Chapter 14: Engineering Features of the New York Subway
From nycsubway.org
Rapid Transit in New York City and in the Other Great Cities · Chamber of Commerce, 1906
Kiosk, N. Y. Subway.
[* Most of the information in this chapter is from the reports issued by the Rapid Transit Commission.] Before the letting of the contract for the construction of the subway, the Board of Rapid Transit Commissioners had prepared complete plans for a railroad, having Broadway from Forty-second street to the Battery as a portion of the route. The adverse decision of the Appellate Division of the Supreme Court prevented the use of that thoroughfare. The preliminary investigations covered the underground conditions of all that portion of the city traversed by the route in regard to soil, foundations, pipes, sewers, and other sub-surface structures. As a result of these studies the Board reached the following general conclusions:
Conclusions of Board. First, that, in order to relieve the congestion of travel, there was needed a railway located either directly along or as near as possible to the major lines of travel.
Second, that, in order to bring the extreme limits of the city into closer relations, provision must be made for the running of trains at higher speed than was possible on any existing elevated railway in New York, or in fact on any intra-urban railway in any other city.
Third, that underground construction should only be considered for those portions of the route along important thoroughfares; and,
Fourth, that a route through private property in the lower portion of the city was neither feasible nor economical.
It was evident that these conclusions could only be secured by the adoption of a route near to and parallel with Broadway; that in general it must follow street lines; and that it must be an underground road of four tracks.
Shallow Excavation Type. The subway is, as far as possible, of the shallow-excavation type; that is, the rail level is as close to the surface of the street as grades and local conditions permit. The design required a flat roof in order to avoid the loss of headroom of an arch, and a re-adjustment of all sewers, pipes, and the like underground structures. This type had been adopted by the Glasgow Central Railway, the Rapid Transit Commission of 1891, the Boston Transit Commission, and in Budapest, Hungary. Heretofore railways of this kind have provided a single service, with all trains stopping at all stations, a limited express service being sometimes obtained by a third track, on which trains could run in the direction of the heaviest travel, and stop at longer intervals than the others. The disadvantages of this system were obvious, and the Board decided that the subway should represent a step in advance. It was therefore determined to construct four tracks over that part of the route where the traffic was greatest, and two and three tracks along the remainder.
Elm Street. In 1897, when the court decided against the Broadway line, the proceedings relating to the widening of Elm street had been finished, and that route suggested itself as an alternative. In January and February, 1897, plans were adopted by the Board. In order that the cost might not exceed $35,000,000, the southern terminus was fixed at the Post Office, and four tracks were laid along Park Row, Center, and Elm streets, Lafayette place, Fourth avenue, Forty-second street, and Broadway to One-hundred-and-fourth street. The line divided at this point, the west side extending to Kingsbridge and the east side to Bronx Park. The route having been decided upon, an investigation was made of the topographical and geological features, the foundations of buildings, sewers, water and gas pipes, conduits, etc. South of Astor place the soil was for the most part coarse sand; to the north it was gneiss rock and gravel. The most serious part of the problem was the handling of the different underground structures. The sewerage system of New York is the "combined" system, by which both house drainage and rainfall are carried away. These, together with the sewers, pipes, and conduits, were almost all near the surface. All of these had to be kept in service and most of them moved to new locations without interruption of their operation.
Routes. While the larger portion of the route was placed practically parallel with the street surface, this was not possible in some localities. In order to avoid interference with the Fourth avenue tunnel of the Metropolitan Railway, the subway was divided from Thirty-third street north into two two-track tunnels, one at each side of, but below the street-car tunnel. In order to overcome depressions, viaducts were adopted on the west side between One-hundred-and-twenty-second street and One-hundred-and-thirty-fifth street and north of Fort George, and on the east side north of Third avenue. The length of each type of construction was as follows: Cut-and-cover, 10.46 miles; tunnel, 4.55 miles; viaduct, 5.8 miles.
Type of Tunnel. The allowed limit of clearance between the street surface and the top of the subway was 30 inches, that being the depth of the yokes of the conduits of the electric railways. The roof was made as thin as possible. To further this aim columns were introduced between each track, so that the roof beams required were only heavy enough to span a single track, an arrangement that would be economical and that, by making the individual members smaller, would facilitate construction. The standard type was a rectangular tunnel consisting of a concrete floor with steel ribs set five feet apart, with arches turned between them. A bed of concrete was first laid down, and on that thin side walls of hollow brick to a height of several feet. On the floor and against the walls was laid a waterproofing of alternate layers of felt and asphalt. On top of this was spread another course of concrete, and on the latter were set the foundations for the center columns and walls. A double row of terra-cotta ducts was placed against the walls, a hollow brick outer wall with the waterproofing being carried up in advance. Then the steel frames were erected, the jack arches turned, and the waterproofing spread over the roof, over which was laid a protecting layer of concrete. The waterproofing was thus protected from outside damage by the thin guard-walls of brick and the top layer of concrete. An admirable feature of the design was that it could be constructed in sections for either the full or part width, with the certainty that the several sections would fit together, the connections between the rigid members being made of plastic and easily molded concrete.
The sections of the tunnels, with the exception of the Murray Hill tunnel, which was a three-center arch designed in order to lower the roof, were semicircular. The same section was used in certain deep cuttings in the two-track lines, where the space above the roof permitted an arch to be constructed. In these instances an arch was found more economical than steel frames.
18th Street Station-N. Y. Subway.
Fulton Street Entrance-N. Y. Subway.
Standard Construction. In the standard construction the center columns were made up of 4 bulb angles, 3 by 4 inches, by 10 pounds per foot, and one web plate 6 by 1/4 inches. These were spaced 5 feet longitudinally of the subway and 12 feet 6 inches transversely, making the total width of the four-track tunnel 50 feet. The roof beams were 42, 60 and 70 pounds, according to the location. The Park avenue tunnels were 24 feet in width by 18 feet in center height, the tracks being 12 feet center to center. The deep tunnels were 25 feet wide by 18 high. In its general features the steel viaduct was similar to other elevated railways, except that it was much stronger in order to provide for the weight of motor cars of 50 tons.
Sewers. As the sewers in the city are commonly placed at a depth of about 13 feet, and as the excavation for the tunnel was to be over 18 feet at the minimum, it followed that a complete reconstruction of the system was necessary, involving the building of 7.21 miles of sewers along the route of the railway, and 5.13 miles of sewers in streets other than that followed by the route. Where sewers were encountered along the route, the method was to build two new sewers, one at each side of the railway and next to the abutting houses, and to diminish to the minimum all cross-connections either over or under the railway. Where they crossed the route they were gathered together and passed beneath the railway in iron pipes, and a new outfall sewer built from the lower end of the cross-connection on a new gradient to such a point as was rendered necessary by the topography of the street to make a new connection with the existing system. This did away with all siphons, with one exception, and left the sewers in a self-cleansing condition. As the sewer at Canal street was below tidewater, such a scheme was not possible. In that case a new route was selected for a new sewer, and the flow diverted to the East River instead of the Hudson, as formerly.
Gas and Water Pipes. Taking care of the pipes was a troublesome task. In general, the small ones, those of 12 inches in diameter and under, were placed on top of the roof, but the large mains were moved to the side of the subway wherever there was not sufficient space on top. At cross streets where the distance between the pavement and roof was insufficient to allow the longitudinal and lateral mains to cross each other, additional space was obtained by constructing a flat metal trough between adjacent roof beams and laying the lateral mains in it. When this space was not sufficient, the large mains were sub-divided into smaller pipes, equal in capacity to the large one. The bottom of the trough was made of 3-inch beams resting on the flanges of the roof beams, with concrete between. In extreme cases where every inch had to be saved a steel plate was set flush with the bottom flanges of the roof beams and supported by angles along the edges.
Almost all of the sewer and pipe work was preliminary to the building of the subway. The reconstruction was accomplished without interrupting the flow of any house connection, catch basin, or other sewer.
Method of Working. Under Park row it had been planned to permit express trains to continue along a possible Broadway extension south, or to be switched back through a tail track. A station on the loop was located in City Hall Park, so as to be conveniently reached from all points, and thereby relieve the pressure at the Brooklyn Bridge station. The construction was carried forward by digging a trench on either side of the four surface tracks (electric conduit) down to grade. Cross drifts were then tunneled beneath the tracks connecting the trenches, and in these drifts timber supports were erected beneath the surface tracks. Then the intervening pillars of unexcavated sand were removed and the roof supported by timber props. The concrete floor and walls were then put in and the steel erected. This method was followed with but slight changes in all localities where similar material was encountered. As Elm street had not been officially opened for traffic, the sub-contractor availed himself of permission to take the whole width of the street and excavate for the entire structure.
Open Cut Through Rock. The section from Lafayette place to Thirty-third street presented considerable difficulty, owing to the presence of rock which in some places came directly beneath the yokes of the electric railway. At first the attempt was made to excavate half the street at a time, confining the traffic to the other half. It was found that this produced almost as much interference with traffic as the building of two railways. The sub-contractors were then permitted to excavate for the full width of the four tracks, supporting the surface tracks over the cut, concentrating the vehicular traffic on the same space and cutting off for the time being access to the abutting houses of each block. Arrangements were made to bridge the excavation at important buildings, and to truck merchandise by hand north or south to the nearest cross street. In order to support the surface railway the contractors placed on the outside of the track and immediately at the side of the excavation a pair of 24-inch rolled beams 40 feet long, and a similar pair in the trench between the surface railway tracks, this trench being afterward roofed over with planks. The ends of the beams were supported on wooden trestles. As the excavation progressed, heavy timber cross beams were inserted transversely beneath the surface tracks, and held by rods from the longitudinal beams. With this arrangement it was possible to remove all the earth beneath the railway and thus leave the whole space from the curb to a point beyond the center line of the street free for the construction of the subway. The width thus secured was sufficient to put in place the center row of columns. Water mains, gas pipes, and electric conduits in the excavation were supported by chains from cross timbers. To remove the excavated material, an overhead cableway on towers was erected longitudinally of the cut at each opening; or, where rock was found in quantities, a derrick was set up. Buckets on the cableway carried the soil forward to the end of the cut and there dropped it into carts. The appliances necessary for handling material took up no more room than the width of the excavation itself.
Work at Union Square. At Union Square, where the surface road was laid directly on the rock, it was decided to move the surface tracks to a new location near the easterly curb, in order to avoid the possibility of injuring them during blasting. Sufficient rock was blasted out for the south bound local and express tracks, and for the intermediate side track to be constructed at this point. The ducts of the street railway, which had to be kept in service in connection with the tracks themselves, contained not only the ordinary low tension feeders, but also high tension cables having a pressure of 6,500 volts each. Although in many cases the rock had to be blasted in direct contact with these ducts, in no instance were the cables broken or the service of the road interrupted.
Except for a short distance at its southern end, the length known as Section 4 was wholly in tunnel. It was impossible to build the four tracks of the subway in one tunnel, for the reason that if this had been tried the arched roof would have interfered with the old Harlem tunnel used by the Metropolitan Railway. Two double-track tunnels were built instead. These were separated a sufficient distance to leave a core of rock between them and directly under the old tunnel. In order that these tunnels might be kept as far away from the upper tunnel as possible, the roof section was designed as a three-center arch. The common shaft method of tunneling was here followed, but the same method of driving was not employed in the two headings. Where the rock was hard and compact enough to be self-supporting a top heading was advanced; in soft, disintegrated rock an upper heading and timbering were used.
N. Y. Subway; 104th Street Junction; Park Ave. and 41st Street; Broadway and Columbia University.
Stations, N. Y. Subway; Bleecker Street; Spring Street; Grand Central Express.
Forty-Second Street. The section from Forty-first to Forty-seventh street presented some unusual features. In order that the travel on Forty-second street, at all times very heavy, might be interfered with as little as possible, the operations were first confined to the south side of the street. In a trench 15 feet wide the steel work for the south bound local track was erected. A drift was then opened north-- across the street-- for a distance of 20 feet and needle beams, consisting of 24-inch 100-pound steel beams, were placed in it, one end resting on the completed roof and the other on the undisturbed rock. The street was supported on these beams by blocking. Beneath the beams the rock was excavated and the southbound express track built.
Rock Work. The section from Forty-seventh street to Sixtieth street and Broadway was almost wholly in rock. The work was done with cableways similar to those used for Fourth avenue. The excavation was confined to the space between the curb and nearest rail of the surface railroad, subsequently drifting under the latter to a point just beyond the center of the street, in which space were erected the side, quarter, and center columns and the roof beams for one-half the subway structure. After the concrete arches had been put in, construction was extended under the remaining half of the street.
Supporting Columbus Monument. At the junction of Broadway, Eighth avenue and Fifty-ninth street is a shaft monument to Christopher Columbus, having a height of 75 feet and a masonry base stepped out in the usual way. The westerly line of the subway excavation passed beneath this base just to the east of the center line of the shaft. It was decided to support the shaft and its base by underpinning. In order to do this the first step was the driving of a tunnel beneath the center of the shaft and to the west of the subway wall; this was filled with masonry. Under the eastern edge of the base was then placed a large girder supported on timber bents, north and south of the monument. The material beneath the base was then dug out, the subway structure built in place, and on top of the subway roof new foundations for the monument were carried up. When this work had been finished the girder was removed.
Broadway, from Sixtieth to One-hundred-and-fourth street, is 102 feet wide between the curbs, and midway throughout is a line of parkways 22 feet wide, on each side of which is a conduit electric railway. The work here was done through the parkways in open cut, under through trusses that had been set on the surface, one on each side of each track. These upheld the conduit while the work was carried on beneath.
The section to One-hundred-and-tenth street was almost entirely in deep tunnel. Work was prosecuted by means of shafts.
The methods of construction above mentioned cover the work of the entire route except the viaduct and Harlem River portions. The former, as has already been stated, differed very little from the ordinary elevated design, except that it was made heavier. This type was departed from at Manhattan street, where a two-hinged steel arch with a span of 180 feet was erected.
Viaduct at Manhattan (125th) Street, N. Y. Subway.
Harlem River Tunnel. The Harlem River tunnel consists of two single-track tubes lined with cast-iron and separated by a vertical partition. The approaches were built in open cut. That portion of the tunnel under the river was constructed by an entirely new method, designed and successfully executed by D. D. McBean, member of the firm that contracted for this section. The Government required that the river be kept open for navigation, but permitted it to be temporarily narrowed. The western half of the tunnel was built in the following way:
N. Y. Subway; Harlem Tunnel—half Section; Typical Underground Construction.
New Method of Submarine Tunneling. A channel was dredged across the river bottom to within a few feet of the full depth of the excavation required for the tunnel. In this channel foundation piles, and a row of specially prepared heavy timber sheeting were driven along each side and across the ends, and cut off to a true plane about 25 feet below the surface of the water. The roof of this chamber was formed of a platform of timber 40 inches in thickness and extending the full width and length of the tunnel section, which was sunk until it rested upon the top of the cut-off sheeting. Simultaneously with pumping the water from under the roof, compressed air was forced into the chamber under a pressure corresponding to the hydrostatic head, or depth of water above the roof. Inside this chamber the west half of the tunnel was built, and then the timber roof was removed.
Harlem River Section N. Y. Subway; Joining Arch and Tubes; Constructing Tubes.
Another simpler and cheaper method was pursued in constructing the easterly half of the tunnel. The sides of the pneumatic working chamber were prepared in the same way, but the sheeting was cut off about 12 feet lower down than in the first case, or exactly on the same level as the spring line of the arch of the tunnel. The top half of the tunnel proper was then built on pontoons which were floated over the tunnel site. The upper half of the tunnel was then lowered until flanges, which had been built upon its sides, rested upon the sheeting. This formed the roof of the working chamber. The foundation and bottom half of the tunnel were then constructed with the aid of compressed air.
Contract No. 2. The Brooklyn extension of the road, known as "Contract No. 2," extends from the Post Office south under Broadway to the Battery, thence under the East River to Joralemon and Fulton streets, and under the latter and Flatbush avenue to the junction of Atlantic and Flatbush avenues. At Bowling Green there is a spur to South Ferry and a loop. On the portion under Broadway it was at first intended to remove the street pavement, and put in a carefully planked roadway. Under this, excavation was to be made from two shafts, one in front of St. Paul's Church yard and the other in front of Trinity Church yard, where their location would not interfere with abutting buildings. It was not deemed wise to keep the gas mains under this roadway, since any leakage of gas, mixing with the confined air, might cause serious explosions. Arrangement was made with the gas company to lay two temporary mains on trestles over the sidewalks the length of the work. At the suggestion of the chief engineer, the contractor made the experiment of removing the soil beneath the pipes and pavement without disturbing the latter, supporting the pipes and pavement on a properly designed system of timbering, and thus use the existing pavement as a temporary cover in lieu of a plank roadway. This scheme worked so well that it was followed on the entire line south of St. Paul's shaft.
Tunneling Under East River. The width of the East River on the line of the crossing between bulkhead lines is 4,150 feet, with a depth at high water of 47 feet. The War Department required a depth of water above the top of the tunnel at low tide of at least 45 feet. This, taken in connection with the irregularities and varying materials of the bed of the river, together with the great volume of river traffic, forced the abandonment of the idea of building the tunnel in the open and floating it into place, and it was decided to do the work by means of shields and compressed air. Two double shafts were sunk, one within the Battery Park loop and the other in Joralemon street, near Henry, from which headings were started for the two tubes. The space between the outside of the shell and the rock was filled with broken stone and cement grout. This section is now nearing completion.
East River Tunnel During Construction-N. Y. Subway.
East River Tunnel— N. Y. Subway.
Station Design. From an operating point of view the stations have been designed in two general classes, local and express. This was accomplished by constructing four tracks from the Post Office to Ninety-sixth street, with two and three tracks in places above that point on both the east and west side lines. The express stations are the Brooklyn Bridge, Fourteenth street, Grand Central, Seventy-second and Ninety-sixth streets, at which stations all trains stop. These stations are located about 1-1/2 miles apart. The other stops, at intervals of one-quarter mile, serve only the local trains. The platforms of all local stations south of Ninety-sixth street are 200 feet long; the platforms of the express stations are 350 feet long. Above Ninety-sixth street all stations have platforms of the longer length. Local stations usually have separate platforms, from which the passengers enter or leave the north or south-bound trains. These are located at the outside of the tracks, and in most cases with no provision for crossing from one platform to the other. At two stations, however, Astor place and Forty-second street and Broadway, underground passageways have been provided, and at five others, One-hundred-and-third, Columbia University, One-hundred-and-sixty-eighth, One-hundred-and-eighty-first and Mott avenue, there are bridges beneath the surface of the street but over the tracks.
Type of Stations. In plan there are five types of local stations. The first includes those from Fiftieth street south. This type has two platforms, one on each side of the street, and made of as great a width as the width of the street permitted, extending from the side of the tracks to the area line of building, which is five feet from the building line. As far as possible these platforms are arranged symmetrically on either side of the cross street where the station is situated, a portion of the cross street being excavated to accommodate the waiting room, in which the ticket office, toilet rooms and service closets are located.
In order to provide the fullest possible facilities for reaching the platforms, each station is provided with eight stairways, four for each platform, located in pairs on the north and south sides of the cross streets. One of these stairways is an entrance and the other an exit. The entrance stairway descends from the sidewalk and reaches the rear of the waiting room where the ticket office is situated. The passenger descends the stairway, buys his ticket and goes forward to the platform. moving always in a straight line and without reversing the direction. The exit stairways lead from the back of the platform directly to the sidewalk. Passengers approach these exit stairways without passing the ticket office or meeting the incoming line of traffic. This stairway accommodation is much in excess of any similar accommodation provided in any station of the elevated railroad.
Second Type. The second type is represented in the local stations on Broadway north of Sixtieth street. In that part of the city Broadway is very wide and the platforms do not come beneath the sidewalks. As the congestion is not so much as in the commercial districts, the platforms are reached by one wide staircase. These, however, can readily be doubled as traffic develops.
Another type is on Lenox avenue from One-hundred-and-sixteenth to One-hundred-and-thirty-fifth street inclusive. Along this section the axis of the subway does not coincide with the center line of the street; so that the west platform is beneath the sidewalk while the other is under the roadway. As these stations are in the commercial districts of Harlem, more stairways have been provided than for the stations on upper Broadway, each platform being provided with two wide stairways.
Deep Tunnel Stations. The deep tunnel stations, at One-hundred-and-sixty-eighth street, One-hundred-and-eighty-first street and at Mott avenue, consist in each case of a wide arch spanning both tracks and the two platforms, access to the platforms being had by shafts, 98, 120, and 46 feet deep, respectively. Each shaft contains a stairway and two elevators, the latter having a capacity of 3,500 passengers per hour. The waiting room containing the ticket office is immediately beneath the sidewalk, and to this a short stairway leads. From the waiting room the elevators descend to a bridge crossing the tracks so that access can be had from this overhead passageway to either platform.
The principal features of the express stations are two large island platforms situated between the express and local tracks. At the Brooklyn Bridge, Fourteenth street and the Grand Central stations, access is had to these platforms by overhead bridges above the tracks but beneath the street surface. The island platforms of the Seventy-second street station are approached by stairways descending from an ornamental house in the center of Broadway. The platforms of the Ninety-sixth street station are reached by a passageway beneath the tracks. At these stations the island platforms serve either the local or express trains.
At the Bridge, Fourteenth, and Ninety-sixth streets, side platforms have been constructed in addition to the island ones; so that at these stations passengers going to or coming from the local trains will not be obliged to go to the island platforms and come in contact with those using the express trains. At the other two stations the width of the street would not permit the erection of such additional platforms.
The stations on the viaduct portion of the road are of the covered type. That at Manhattan Valley has three tracks with two platforms. As these are 55 feet above the street a double moving stairway has been installed. This leads from a house in which are the ticket windows and waiting rooms, and thence to a platform directly beneath the tracks and connected by a short stairway with both platforms.
City Hall Station, Platform and Mezzanine, N. Y. Subway.
Twenty-Third Street Station- N. Y. Subway.
Stations Near the Surface. All of the stations have been constructed as near the street surface as possible. This made two things possible; it permitted the stations to be supplied with natural light in the daytime, and reduced the length of the stairways to a minimum. Wherever the platforms are beneath sidewalks, the sidewalks are made of glass. Of the 37 subway stations, 20 are so well provided with natural light that very little artificial illumination is required during daylight hours. The length of stairways is not much more than one-half of the average length of stairways leading to elevated stations.
Lighting. In general the lights in the stations are incandescent lamps placed in recesses in the ceilings. Current for these is obtained from two sources, a regular lighting circuit and the track circuit. The latter is for use only in cases of emergency. All lights are controlled from a switchboard in the ticket booth.
The stations are made damp-proof by an inner and outer shell. The sidewalls and ceilings of the outer shell are built of steel columns and beams with concrete filled between them, and a layer of concrete forms the foundation of the floor. Water-proofing, protected by an outer lining of masonry, envelops the entire structure. This forms the constructive work of the stations, as distinguished from the finishing work of the inner shell which includes the sidewalls, ceilings and floors. The floors are all alike, and are made of concrete divided into plaques 3 feet square. The floors are graded so as to drain to catch basins which are connected with the sewer.
Design of Stations. The base of the walls at stations is built of a hard buff-colored brick, forming a double wainscot that extends around the whole platform 21 feet high. This wall is set back from the face of the brick to allow for a finish of glass or of glazed tile. At the top of the tile a cornice of faience or terra cotta is built into the wall, and at intervals of 15 feet, or opposite the platform columns, the cornice line is broken by an ornamental symbol designating the name of the station. At certain intervals a large tablet, consisting of a dark background of glazed mosaic, carries the name of the station in white mosaic, or gold letters or distinct figures. The finished floors and walls are joined with a sanitary cove base, so that corners where dust might lodge are avoided. The materials used in the wall treatment between the cornice and wainscot are glass, or glazed tile or ceramic-tile mosaic. The glass or glazed tile is 3 by 6 inches and covers most of the surface. The ceramic mosaic work is used for ornamentation. It is made up into narrow bands of single colors, ornamental friezes, pilasters and name tablets. Under each cornice plaque there is usually a pilaster design separating the wall into panels about 15 feet long. The panels are bordered with mosaic bands and friezes.
There are two kinds of ceilings; one is a flat suspended ceiling which covers all the steel and concrete in the roof; the other is arched and is also suspended, but only between the roof beams. Ornamental moldings are used to divide the ceiling into panels. Heat is furnished to all stations by electric heaters.
At appropriate stations suggestive designs have been worked into the walls. Thus there is the "Caravel" at Columbus Circle; the "Beaver" at Astor place, and the Arms of Columbia University at the Columbia University station.
Elevated Structure-N. Y. Subway.
Typical Four-Track Construction. Spring St. Station-Five Tracks. N. Y. Subway.