270 TUNNKLING 



torn is a more serious occurrence. It seldom happens unless 

 there is a cavity beneath the floor, due either to natural causes 

 or to the fact that mining operations have gone on in the hill 

 or mountain penetrated by the tunnel. When the bottom of 

 the tunnel sinks, three cases may be considered : (a) when the 

 sinking is limited to the middle of the tunnel floor; (6) when 

 only a portion of the foundation masonry is affected ; and, (c) 

 when the entire lining is disturbed. In the first case repairs 

 are easily made by filling in the cavity with new material. In 

 the second case the unimpaired portion of the masonry is tem- 

 porarily supported by shoring while the injured portion is re- 

 moved and rebuilt on a firm foundation. The remaining cavity 

 is then filled. In the case of the complete failure of the lining, 

 the method of repairing employed when the roof falls, and 

 described below, is usually adopted. 



(3) The most dangerous of all failures is the falling of the 

 tunnel roof. In such casualties two cases may be considered : 

 (a) When the falling mass completely fills the tunnel section, 

 and () when it fills only a portion of the section. 



When the whole section is filled by the fallen material, the 

 problem may be considered as the excavation of a new tunnel 

 of short length inside the old tunnel, and under rather more 

 difficult conditions. The first task, particularly if men have 

 been imprisoned behind the fallen material, is to open com- 

 munication through it between the two uninjured portions of 

 the tunnel. It is advisable to do this even when there is no 

 danger to life because of imprisoned workmen, since it enables 

 the work of repairing to be conducted from both directions. 

 The excavation of a passageway through the fallen material 

 is rendered difficult, both because the fallen material is of an 

 unstable character, and also because it is usually filled with the 

 lining masonry, timbering, etc. When, therefore, the accident 

 has happened l>efore the full section of the original material 

 has been removed, the first heading or drift is driven through 

 this original material rather than through the fallen debris. 



