TUNNELS THROUGH HARD ROCK 91 



cast-iron pipe 7| in. in diameter, up to the front of the excava- 

 tion. The joints of the pipes were made with turned faces, 

 grooved to receive a ring of oakum which was tightly screwed 

 and compressed into the joint. To ascertain the amount of 

 leakage of the pipes, they and the tanks were filled with air 

 compressed to 6 atmospheres, and the machines stopped ; after 

 12 hours the pressure was reduced to 5.7 atmospheres, or to 

 95 % of the original pressure. 



Sommeilier's percussion drilling machines were used in the 

 excavation of this tunnel. They were provided with 8 or 10 

 drills acting at the same time, and mounted on carriages running 

 on tracks. These were withdrawn to a safe place during the 

 blasting, and advanced again after the broken rock was removed 

 from the front and the new tracks laid. 



Machine shops were built at both ends of the tunnel for 

 building and repairing the drilling machines, bits, tools, etc. 

 A gas factory was built at each end for lighting purpose. 



Strutting. The roof of the drift was strutted by means of 

 longitudinal planks supported by cap-pieces laid across the line 

 of the tunnel and resting on vertical props close to the sides 

 of the excavation. This strutting was necessitated in order to 

 prevent the fall of the rock from the upper part of the section. 

 For the upper portion of the profile no continuous strutting 

 was required, but at places where the rock was fissured or 

 disintegrated a polygonal strutting was employed. This 

 consisted of a sill laid across the axis of the tunnel and just 

 above the roof of the drift On this sill two inclined props 

 were placed supporting a cap-piece. Close to the feet of these 

 two inclined props other props were inserted abutting against 

 wooden blocks close to the faces of the excavation. These 

 blocks were of tra|>ezoidal shape, the smaller side being near 

 the excavation, while the longer ones abutted against the 

 props. Between two consecutive wooden blocks small IKMUIIH 

 were inserted as close as possible to the excavation, and in 

 such a manner as to assume the form of a polygon. Planks 



