102 TUNNELING 



6 ft. 6 ins. is excavated and timbered. Finally the crown is 

 cleared out, heavy wooden centers are put in, the arch is turned, 

 and all timbers are withdrawn except the top poling-boards, 

 supporting the loose rock. 



The masonry for the side walls is obtained either from the 

 tunnel itself or from a neighboring quarry, and varies in char- 

 acter according to the pressure ; but the face of the arch is al- 

 ways of cut or artificial stones, the latter being of 7 to 1 cement 

 concrete. Where the alinement heading, or the "gallery of 

 direction," joins the curving portion of tunnel No. 1, the section 

 is very much greater, and necessitates special timbering. 



Transport (Italian Side). A small line of railway, 2 ft. 7$ 

 ins. gauge, with 40-lb. rails, enters all three portals ; but since 

 the construction of a wooden bridge over the Diveria, the route 

 through the " gallery of direction," across heading No. 2, to 

 tunnel No. 1, is used exclusively; this railway leads to the face 

 in both headings, and, where convenient, from one heading to 

 the other by the cross-galleries. Different types of wagons are 

 in use ; but in general they are four-wheeled, non-tipping box 

 wagons, supplied with brakes and holding 2 cu. yds. of debris. 

 A special type of locomotive is used, designed to pass round 

 curves of 50 ft. radius, and supplied with a specially large boiler 

 to avoid firing in the tunnel. 



Method of Working. The drilling-machines employed are of 

 the Brandt type, Fig. 57, and are mounted in the following 

 manner : A small four-wheeled carriage supports at its center 

 a beam, the shorter arm of which carries the boring mechanism 

 and the longer a counterpoise ; near its center is the distributor. 

 In the short arm is a clamp holding the rack-bar or butting 

 column, which is a wrought-iron cylinder with a plunger con- 

 stituting a ram, and is jammed by hydraulic pressure between 

 the walls of the heading, thus forming a rigid support for the 

 boring-machine, and an efficient abutment against the i eaction 

 of the drill. This rack-bar can be rotated on its cl inp in a 

 plane parallel to the axis of the beam. Three or four separate 



