Chapter 06. Sewers, Pipes, and Conduits
The New York Rapid Transit Railway Extensions · Engineering News, 1914
Electric Conduits. Four-way vitrified ducts with round holes (3.5 in.) are used almost entirely for electrical conduits in the subway; they are located in the side bench walls, as shown in the various cross-sections, usually a double tier 5 high, making 40 single ducts.
The specifications require that the outside dimensions (of 4-way ducts) be not less than 9.25 in. or more than 10 in. with square outer lines and that the outside walls and webs be 0.75 in. thick.
A linked mandrel is used for laying and the joints are wrapped with muslin wraps soaked in cement grout. The mandrel is arranged so that the back part of it holds the joint last made, while the forward end holds the joint being made. One or two other forms of wraps have been tried but with little success. The whole bank of ducts is usually laid up, then the outer face of concrete, and top of the bench wall put in place. This is usually done before rodding to insure the stability of the duct bank.
Wherever it is at all possible, connections are made from all splicing chambers to the street so that cables may be placed or withdrawn from the street surface, on account of the difficulty of handling them in the subway after operation has been started. Where space allows, a regular manhole is built from the splicing chamber to the street surface, but where this is not possible a 10-in. feed pipe is built in. The splicing chambers are, of course, set back and project outside the ordinary cross-section, a typical form being shown in Fig. 24.
Fig. 24. Horizontal section of subway wall at cable splicing chamber and vertical section through manhole. (Click image to enlarge.)
The prices for these conduits as shown by contracts already awarded are approximately 10 to 15 cents per lin.ft. of single duct.
Fig. 25. Isometric view of underground piping at the intersection of Broadway and Fulton St. in 1890.
Existing Underground Structures. The construction of the subway, of course, necessitates many changes in the existing underground structures, such as sewers, water and gas pipes, electric-wire conduits, etc. The isometric drawing, Fig. 25. shows conditions at Fulton St. and Broadway on the line of the present subway as they existed 24 years ago. Since that time very great additions have been made to the underground street piping. The picture, however, gives some idea of what is found at many street intersections, though, as a matter of fact, the general appearance of one of these cross streets in the downtown section is more that of an intricate and confused jumble of pipes and cables of all descriptions instead of the orderly appearance of the drawing. Fig. 30 shows part of the plan of the intersection of Prince St. and Broadway and is fairly typical of conditions in the lower part of the city.
Fig. 30. A typical rearrangement of underground piping at a street intersection, old and new plans of underground piping at Broadway and Prince Street. (Click image to enlarge.)
It would seem at first thought that certain large trunk sewers and other important subsurface structures might have considerable influence on the location of the rapid-transit lines, but it has generally been found easier or less costly to change these than to change the position of the subway. This latter, of course, is usually located as near the surface as possible, and any change would mean lowering it, which would be undesirable from the standpoint of those who have to use it, and, of course, would mean increased excavation with proportionately greater expense, so that generally speaking, the subway is located regardless of any of these structures and these latter moved if necessary. There are a few instances where it has not been practical to move some of the large trunk sewers, and these will be noted further on.
This work connected with the changes, etc., required in the existing underground structures is all taken care of by two divisions, each covering the whole city in its own special department. One under C. N. Green has charge of the relocation of all pipes, conduits, etc., and the other under L. D. Fouquet has charge of all the sewer relocation, design and reconstruction. The importance of this part of the work may be gathered from the fact that there are employed in these two departments alone, taking care of work in a way entirely outside of the main construction, some 150 engineering assistants, the changes in the sewers alone involving an expenditure of $6,000,000 to $7,000,000 and the construction of some 60 miles of new sewers.
Gas Mains. One of the most important developments in taking care of the pipes, etc., on the new work has been the bypassing of all the gas mains, that is, the construction of new pipes for gas, on the surface of the street and the stoppage of the flow in the pipes underneath before commencing the excavation. This, of course, involves also new temporary house connections as well, but the danger of the accumulation of gas underneath the decking is thereby eliminated. The very great danger from this source was demonstrated by the recent explosions at 23rd St. and 5th Ave. (Eng. News, Mar. 12, 1914), which while generally attributed by the public press to the subway construction, were caused if at all only very indirectly by this work. The following description of the method of dealing with the gas mains is contributed by C. N. Green.
"The present specifications for subway construction call for the street to be planked or decked over in the business sections or where traffic is heavy, so that business may be carried on as usual and with as little inconvenience to the public as possible. This decking then forms a temporary street surface under which the excavation is carried on. In time dirt and the sweeping of the street make the decking tight and prevent a circulation of air that would free the excavation of gas if a main should leak."
"Cast iron is used for gas and water mains when they are laid underground for the reason that it lasts much longer under such conditions than wrought iron. It is difficult, however, to keep the lead-calked joints of such pipe tight even when they are undisturbed, but when they are moved in excavating and hung up on the timbering, some joints are sure to be strained and begin to leak."
"All gas mains are, therefore, 'killed' where they would be underneath a closely decked street, except in rare cases where small transverse cast-iron mains are replaced under the decking by wrought-iron pipes with screw connections. A temporary system of wrought-iron pipes or bypasses for gas distribution is laid in the gutters and connected with the live mains in the transverse streets and the house and street-lamp services are transferred from the cast-iron mains below the street surface to the temporary system. There is then no live main or pipe containing gas below the street surface." (Figs. 26 to 29.)
Fig. 29. Typical method of placing gas main at curb.
"An 8-in. gas pipe broken off in the excavation might, under existing conditions, deliver 1000 cu.ft. of gas per minute. A 10% mixture of gas and air will perhaps not always produce a maximum explosive effect, but this is assumed for convenience and is very near a maximum. This would make 10,000 cu.ft. of explosive mixture per minute. A subway cut 25 ft. deep, 60 ft. wide, would contain 390,000 cu.ft. in a city block, so that theoretically in 39 minutes the block would be filled with an explosive mixture. This, of course, would not be absolutely true as the gas would not diffuse itself with such rapidity and the mixture would be higher in gas near the break in the main and perhaps be too low in gas at the farthest point to explode; if the mixture should explode, however, the gas would burn where there is an excess and probably set fire to the timber with results as disastrous as the explosion itself."
"Gas mixed with air forms a very explosive compound which only needs a spark to ignite it. This spark might be furnished by the underground trolley of the surface railway, by one of the numerous cables exposed during the excavation, by a lighted match thrown away, by fire engines, by the shoe of a horse, or by blasting, etc. The violence of such explosions has been frequently shown when manhole heads have been blown into the air, and by numerous sewer explosions which have occurred in the last few years. Philadelphia and Boston had examples in the construction of their subways and various minor instances have attested to the power of such a mixture."
"Knowing the danger from gas, both the gas company and the engineers of the Public Service Commission have taken every precaution to avert it. It was, therefore, decided not to leave any cast-iron mains, carrying gas, under the decking unless properly ventilated by means of gratings or protected by watchmen. Where mains crossing the subway excavation could not be cut out of service temporarily, wrought-iron pipe bypasses were to be built over the street or in the case of small pipes carried across under the decking. All connections between the cast-iron mains and the wrought-iron bypasses were to be carried far enough back into solid earth to avoid the danger of breaking off the pipe if a slide or cave-in should occur."
"In mains to be 'killed,' the flow of gas was first stopped by inserting bags in the pipe and readings were taken to see if the pressure prescribed by the Public Service Commission could be maintained. The pressures were taken continuously for several days and if these readings were satisfactory the longitudinal mains were cut off and capped at intervals of two blocks. In the meantime the bypass pipes were laid in the gutter and connected up. These were 6, 8, 10 or 12 in. in diameter, depending on the requirements of the different districts. Depending on conditions, one or two lengths of pipe were used to carry back in the transverse street and the turn at the curb was made with a pipe bent to fit the radius of the curb corner, Fig. 26. The pipes in each transverse street were then cut and capped about 10 ft. back of the sheeting line on both sides of the proposed excavation. Connection was then made with the largest pipe underground on each side of the street and the bypass continued underground across the transverse street."
Fig. 26. A temporary gas main run next to the street curb.
"Generally there are from one to five pipes on each side of the transverse streets. Where there is more than one pipe on the same side of the street, each is connected back of the caps with a 1.5 in. circulation connection, the vertical legs of which are tapped into the top of the pipe."
"In laying the first bypasses little attention was paid to the necessity of keeping the tops of the pipes level with or below the curb. As a consequence, numerous accidents occurred, due to persons slipping or tripping over the pipe. In one instance where the pipe did not lie close to the curb, a boy's leg was broken by being caught between it and the curb. Now all pipes are laid close to the curb, with the top not higher than the curb even if necessary to remove the gutter stones, and the remaining space is filled with concrete."
"Extra heavy wrought-iron pipe has been used generally, so that when the bypasses are moved the pipe can be used to relay the cast-iron mains over the subway. The gas company has found this desirable, as the vibration from the passing trains loosens the calking in cast-iron pipes."
"In Lower Broadway from Canal St. south are two mains 16 and 20 in. in diameter, respectively, supplying the lower part of the city. These mains were bypassed during the building of the subway from Vesey St. to Canal St. Wrought-iron pipes with flanged connections were laid about 14 ft. above the sidewalk. The trestle bents were so placed as to avoid entrances and interfere with business as little as possible. The cross streets where trestle bents could not be erected, the pipe was trussed with wire rope anchored to the pipe with clamps."
"In 138th St. are one 16-in., one 20-in. and one 24-in. gas mains forming a cross-town connection between the works and the lower west side of the Bronx. As these were within the sheeting lines, they were 'killed', and to take their place two 24-in. riveted steel pipes were laid on trestles, one on each side of the street."
"In various locations are large feeder or pumping mains crossing the line of the subway, and these mains generally were bypassed overhead, giving about 14 ft. clearance for the street cars and other vehicles. A description of one would be typical of all. Fig. 27 shows the bypass for the 36-in. main crossing Lexington Ave. at 112th St. The bypass pipe is a 30-in. wrought-iron riveted pipe carried on two gallows-frame supports back of the building line in the side street. Between these frames the pipe is suspended from two wire ropes carried over the gallows frames and each anchored back to a deadman. The deadman consisted of an inclined I-beam carried well below the street surface and its lower and embedded in a block of concrete. The main in the street was bagged off, the pipe cut and a three-way and valve inserted in the line of the pipe. This operation was repeated on the other side of Lexington Ave. The bypass was then connected to the three-ways on either side of the avenue, the bags removed and the valve closed to throw the gas into the bypass and kill the main underground."
Fig. 27. Overhead gas main at Lexington Ave. and 112th St.
"The cost of bypassing service mains, using 6- or 8-in. wrought-iron pipe laid in the gutter, varies but little from $50,000 per mile of street bypassed. The distribution mains requiring 16- to 30-in. bypasses in general run across the island from the east to west, thus crossing most of the subway lines nearly at right angles. The cost of carrying a distribution main across the street on trestle may vary from $1000 to $10,000, depending on local conditions. The average cost, however, should be about $2500."
"After the construction of the subway and the restoration of the underground pipe, the bypasses are removed and the street restored to its original condition."
Sewers. The work of the department in charge of the necessary sewer relocations commences as soon as a new route is proposed, as, although the subways are generally located with little regard for existing subsurface structures, minor changes and adjustments in elevations and gradients are quite often found to be desirable. General studies of the sewer situations are, therefore, necessary from the beginning. The sewer changes are worked out in consultation with the city authorities and the plans are made part of the contract drawings. This same department makes the preliminary studies, final plans and supervises construction.
Generally the existing sewer line is located in the center of the street. The construction of a subway therefore usually involves its complete elimination, and the substitution of two lines, one on either side. In Manhattan also the main trunk sewers and intermediate main lines are generally located in the cross streets running east and west to the Hudson or East River. The construction of a subway on one of the main north and south avenues therefore cuts these all off, as they are nearly always located below the level of the roof.
On what may be referred to as the down stream side of the avenues the problem is usually comparatively simple. A new line is laid between the subway and the buildings, connecting at the cross streets to the existing sewers, which, however, of course only get part of their former flow. On the up stream side, however, not only do the buildings adjacent to the subway have to be taken care of, but also the flow from the cross drains which have been cut off. The least important of these cross sewers are, therefore, collected in a main laid parallel to the subway and carried to some convenient crossing point, where either the subway can be lowered, to pass the sewer over the top, or where topographical conditions permit the sewer to go under and continue with sufficient fall to the point of discharge into the river. The conditions at 30th St., New York, are quite typical of this condition, Fig. 31. The construction of this one line, giving a new outlet all the way to the North River, cost over $500,000.
Fig. 31. Typical sewer reconstruction work on 30th Street and Seventh Avenue. (Click image to enlarge.)
In a very few instances there have been large trunk sewers which could not be changed and which have necessitated a very considerable adjustment of the gradients of the subway to enable the line to pass them. At Canal St. and Broadway and at Duane St. and West Broadway, Manhattan, the subways were depressed to go under the sewer, while at Brook Ave. and 138th St., in the Bronx, the surface of the street was raised 5 ft. to enable the subway to pass over the top of the sewer.
Generally speaking, however, the large sewers where they have been encountered have been passed under the subway by means of siphons, and while this is not generally considered desirable for sewers, those so far built seem to be working satisfactorily. The general principle on which they are designed is much the same for all; that is, a comparatively small pipe for the so called "dry-weather" flow, with one or two larger pipes for the storm flow. The plan and section shown, Fig. 32. of the siphon at 110th St. and Lexington Ave. is quite typical. Most of the siphons have been built with easy slopes for the drop or rise, but in one instance in Brooklyn, at Hudson St., perpendicular raises were required on account of the cramped conditions. In this case a wide, very shallow additional safety overflow was provided over the roof of the subway.
Fig. 32. Inverted siphon carrying sewer under subway at 110th Street. (Click image to enlarge.)
Cross-sections of particular forms of construction not usually met with in sewer work, but required by the exigencies of limited clearance in many cases in connection with the subways, are shown in Fig. 33. An interesting temporary expedient was adopted on the Fourth Ave. Subway in Brooklyn. At one place on this line it was necessary to take care of quite a large volume of sewage until such time as a new relief trunk sewer could be built by the city. The subway at this point was built for six tracks, so one whole bay at one side for a length of 2200 ft. was isolated by being walled in, waterproofed and turned into a sewer until such time as the relief sewer was built.
Fig. 33. Typical cross-section of new sewers. (Click image to enlarge.)
The Duane St. sewer in Manhattan is typical of certain conditions which have to be met and where advantage was taken of the peculiar topography of New York and the long established habit of drainage into the rivers on both sides of the city. The drainage from Centre St., through which the so called Loop line runs from Duane to Delancey, was to the East River. This was cut off by the construction of the Loop, which was too deep to permit the construction of the sewer underneath, so a deep-level sewer was built under the original subway, through Duane St. to the North River, thus reversing the flow from what was formerly the up-stream side of Centre St. It is this new sewer that the Seventh Ave. route in West Broadway has to go under, as referred to above.
Fig. 33. Typical cross-section of new sewers. (Click image to enlarge.)
The numerous questions which come up in connection with the relocation of these existing underground structures are only hinted at above, but enough is given to show at least generally the importance of this part of the work and the skill and ingenuity often necessary in working this out.