86 



TUNNELING 



others are bored " straight " or parallel with the vertical plane 

 of the tunnel. 



The width of the advanced gallery or heading depends 

 upon the quality of the rock. In hard rock American engi- 

 neers give it the full width of the tunnel section ; but this 

 runnot be done in loose or fissured rock, which has to be sup- 

 ported, the headings here being usually made about 8x5 ft. 

 The wider heading is always preferable, where it is possible, 



PIGS. BO to 63. Sketches Illustrating American Center-Cut Method of Blasting Tunnels. 



since more room is available for removing the rock, and deeper 

 holes can be bored and blasted. 



With the preceding general discussion of tunneling through 

 rock we may proceed to a detailed consideration of the con- 

 struction of typical examples of rock tunnels. For this pur- 

 pose the Mont Cenis and Simplon tunnels are selected as 

 examples of rock tunnels driven by a drift, and the St. Goth- 

 ard and Busk tunnels as examples of rock tunnels driven by 

 headings. 



