ACCIDENTS AND REPAIRS IN TUNNELS 269 



under any of these conditions is to construct the masonry as 

 soon as possible. 



When collapse is likely to occur after the masonry is com- 

 pleted, its probability is generally indicated by the cracking 

 and distortion of the lining. A study of the cause is quite 

 likely to show that it is the percolation of water through the 

 material surrounding the lining which causes cavities l>ehind 

 the lining in some places, and an increase of the pressures in 

 other places. When it is certain that this water comes from 

 the surface streams above, these streams may often be diverted 

 or have their beds lined with concrete to prevent further perco- 

 lation. When percolating water is not the cause of the trouble, 

 a usually efficient remedy is to sink a shaft over the weak point, 

 and refill it with material of more stable character. These, 

 and the remedies previously suggested, are designed to prevent 

 failure without resorting to reconstruction. When they or 

 similar means prove insufficient, reconstruction or repairs have 

 to be resorted to. 



Repairing Failures. Tunnels may collapse in several ways : 

 (1) The front and sides of the excavation may cave in; (2) 

 the floor or bottom may bulge or sink ; (3) the roof may fall 

 in ; (4) the material above the entrances may slide and fill 



tin-in Up. 



(1) One of the most common accidents is the caving of the 

 front and sides of the excavation. This may often l>e prevented 

 !.\ taking care that the face of the excavation follows the natu- 

 ral slope of the material instead of U ing more or less nearly 

 \.itioal. When, however, caving does occur it may usually 



liirc<l ly removing the fallen material, strongly shoring 

 the cavity, and filling in lehind with stone. tiniU'r, or fascines. 



(2) The bulging or rising of the bottom of the tunnel may 

 usually IK* considered as a consequence of the squeezing together 

 of the side walls. It usually occurs in very loose soils, and is 

 chiefly important from the fact that the reconstruction of the 

 side walls is made necessary. The sinking of the tunnel bot> 



