[Ill.u.s.tration: DUCTS IN SIDE WALLS--EIGHT ONLY OF THE SIXTEEN LAYERS ARE SHOWN]

[Ill.u.s.tration: REINFORCED CONCRETE CONSTRUCTION]

[Ill.u.s.tration: ROOF SHOWING CONCRETE-STEEL CONSTRUCTION--LENOX AVENUE AND 140TH-141ST STREETS]

[Ill.u.s.tration: SECTION OF SUBWAY AT PEARL STREET This construction was made necessary by encountering a layer of Peat resting on Clay]

[Ill.u.s.tration: SURFACE RAILWAY TRACKS SUPPORTED OVER EXCAVATION ON UPPER BROADWAY]

[Ill.u.s.tration: 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 sh.o.r.ed up temporarily, and in other places the subway pa.s.ses 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. s.p.a.ce 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.

[Ill.u.s.tration: 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 s.p.a.ce 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.

[Ill.u.s.tration: 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 pa.s.s 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 subst.i.tute 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: MOVING WEST SIDE WALL TO WIDEN SUBWAY FOR THIRD TRACK--135TH STREET AND BROADWAY]

[Ill.u.s.tration: SUBWAY THROUGH NEW "TIMES" BUILDING, SHOWING INDEPENDENT CONSTRUCTION--THE WORKMEN STAND ON FLOOR GIRDERS OF SUBWAY]

[Ill.u.s.tration: COLUMNS OF HOTEL BELMONT, Pa.s.sING 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.)

[Ill.u.s.tration: SMALL WATER MAINS BETWEEN STREET SURFACE AND SUBWAY ROOF, SUBSt.i.tUTED FOR ONE LARGE MAIN--125TH STREET AND LENOX AVE.]

[Ill.u.s.tration: 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 pa.s.ses 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.

[Ill.u.s.tration: THREE PIPES SUBSt.i.tUTED FOR LARGE BRICK SEWER AT 110TH STREET AND LENOX AVENUE]

[Ill.u.s.tration: SEWER SIPHON AT 149TH STREET AND RAILROAD AVENUE]

[Ill.u.s.tration: 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.)

[Ill.u.s.tration: CONCRETE SEWER BACK OF SIDE WALL, BROADWAY AND 56TH STREET]

[Ill.u.s.tration: 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 bas.e.m.e.nt 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 pa.s.sing through the subway station.

[Ill.u.s.tration: DIFFICULT PIPE WORK--BROADWAY AND 70TH STREET]

At 42d Street and Park Avenue the road pa.s.ses 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 f.l.a.n.g.es 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 f.l.a.n.g.es 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: THREE-TRACK CONCRETE ARCH--117TH STREET AND BROADWAY]

A notable example of sewer diversion was at Ca.n.a.l 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.

[Ill.u.s.tration: 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 pa.s.sed 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 subst.i.tutes 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.

[Ill.u.s.tration: TWO COLUMN BENT VIADUCT]

[Ill.u.s.tration: 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.