114 TUNNELING 



CHAPTER XL 



TUNNELS THROUGH HARD ROCK (Continued). 



EXCAVATION BY HEADINGS. ST. GOTH- 



ARD TUNNEL. BUSK TUNNEL. 



THE more common method of tunneling through hard rock 

 is to begin the work by a heading, instead of by a drift. This 

 heading may be of small dimensions, and the remainder of the 

 section may also be removed in successive small parts, or it may 

 be the full width of the section, and the enlargement of the 

 section be made in one other cut. 



General Discussion. When the tunnel is excavated by means 

 of several cuts, which is the method usually employed in 

 Europe, the sequence of work is as indicated by Fig. 57. 

 Work is begun by driving the center top heading No 1, whose 

 floor is at the level of the bottom of the roof arch, and which is 

 usually excavated by the circular cut method. This heading is 

 widened by removing parts No. 2 until the top part of the sec- 

 tion is removed, then the roof arch is built with its feet rest- 

 ing on the unexcavated rock below. The lower portion of the 

 section or bench is removed by first sinking the trench No. 3, 

 after which part No. 4 is taken out, and then part No. 5, and 

 the side walls built. Part Xo. 6 for the culvert is finally 

 opened. The heading is, as a rule, diiven far in advance, but 

 the excavation of each of the other parts follows the preceding 

 one at a distance behind of about 300 ft. 



The strutting, when any is required, is usually the typical 

 radial strutting of the Belgian method of tunneling. The 

 masonry lining is constructed practically the same as in tunnels 

 excavated by a drift. The hauling is done on a single track 

 laid in the heading No. 1, which separates into double tracks 



