ACCIDKMS AND REPAIRS IN TUNNELS 275 



220 ft. from the south portal of the Giovi tunnel, a disturbance 

 of the masonry lining for a length of about 52 ft. was observed. 

 Accurate measurements showed that the lining was not sym- 

 metrical with respect to the vertical axis of the sectional profile. 

 It was concluded that owing to some disturlianre of the sur- 

 rounding soil unsymmetrical vertical and lateral pressures were 

 acting on the masonry. Close watch was kept of the dis- 

 torted masonry, which for some time remained unchanged 

 in position. In 1872, however, new crevices were observed 

 t > have developed, and shortly afterwards, in January, 1873, 

 tlie injured portion of the masonry caved in, obstructing 

 the whole tunnel section. The fallen material consisted 

 chiefly of clay in a nearly plastic state. The surface of the 

 ground above was observed to have settled. Investigation 

 showed also that the cause of the caving was the percolation of 

 water from a nearby creek. The water had soaked the ground, 

 and decreased its stability to such an extent that the masonry 

 lining was unable to withstand the increased vertical and lateral 

 pressures. 



The mode of procedure decided upon for repairing the 

 damage was: (1) To open at least one track for the temporary 

 accommodation of traffic ; (2) To remove permanently the causes 

 which had produced the collapse ; (3) To build a new and 

 much stronger lining. Close to the western side wall, which 

 was still standing, the debris was removed, and the opening 

 strongly strutted in order to allow the laying of a single 

 track to reestablish communication. At the same time a shaft 

 was sunk from the surface above the caved portion of the tunnel, 

 for the double purpose of facilitating the removal of the 

 fallen material and of sifl'ording ventilation. The depth of the 

 surface above the tunnel was 41.> ft., which made the construc- 

 tion of the shaft a comparatively easy matter. The shaft itself 

 wasfi.} ft. wide and 1 * ft. \ni\g. with its longer dimensions parallel 

 to the tunnel, and it was lined with a rectangular hori/.ontal 

 frame and vertical-poling board construction. After tern- 



