METHODS AND COST OF LINING TUNNELS AND SUBWAYS.
Fig. 126.—Section Showing Lining for Capitol Hill Tunnel. Washington, D. C.
Tunnel lining work is of two distinct classes: Lining work, done during original construction and relining of tunnels in service. The methods of work to be adopted and the cost of work will be different in the two cases. In relining work the costs are increased by the necessity of providing for the movement of trains and by the delays due to these movements and also by the labor of removing the old lining and, often, of enlarging the excavation. Comparatively few published figures are available on the cost of concrete tunnel lining, and such as exist are commonly incomplete. The common practice is to record the cost as so much per lineal foot of tunnel. This should be done, but the record should also show the cost per cubic yard of concrete in the lining. The notions of engineers vary as to the proper thickness of lining to use and this dimension also varies with the character of the ground. One tunnel lining may easily contain twice as many cubic yards of concrete per lineal foot of lining as another tunnel contains.
The two problems in form construction for tunnel work are: First, to construct the form work so that it does not interfere with train movements, and, second, to construct it so that it can be taken down, transported and re-erected and thus used over and over. The examples of practice given in the succeeding sections are the best instructions that can be laid before the reader in regard to possible ways of solving these problems and, also, the problem of handling the concrete and other materials to the work.
Fig. 127.—Traveling Derrick for Constructing Side and Center Walls, Capitol Hill Tunnel.
Fig. 128.—Steel Forms for Side Walls for Capitol Hill Tunnel.
METHOD OF LINING CAPITOL HILL TUNNEL, PENNSYLVANIA R. R., WASHINGTON, D. C.—The tunnel through Capitol Hill for the Pennsylvania R. R. approach to its new Union Station at Washington, D. C, is a two-track, double tube tunnel 4,000 ft. long through earth. Figure 126 shows the lining construction; it consists of stone masonry center wall, mass concrete inverts and side walls and a brick roof arch backed with concrete. For building the center and side walls the traveling derrick shown by Fig. 127 was employed. This traveler moved ahead with the work on a 14-ft. gage track and it handled the stone and concrete buckets from the material cars to the workmen on the walls. In connection with the derrick in the concrete side wall construction use was made of steel plate forms for the inside faces of the walls. These forms were made of 4×10 ft. sections of steel plate, constructed as shown by Fig. 128, and connected together by bolting through the flanges. The steel forms were erected by hand in advance of the derrick, 20 ft. of form on each side at a time. The concrete buckets were brought into the tunnel on cars hauled by electric motors from the mixing plant at the portal, and the buckets were lifted by the derricks and emptied into the forms. The side walls were concreted to the springing line and then the five-ring brick roof arches were constructed on traveling centers and in 20-ft. sections. The remainder of the concrete was then placed over the arches by means of the special back-filling machine, shown by Fig. 129. This machine also handled the earth used to fill behind the masonry. It consisted of a platform mounted on wheels and of the same general construction as the derrick platform. On the forward end of this platform a stationary hoist was mounted and behind this a belt conveyor platform.