677. The advantages of concrete in subway construction and in tunnel lining are now well established. In subways built in open cut, the side walls and invert are of concrete built in place, while the roof is frequently made with I-beams with concrete arches turned between them. The I-beams are supported directly on the side walls, which are usually made monolithic with the invert.

678. Special precautions have to be taken to exclude water from a subway, and for this purpose tarred felt and Portland cement plaster are employed.

The specifications for the New York Rapid Transit Subway 2 were carefully framed to secure a waterproof construction. On the sub-grade was placed a layer of concrete, smooth and level on top. This was covered by alternate layers of hot asphalt and felt, from two to six layers of each being used as deemed necessary for the conditions encountered. The remainder of the concrete forming the floor was then laid upon the top layer of asphalt. In dry, open soil the felt was not required, and in dry rock excavations above water level both the asphalt and felt were omitted. Similar provisions were made for waterproofing the side walls and roof, resulting in a complete layer of asphalt and felt imbedded in concrete about the entire tunnel, the waterproofing being protected both inside and out by concrete.

1 Engineering News, Feb. 18, 1904.

2 Abstracted in Engineering News, Feb. 13, 1903.

679. In the construction of the Boston Subway 1 the portion built in open cut was made as follows: The work was divided into sections of convenient length, about twelve feet, so that work on a section could be carried on continuously until completed. Upon the prepared grade were laid three thicknesses of tarred felt with six-inch lap joints, well pitched between the layers, and the top of the upper layer thoroughly covered with the pitch. When the latter had hardened, the invert was laid over the entire width of the section.

At each side a back wall six inches thick was built up to a convenient height and braced. The forms were then removed and the face of this back wall was plastered with rich Portland cement mortar. The main side walls were then built up between this layer of plaster and the forms defining the interior face of the wall. This portion of the subway had an arch roof, two feet thick at the crown, which was laid on wooden centers. The exterior of the roof was plastered like the side walls, and then covered with four inches of concrete to protect the plaster from injury. The centers were removed after from ten to thirty days; the span of the arch was about twenty-three feet.

680. Tunnel Lining In Firm Earth

In building tunnels in earth that is sufficiently firm not to require extensive timbering, concrete is well adapted for lining. An instance of this is furnished by the extensive system of tunnels constructed for telephone and telegraph service under the streets of Chicago.2 The trunk conduits for this system are about thirteen by fourteen feet inside, and the laterals about six by seven feet, all of the five center horseshoe form.

1 Annual Report Boston Transit Commission, 1900; also described in Engineering News, April 4, 1901.

2 Mr. George W. Jackson, Engineer, Proc. W. Soc. Engrs., 1902; also in Engineering News, Feb. 19, 1903.

The excavation was in hard clay which stood up well. Shafts were located in basements of buildings rented for the purpose, and in these basements were placed the compressed air plants, material bins, concrete mixers, etc. The large air locks, some of which would hold ten small construction cars, were placed at the bottoms of the shafts. Work was done in three shifts, working eight hours each. The two night shifts could excavate about twenty-one feet of lateral tunnel in the sixteen hours, and the day shift placed the lining.

681. The concrete was in general composed of five parts of broken stone and screenings, or of mixed gravel and sand, to one part Portland cement. For intersections but four parts aggregate were used. This should make a very strong concrete. The centers for the smaller conduits were made of three-inch channels, each rib being in five parts bent to the proper form and connected by flange plates bolted to the inside of the channels at the ends. These ribs were placed three feet apart, and two-inch plank used for lagging.

The ribs for the trunk sewers were of similar construction, but with heavier channels braced with angles. Steel lagging was used, made of plates about twelve by thirty-six inches, stiffened by 1 1/2 inch angles on four edges. There were also provided bulkheads or steel end plates of voussoir shape, twelve inches along the intrados and twenty inches high, for the purpose of retaining the end of each section of lining and permit thorough tamping. These bulkhead sheets, or "end flights," were also stiffened along three edges, and could be attached to the webs of the channel ribs by short bolts.

The concrete was mixed at the shaft head and conveyed to the work in cars twenty inches wide and four feet long, running on a fourteen-inch gage track. The floor of the tunnel was first laid in the excavation, the steel ribs then put in place on the floor, and the lagging placed at the bottom and built up the sides just ahead of the concrete. When near the crown, short pieces of lagging three feet in length covering but two ribs were used, and the concrete rammed in from the end of these short sections until they were complete, and then another row of short pieces placed and the operations repeated.

The concrete floor of laterals was designed to be thirteen inches, and the sides and arch ten inches thick, but in all cases the entire space between the lagging and the sides of the excavation was filled with concrete.

682. In such work as this only the best materials should be used, and, as early strength is desired, the use of Portland cement is general in order that the centers may be removed within a reasonable period. The ends of the sections into which the work is divided should, if possible, be brought up square, the bulkhead sheets described above being an ingenious and effective method of providing for this. Where it is not practicable to finish with a square end over the entire area of section, then the work on the sides should be stepped back from the bottom toward the crown, each step being bounded by planes corresponding to coursing and heading joints in a masonry arch.