Chapter 05. Ventilation, Drainage, and Waterproofing
The New York Rapid Transit Railway Extensions · Engineering News, 1914
Every effort has been made to so design the new subways, that the excessive heating which occurs at times in the summer in the present subway may be avoided. The tracks are to be divided so that trains going in one direction will be in a separate tube or tunnel from those going the opposite way; by this means it is expected to utilize the movements of the trains (the so called piston action where there is only one track in a single tube) to push the air ahead and out through the openings which are provided for this purpose.
The original subway is completely surrounded by an envelope of waterproofing, and it has been thought that this has prevented the dissipation of some of the heat generated by the motors, brakes, friction, etc., into the ground surrounding the structure. On the new lines waterproofing will generally only be used where actually necessary to keep out water, that is, below the groundwater line, in earth, and on the roof.
Openings in the roof of the tunnel, with sidewalk gratings, are provided over the station platforms, and generally one about half way between each station and one at each end of the stations on the side toward the approaching train, these latter being expected to take care of most of the draft caused by the train, instead of allowing it to create a current at the platform and up the stairways. The general form of these openings, and the details of their construction are shown in Fig. 22, and a typical arrangement of location in Fig. 21 (7th Ave., 17th to 24th St.). The dimensions and numbers of these openings have been so fixed that it is expected that the current of air coming through the gratings in the sidewalk will be barely noticeable to pedestrians.
Fig. 21. Plan of portion of Seventh Avenue Subway, showing provision for ventilation. (Click image to enlarge.)
Fan chambers, which are all arranged so that they may be also used as emergency exits to the streets, are provided, one about midway between each station. They are so arranged that they will draw the air from the tunnel at points intermediate between the stations and blow it out through the gratings already described, thus, of course drawing fresh air in at the stations. It is expected that the openings alone combined with the action of the trains will ordinarily provide sufficient ventilation, the fans being used only occasionally when circumstances require.
The piston action of the trains in the single-track tube of the Hudson & Manhattan R.R. has been noticeably efficacious in promoting efficient ventilation, but except on the Fourth Ave., Brooklyn line, where there is a parked space in the center of the street and where walls are provided between each track, it was not considered practicable to divide all the tracks of the new four-track lines so that each would be in a separate tube, on account of the difficulty of providing outlets for the center tracks. It would be impractical, of course, to provide openings in the roadway of the streets, and in order that the openings in the sidewalks might be used, the center wall only was built dividing the traffic going in opposite directions, but leaving the two tracks on one side in the one space. If this does not produce the required movement of the air, that is, actual propelling movement, not mere stirring up as in the present subway, the fans must be utilized to supplement it. Openings about 2 ft. wide and 8 ft. high are provided at every 10 ft. in the center wall as a means of communication between the two sides, and as refuge niches, and these may tend to reduce the piston effect to some small extent.
Although this arrangement in the four-track section will reduce somewhat the positive piston action of the trains it will be beneficial to the extent that it will tend to reduce the air resistance, which has been shown (see J. V. Davies, "Air Resistance in Tube Tunnels," Trans. Am. Soc. C. E., Vol. LXXV, 1912.) to be by no means a negligible factor in cost of operation in single-track tubes, though this cost may be offset by the benefits of more efficient ventilation.
Fig. 22. Partial cross-section and plans of double-deck subway, showing typical arrangement of ventilation outlets. (Click image to enlarge.)
The actual effect of all these different items and of the size of the cross-section both on the efficiency of ventilation, as well as the cost of operation, is something of which little is actually known, but in view of the enormous expenditures which are being made and which undoubtedly will continue to be made in underground railways for rapid transit, in subaqueous tunnels, etc., it is hoped that further experiments along the lines of these already referred to and others of like nature may be continued.
As will be seen by the diagrams the object of the design of the openings has been to provide at the track level a space into which the air pushed ahead of the train may expand and be detained, instead of being pushed by, and then provide an opening above through which it may escape to the surface, there being apparently little reason to doubt the efficacy of this proposed scheme.
Drainage and Waterproofing. In the general clauses of the specifications it is stated that "it is the very essence of these specifications to secure a railroad structure underground which shall be free from the percolation of ground or outside water. The mixing and placing of the concrete and the placing and protection of the waterproofing shall be with this end in view."
"In general, waterproofing of the structure will be limited to the roof and sidewalls at the stations and over the roof between stations, and to those surfaces near ground water or mean high water if ground-water level is found for any reason to be below mean high water. At other places free drainage shall be provided by pipe drain, hollow tile or broken stone."
The specifications provide for the use of fabric waterproofing, laid in hot pitch or asphalt, and in from three to six thicknesses or plies, and for brick or hollow tile, laid in pitch or asphalt mastic. The latter to contain "one third pure bitumen, and sand and cement or lime dust in proportions governed by local requirements and weather conditions."
At temperatures of 50 degrees to 70 degrees, the proportions used are usually one-third asphalt, one-third cement, one-third sand; in colder weather the proportion of asphalt is increased as required up to a maximum of 60%, though 50% is seldom exceeded. Lime dust is apparently not used in place of the cement, as it appears to require a larger proportion of asphalt to make it workable.
The fabric waterproofing is generally used on the roof or other horizontal surfaces where it may be required, and the brick in mastic on the sidewalls or on any vertical surfaces and under the floor when required there. At stations brick and mastic are used over the roof.
In the concrete specifications, the following clauses apply to the waterproofing:
- The proportions of cement and sand and stone (or gravel) used in making protective concrete outside of waterproofing lines on sides and roof, shall be as follows: 1 part of cement, 4 parts of sand and 8 parts of stone.
- Concrete to which waterproofing is to be applied shall be made smooth at the time of laying and shall be carefully protected from injury by barricades or otherwise, if necessary, until thoroughly set.
- It is intended to obtain concrete impervious to water; the concrete shall be mixed and deposited with this end in view, and on the roof of the railroad, if waterproofing is not used, top surface of the concrete shall be carefully troweled as may be directed in order to add to its imperviousness.
Reference has already been made to the fact that on account of the supposed influence of the waterproofing envelope enclosing the present subway, in retaining the heat, that waterproofing is only carried out in the new lines where the evident necessity shows the need of protection to keep the structure reasonably dry. Much greater reliance is being placed on the provision of free drainage to take care of small quantities of water, than has been done heretofore, this being in line with recent experience.
The question of waterproofing tunnels is comparatively modern and its importance is due principally to the development of electric traction and of the numerous underground lines for urban rapid transit. On that section of the Pennsylvania Railroad's New York tunnels, which passes under the Bergen Hill (see "The Bergen Hill Tunnels of the P.R.R.", Trans., Am. Soc. C. E., Vol LXVIII, p. 146) on the Jersey side of the Hudson River, and where there was a considerable seepage of ground water, the ample and careful provision of free drainage, without the general use of waterproofing has resulted in a remarkably dry structure.
On all the new subway lines, drain pipes are laid in the floor (under the center of each track) which lead to sumps at pump chambers, from which the drainage is discharged by automatic electric pumps into convenient sewers.
These floor drains have grating openings in the concrete floor every 50 ft. and the floor grades are arranged (irrespective of the track grades) so that there is a summit between each grating (in the case of steep grades 2 or 3%, the summit is just below the grating), 4-in. pipes lead to these center drains from the sides, and, if necessary, part or all the way up, to take care of any seepage there may be. In the case of the Lexington Avenue rock tunnels, these side drain pipes generally reach up to the bottom of the loose rock packing over the roof. See Figs. 12 and 14.
Speaking generally, there are two typical methods of waterproofing. The first where the structure is in earth, where the water level (mean high water or ground water) is above the bottom of the structure. In these cases the waterproofing is carried across the bottom and up the sides to about 2 ft. above the level of the water. See Fig. 15. This drawing also shows the waterproofing carried over the roof, as the section is taken at a station.
The second where the structure is partly in rock and where the water level is above the rock. In these cases the waterproofing is carried from a point 2 ft. above the water level to the rock or more commonly sealed into a trench in the sand wall, as shown by the sketch, Fig. 23. When the water level is below the top of the rock, waterproofing is not generally used except over the roof (see Fig. 12), and in the case of the Lexington Ave. tunnels, even this is omitted.
Fig. 23. Example of waterproofing structure in rock with brick wall laid in mastic. (Click image to enlarge.)
The necessity or otherwise of using the waterproofing is, of course, governed entirely by the local subsurface conditions, the plans provide for what may reasonably be expected, based on the results of the borings, but the judgment of the field engineers is relied on largely to modify this to conform to actual conditions developed as the work progresses.
In that section of Lexington Ave. south of 100th St., where the structure is in rock tunnel and wholly above the water level, no waterproofing at all is used; on the other hand. just above this point, at about 102nd St., though the structure is wholly in rock, the water level is about 10 ft. above the bottom of the structure and the brick in mastic is, therefore, carried down below the floor at the sides, and the top is covered with 1 ply of fabric and two layers of brick in mastic. Burlap coated with an asphalt compound is generally used for the fabric, but where there is water, as in the bottom under the floor or in depressed bays, etc., one layer of felt is used first.
At stations the waterproofing (3-ply fabric or 1-ply and two layers of brick in mastic) is carried over the roof and down the sides to below the track level, in order to prevent any damage to the decorations, as well as to protect the offices and passengers.
When the floor is to be waterproofed, a 6-in. concrete base is laid in th bottom of the excavation, and two layers of brick laid flat in mastic laid on this; at the sides, if the sheeting is to be left, the two courses of brick in mastic are generally laid right against it, otherwise, where the sides require waterproofing, the steel is first erected, then a hollow-tile or concrete-sand wall is built behind it, on which the waterproofing fabric is hung and then the concrete sidewalls are built. Loose rock is packed behind the hollow tile as it is built up.
In many cases the protection wall of 4-in. hollow tile, or a concrete sand wall is built, then the steel is erected before the brick in mastic is laid up. The steel columns then act as braces for the rough board forms necessary to support this latter until the mastic hardens. These boards are usually painted with a good thick coat of cement grout to prevent their sticking to the mastic, the grout sticks to the mastic and the boards are easily removed.
An inspection of the various bids made up to the present time shows the general average prices for the above work on the contracts awarded to be approximately as follows:
- Waterproofing, 1 ply per sq.yd, $0.50 to $0.60
- Waterproofing, 2 ply per sq.yd 0.80 to 0.90
- Waterproofing, 3 ply per sq.yd 1.10 to 1.20
- Waterproofing, 4 ply per sq.yd 1.35 to 1.50
- Waterproofing, 5 ply per sq.yd 1.55 to 1.75
- Waterproofing, 6 ply per sq.yd 1.75 to 2.00
- Brick in mastic, per cu.yd. 27.00 to 30.00
- Vitrified drain pipes, 4 in. per lin.ft $0.40
- Vitrified drain pipes, 6 in. per lin.ft 0.50
- Vitrified drain pipes, 5 in. per lin.ft 0.75
- Vitrified drain pipes, 10 in. per lin.ft 0.90
- Vitrified drain pipes, 12 in. per lin.ft 1.10
- Cast-iron drain pipes, 4 in. per lin.ft $0.75 to $1.00
- Cast-iron drain pipes, 6 in. per lin.ft 1.00 to 1.25