178 



TINNKLIM; 



altogether. The method of construction by the pilot system is 

 as follows : 



Shafts large enough for the proper conveyance of materials 

 from and into the tunnel are sunk at such places on the line of 

 work as are most convenient for the purpose. From these 

 shafts a small tunnel, technically a pilot, about 6 ft. in diameter, 

 composed of rolled boiler iron riveted to light angle irons on 

 four sides, perforated for bolts, and bent to the required radius 

 of the pilot, is built into the central part of the excavation on 

 the axis of the tunnel. This pilot is generally kept about 30 ft. 

 in advance of the completed excavation, as shown by Fig. 105. 

 The material around the exterior of the pilot is then excavated, 

 using the pilot as a support for braces which radiate from it and 



Bracing." ' Arch Construction. 



FIG. 105. Sketch Showing Pilot Method of Tunneling. 



secure in position the plates of the outside shell which holds 

 the sand, gravel, or other material in place until the concentric 

 rings of brick masonry are built. Ribs of T-iron bent to the 

 radius of the interior of the brick work, and supported by the 

 braces radiating from the pilot, are used as centering supports 

 for the masonry. On these ribs narrow lagging-boards are laid 

 as the construction of the arch proceeds, the braces holding the 

 shell plates and the superincumbent mass being removed as the 

 masonry progresses. The key bricks of the arches are placed 

 in position on ingeniously contrived key-boards, about 12 ins. in 

 width, which are fitted into rabbeted lagging-boards one after 

 another as the key bricks are laid in place. After the masonry 

 has been in place at least twenty-four hours, allowing the cement 



