118 TUNNELING 



drained of its moisture, and the discharge from the end receiver 

 into the tunnel delivery pipes was not affected by the pulsations 

 of the compressors. The delivery pipe decreased from 8 in. 

 in diameter at the receiver to 4 ins. in diameter, and finally to 

 2^ ins. in diameter, at the front. 



The drills employed were of various patterns. ' The first one 

 employed was the Dubois & Franois " perforator," in which the 

 drill-bit was fed forward by hand. This was replaced by Fer- 

 roux drills having an automatic feed. Jules McKean's " perfo- 

 rator " was employed at the north end of the tunnel. All of 

 these drills were of the percussion type, and were mounted on 

 carriages running on tracks. Their comparative efficiency was 

 officially tested in drilling granitic gneiss with an operating 

 air pressure of 5.5 atmospheres with the following results : 



NAME OF DRILL. PENETRATION INS. PER Mix. 



Ferroux 1.6 



McKean 1.4 



Dubois & Francois 1.04 



Souminelier 0.85 



The heading was excavated by the circular cut method, the 

 holes being driven as follows : Near the center of the heading 

 three holes were first drilled, converging so as to inclose a 

 pyramid with a triangular base. Around these center holes 

 from 9 to 13 others were driven parallel to the tunnel axis. 

 The center holes were blasted first, and then the surrounding 

 holes. From 3 to 5 hours were required to drill the two sets 

 of holes, and from three to four hours were required to remove 

 the blasted rock. The number of holes drilled in removing 

 each of the various parts was as follows : 



Part No. 1 6 to 9 



Part No. 2 6 to 10 



Part No. 3 2 



Part No. 4 6 to 9 



Part No. 5 3 



Part No. 6 6 to 9 



Part No. 7 1 



Total for full section . 39 tolo 



