TUNNELING 



dynamo of 20 arc light capacity, one arc light being placed on 

 each side of the tunnel at all working-places. Each lamp 

 carried a coil of wire 20 or 30 ft. long to allow it to be shifted 

 from place to place without delay. 



Mullan Tunnel. This tunnel is 3,850 ft. long, and crosses 

 the main range of the Rocky Mountains, abou-t 20 miles 

 west of Helena, Mont. The tunnel is on a tangent throughout, 

 and has a grade of 20 % falling toward the east. The summit 



of the grade, west of the tun- 

 nel, is 5,548 ft. above sea 

 level, and the mountain above 

 the line of the tunnel rises 

 to an elevation of 5,855 ft. 

 Owing to the treacherous 

 nature of the material through 

 which the tunnel passed, it 

 had been a constant menace 

 to traffic ever since its con- 

 struction in 1883, and numer- 

 ous delays to trains had been 

 caused by the falls of rock 

 and fires in the timber lin- 

 ing. For these reasons it was 

 finally decided to build a per- 

 manent masonry lining, and 

 work on this was begun in July, 1892. 



The original timbering consisted of sets spaced 4 ft. apart 

 c. to <?., with 12 x 12 in. posts supporting wall plates, and a 

 five-segment arch of 12 x 12 in. timbers joined by H-in. 

 dowels. The arch was covered with 4-in. lagging, and the 

 space between this and the roof was filled with cordwood. 

 Except where the width had been reduced by timbering placed 

 inside the original timbering to increase the strength, the clear 

 width was 16 ft., and the clear height 20 ft. above the top of 

 the rail. Fig. 145 shows the timbering and also the form 



FIG. 145. Relining Timber Lined Tunnel, 

 Great Northern Ry. 



