TUNNELS THKOUUH HAKD BUCK. 



CHAPTER IX. 



TUNNELS THROUGH HARD ROCK; GENERAL 

 DISCUSSION; EXCAVATION BY DRIFTS. 

 MONT CENIS TUNNEL. 



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 ma- 

 chine is the power drill, which requires eitlier air or hydraulic 

 pressure for its operation according to the special tyjnj em- 

 ployed. Under present conditions, therefore, the engineer is 

 limited either to air or water under compression for the trans- 

 mission 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 advan- 

 tages 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 com- 

 pressed air and hydraulic pressure for its transmission. 



Whether the engineer should adopt water-power or steam '> 

 generate the power required for his excavating maHiine-x -I 

 pends upon then relative availability, cost, and suitability to the 



