—In short tunnels niches are employed simply as places of refuge for trackmen and others during the passing of trains, and are of small size. In long tunnels they are made larger, and are also employed as places for storing small tools and supplies employed in the maintenance of the tunnel. Niches are simply arched recesses built into the sides of the tunnel, and lined with masonry; [Fig. 48] shows this construction quite clearly. Small refuge niches are usually built from 6 ft. to 9 ft. high, from 3 ft. to 6 ft. wide, and from 2 ft. to 3 ft. deep. Large niches designed for storing tools and supplies are made from 10 ft. to 12 ft. high, from 8 ft. to 10 ft. wide, and from 18 ft. to 24 ft. deep, and are provided with doors. Refuge niches are usually spaced from 60 ft. to 100 ft. apart, while the larger storage niches may be located as far as 3000 ft. apart. The niche construction shown by [Fig. 48] is that employed on the St. Gothard tunnel.
Entrances.
—The entrances, or portals, of tunnels usually consist of more or less elaborate masonry structures, depending upon the nature of the material penetrated. In soft-ground tunnels extensive wing walls are often required to support the earth above and at the sides of the entrance; while in tunnels through rock, only a masonry portal is required, to give a finish to the work. Often the engineer indulges himself in an elaborate architectural design for the portal masonry. There is danger of carrying such designs too far for good taste unless care is employed; and on this matter the writer can do no better than to quote the remarks of the late Mr. Frederick W. Simms in his well-known “Practical Tunneling”:
“The designs for such constructions should be massive to be suitable as approaches to works presenting the appearance of gloom, solidity, and strength. A light and highly decorated structure, however elegant and well adapted for other purposes, would be very unsuitable in such a situation; it is plainness combined with boldness, and massiveness without heaviness, that in a tunnel entrance constitutes elegance, and, at the same time, is the most economical.”
Fig. 49.—East Portal of Hoosac Tunnel.
[Fig. 49] is an engraving from a photograph of the east portal of the Hoosac tunnel, which is an especially good design. The portals of the Mount Cenis tunnel were built of samples of stone encountered all along the line of excavation. The stones were cut and dressed and utilized for walls and voussoirs. The only ornament that is usually allowed on the portals is the date of the opening of the tunnel prominently cut in the stone above the arch.
Table II.
Showing Thickness of Masonry Lining for Tunnels through Soft Ground.
| Character of Material. | Keystone. | Springers. | Invert. |
|---|---|---|---|
| Ft. | Ft. | Ft. | |
| Laminated clay, first variety | 2.15 to 3 | 2.75 to 3.5 | 1.6 to 2.5 |
| Laminated clay, second variety | 3 to 4.5 | 3.5 to 5.5 | 2.5 to 4 |
| Laminated clay, third variety | 4.5 to 6.5 | 5.5 to 8.1 | 4 to 4.5 |
| Quicksand | 2 to 3.28 | 2 to 4.1 | 1.33 to 2.5 |