The total cost of New York's Dual Subway System, including equipment, real estate, power house construction and changes, interest during construction, etc., will be about $400,000,000. Of this sum about $60,000,000 is contributed by the Brooklyn Company, about $146,000,000 by the Interborough, and the remainder, ap proximately $194,000,000, by the City of New York.
The general type of subway adopted for New York was of the fiat roof shallow form, bringing the rail and platform level as close to the surface of the street as possible. The ir regular topography of the city, however, has compelled a departure from this type in several instances, so that a portion of the road is in ordinary deep tunnel, and the portion beneath the East River, and the approaches thereto, is of the tubular type, as adopted in London.
While this work of subway construction had been going on in other cities, London had made many additions to its subway construction by building detached and separately operated lines on the tubular principle, there having been a decided popular prejudice against any form of construction that would threaten to interfere with surface travel during construction, or that would invade the vested rights of the several local governments, which form the constituent parts of the government of the Metropolitan district, or what is commonly known as London.
The construction of sub ways presents a wide field for the display of engineering skill and ingenuity, as a modern line demands the application of nearly all the principles on which engineering science is based. At the time when the first railroad was built in London, iron was expensive, and steel, as a ma terial of construction, unknown, and recourse was had to masonry, except in cases where the use of iron girders was obligatory. The struc ture was, therefore, a continuous archway, and calls for no extended description.
When the City and South London Railway was begun, cast iron was sufficiently cheap to be used. At that time the building of a line so near the surface as to disturb it, even tem porarily, was out of the question, owing to the opposition of various interests. Mr. Greathead therefore decided to make a deep tunnel, which he built by means of a such as is com monly employed in deep turned work in soft soil.
The advantage attending tubular construction is that it can be carried on without the ruption of surface traffic, but the disadvantages are that elevators must be installed to carry passengers between the platform and street sur face, a distance varying usually from 50 to 100 feet, with the attending cost and delay, and the greater difficulty of properly ventilating the deep tubes. On the other hand, a subway built close to the surface of the street, whether by tunneling or in open excavation, introduces many complex difficulties, as there are involved the interference with street travel, and the care and at times extensive reconstruction of other sub-surface structures, such as sewers, pipes or electric conduits.
In Boston these difficulties were avoided in some instances by tunneling with the aid of a semi-circular shield, and elsewhere reduced, as a rule, by keeping the roof of the subway suf ficiently low to pass beneath the water and gas mains, so that they were left in their previous positions. When tunneling methods were not employed, surface travel was maintained by permitting the contractors to break the con tinuity of the surface during the night only. The first step was to make an excavation across the street and with .a length of about 12 feet. This excavation, roofed over with wood during the day, was carried down to the foundation level and a section of the subway structure built. Such an excavation was called a After the first slice was well advanced, a second was begun, and an intervening slice interval left undisturbed. When a series of alternate slices were completed the intervening excavation was removed.
In Paris, subway construction was much aided by the width of the streets and the placing of nearly all sub-surface structures beneath the sidewalks. In New York, in order to reduce to the minimum the distance between the platform and street surface so as to give the shortest flight of steps for passengers to climb, it was planned to build the old subway regardless of sewers and pipes, even though extensive read justment was necessary. A general design of a flat roof was adopted, the top of which was permitted to come within 30 inches of the top of the paving, a distance just sufficient to con tain the yokes carrying the sub-conduit of the electric surface tramway. In order to support the flat roof with its superincumbent load, a steel framework was devised, consisting of horizontal steel beams in the roof resting on rows of columns between the tracks and verti cal steel beams at the sides. These steel frames were set at intervals of five feet, and provide a series of supporting ribs. Between the roof beams and again between the wall beams, arches of concrete were formed. The floor of the structure is also of concrete, reinforced with steel beams, whenever an upward hydrostatic pressure was present. At the sides and immedi ately next to the wall columns were placed two vertical rows of terra-cotta ducts, into which were drawn, through manholes leading to the street surface, the lead-covered cables to carry the power current for operation. In order to keep moisture from the subway a layer of water-proofing material consisting of felt and asphalt was laid in the floor concrete and then carried up outside of the ducts and over the roof, making a continuous envelope. Immedi ately outside of the waterproofing was laid a thin protective wall of brick in concrete in order to guard it against damage. For the special difficulties incident to the tunnel construction of the New York subways see TUNNELS AND TUN NELING — New York Rapid Transit Tunnels.