lib'C TUNNELING 



CHAPTER XXII. 



ACCIDENTS AND REPAIRS IN TUNNELS DURING 

 AND AFTER CONSTRUCTION. 



IN the excavation of tunnels it often happens that the dis- 

 turbance of the equilibrium of the surrounding material by the 

 excavation develops forces of such intensity that the timbering 

 or lining is crushed and the tunnel destroyed. To provide 

 against accidents of this kind in a theoretically perfect manner 

 would require the engineer to have an accurate knowledge of 

 the character, direction and intensity of the forces developed, 

 and this is practically impossible, since all of these factors differ 

 with the nature and structure of the material penetrated. The 

 best that can be done, therefore, is to determine the general 

 character and structure of the material penetrated, as fully as 

 practicable, by means of borings and geological surveys, and 

 then to employ timbering and masonry of such dimensions and 

 character as have withstood successfully the pressures devel- 

 oped in previous tunnels excavated through similar material. 

 If, despite these precautions, accidents occur, the engineer is 

 compelled to devise methods of checking and repairing them, 

 and it is the purpose of this chapter to point out briefly the 

 most common lands of accidents, their causes, and the usual 

 methods of repairing them. 



Accidents During Construction. Accidents may happen both 

 during or after construction, but it is during construction, when 

 the equilibrium of the surrounding material is first disturbed, 

 and when the only support of the pressures developed is the 

 timber strutting that they most commonly occur. 



