TABLE III.
Showing Thickness of Masonry Lining for Side Tunnels through Soft Ground.
| Character of Material. | Keystone. | Springers. | Invert. |
|---|---|---|---|
| Ft. | Ft. | Ft. | |
| Laminated clay, first variety | 1.6 to 2.3 | 1.8 to 3 | 1.5 to 2 |
| Laminated clay, second variety | 2.3 to 3 | 3 to 4.1 | 2 to 2.6 |
| Laminated clay, third variety | 3 to 4 | 4.1 to 5 | 2.6 to 3.29 |
| Quicksand | 1.6 to 2.5 | 1.3 to 2 | 1.3 to 2 |
CHAPTER IX.
TUNNELS THROUGH HARD ROCK; GENERAL DISCUSSION; REPRESENTATIVE MECHANICAL INSTALLATIONS FOR TUNNEL WORK.
The present high development of labor-saving machinery for excavating rock makes this material one of the safest and easiest to tunnel of any with which the engineer ordinarily has to deal. To operate this machinery requires, however, the development of a large amount of power, its transmission to considerable distances, and, finally, its economical application to the excavating tools. The standard rock excavating machine is the power drill, which requires either air or hydraulic pressure for its operation according to the special type employed. Under present conditions, therefore, the engineer is limited either to air or water under compression for the transmission of his power. Steam-power may be employed directly to operate percussion rock drills; but owing to the heat and humidity which it generates in the confined space where the drills work, and because of other reasons, it is seldom employed directly. Electric transmission, which offers so many advantages to the tunnel builder, in most respects is largely excluded from use by the failure which has so far followed all attempts to apply it to the operation of rock drills. As matters stand, therefore, the tunnel engineer is practically limited to steam and falling water for the generation of power, and to compressed air and hydraulic pressure for its transmission.
Whether the engineer should adopt water-power or steam to generate the power required for his excavating machinery depends upon their relative availability, cost, and suitability to the conditions of work in each particular case. Where fuel is plentiful and cheap, and where water-power is not available at a comparatively reasonable cost, steam-power will nearly always prove the more economical; where, however, the reverse conditions exist, which is usually the case in a mountainous country far from the coal regions, and inadequately supplied with transportation facilities, but rich in mountain torrents, water-power will generally be the more economical. In a succeeding chapter the power generating and transmission plants for a number of rock tunnels are described, and here only a general consideration of the subject will be presented.
Steam-Power Plant.
—A steam-power plant for tunnel work should be much the same as a similar plant elsewhere, except that in designing it the temporary character of its work must be taken into consideration. This circumstance of its temporary employment prompts the omission of all construction except that necessary to the economical working of the plant during the period when its operation is required. The power-house, the foundations for the machinery, and the general construction and arrangement, should be the least expensive which will satisfy the requirements of economical and safe operation for the time required. It will often be found more economical as a whole to operate the machinery with some loss of economy during the short time that it is in use than to go to much greater expense to secure better economy from the machinery by design and construction, which will be of no further use after the tunnel is completed. The longer the plant is to be required, the nearer the construction may economically approach that of a permanent plant. As regards the machinery itself, whose further usefulness is not limited by the duration of any single piece of work, true economy always dictates the purchase of the best quality. Speaking in a general way, a steam-power plant for tunnel work comprises a boiler plant, a plant of air compressors with their receivers, and an electric light dynamo. When hydraulic transmission of power is employed, the air compressors are replaced by high-pressure pumps; and when electric hauling is employed, one or more dynamos may be required to generate electricity for power purposes, as well as for lighting. In addition to the power generating machines proper, there must be the necessary piping and wiring for transmitting this power, and, of course, the equipment of drills and other machines for doing the actual excavating, hauling, etc.