274 



TUNNKL1N*; 



such as the bulging inward of a portion of the lining without 

 total collapse. In the first case the first duty of the engineer 

 is to open communication through the fallen debris, so 

 that passengers at least may be transferred from one part of the 

 tunnel to the other and proceed on their way. This is done 

 by driving a heading, and strongly timbering it to serve as a 

 passageway. If the tunnel is single tracked this heading is 

 afterwards enlarged until the whole section is opened. In 

 double-track tunnels the method generally adopted is to open 

 first one side of the section and timber it strongly, so as to clear 

 one track for traffic. While the trains are run- / 

 ning through this temporary passageway the 

 other half of the section is opened and re- 

 paired ; the traffic is then shifted to the 

 new permanent track, arid the temporary 

 structure first employed is replaced 

 with a permanent lining. 

 When the accident is such 

 that the repairs can be 

 made without ob- 

 structing traffic en- 

 tirely, Various FlG 140 _ Extendlug T unn el through Landslide at Portal. 



modes of procedure 



are followed. In all cases great care has to be exercised to 

 prevent accident to the trains and to the tunnel workmen. 

 The work should be done in small sections so as to disturb as 

 little as possible the already troubled equilibrium of the soil ; 

 the strutting should be placed so as to give ample clearing 

 space to passing trains, and the trains themselves should be run 

 .at slow speeds past the site of the repairs. To illustrate the 

 two kinds of accidents and the methods of repairing them, 

 which have been mentioned, the accidents at the Giovi tunnel 

 in Italy and at the Chattanooga tunnel in America have been 

 selected. 



Giovi Tunnel Accident In September, 1869, at a point about 



