Fig. 157.—Shortening Tunnel Crushed by Landslide at Portal.
Accidents After Construction.
—Accidents after the completion of the tunnel may be divided into two classes: first, those which entirely obstruct the passage of trains, of which the collapse of the roof is the most common; and second, those which allow traffic to be continued while the repairs are being made, 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 débris, 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 running through this temporary passageway the other half of the section is opened and repaired; the traffic is then shifted to the new permanent track, and the temporary structure first employed is replaced with a permanent lining. When the accident is such that the repairs can be made without obstructing traffic entirely, various 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.
Fig. 158.—Extending Tunnel through Landslide at Portal.
Giovi Tunnel Accident.
—In September, 1869, at a point about 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 symmetrical with respect to the vertical axis of the sectional profile. It was concluded that owing to some disturbance of the surrounding soil unsymmetrical vertical and lateral pressures were acting on the masonry. Close watch was kept of the distorted masonry, which for some time remained unchanged in position. In 1872, however, new crevices were observed to have developed, and shortly afterwards, in January, 1873, the 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 débris was removed, and the opening strongly strutted in order to allow the laying of a single track to reëstablish 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 affording ventilation. The depth of the surface above the tunnel was 41.6 ft., which made the construction of the shaft a comparatively easy matter. The shaft itself was 61⁄2 ft. wide and 18 ft. long, with its longer dimensions parallel to the tunnel, and it was lined with a rectangular horizontal frame and vertical-poling board construction. After temporary communication had been opened on the western track of the tunnel, the remainder of the fallen earth was removed and the excavation strutted. The new masonry lining was then built.
To remove permanently the cause of the cave-in, which was the percolation of water from a close-by stream, this stream was diverted to a new channel constructed with a concrete bed and side walls.
The failure of the original lining occurred by cracks developing at the crown, haunches, and springing lines. The new lining was made considerably thicker than the original lining, and at the points where failure had first occurred in the original arch cut-stone voussoirs were inserted in the brickwork of the new arch as described in [Chapter XIII].