoisted from the openings
to the platforms and passed through chutes to wagons. On the street
surface, over and in advance of the excavations, temporary plank decks
are placed and maintained during the drifting and tunneling
operations, and after the permanent subway structure has been erected
up to the time when the street surface is permanently restored. The
roof of the subway is about 5 feet from the surface of the street,
which has made it necessary to care for the gas and water mains. This
has been done by carrying the mains on temporary trestle structures
over the sidewalks. The mains will be restored to their former
position when the subway structure is complete.
From Bowling Green, south along Broadway, State Street and in Battery
Park, where the subway is of reinforced concrete construction, the
"open cut and cover" method is employed, the elevated and surface
railroad structures being temporarily supported by wooden and steel
trusses and finally supported by permanent foundations resting on the
subway roof. From Battery Place, south along the loop work, the
greater portion of the excavation is made below mean high-water level,
and necessitates the use of heavy tongue and grooved sheeting and the
operation of two centrifugal pumps, day and night.
The tubes under the East River, including the approaches, are each
6,544 feet in length. The tunnel consists of two cast-iron tubes
15-1/2 feet diameter inside, the lining being constructed of cast-iron
plates, circular in shape, bolted together and reinforced by grouting
outside of the plates and beton filling on the inside to the depth of
the flanges. The tubes are being constructed under air pressure
through solid rock from the Manhattan side to the middle of the East
River by the ordinary rock tunnel drift method, and on the Brooklyn
side through sand and silt by the use of hydraulic shields. Four
shields have been installed, weighing 51 tons each. They are driven by
hydraulic pressure of about 2,000 tons. The two shields drifting to
the center of the river from Garden Place are in water-bearing sand
and are operated under air pressure. The river tubes are on a 3.1 per
cent. grade and in the center of the river will reach the deepest
point, about 94 feet below mean high-water level.
The typical subway of reinforced concrete from Clinton Street to the
Flatbush Avenue terminus is being constructed by the method commonly
used on the Manhattan-Bronx route. From Borough Hall to the terminus
the route of the subway is directly below an elevated railway
structure, which is temporarily supported by timber bracing, having
its bearing on the street surface and the tunnel timbers. The
permanent support will be masonry piers built upon the roof of the
subway structure. Along this portion of the route are street surface
electric roads, but they are operated by overhead trolley and the
tracks are laid on ordinary ties. It has, therefore, been much less
difficult to care for them during the construction of the subway. Work
is being prosecuted on the Brooklyn Extension day and night, and in
Brooklyn the excavation is made much more rapidly by employing the
street surface trolley roads to remove the excavated material. Spur
tracks have been built and flat cars are used, much of the removal
being done at night.