METHODS OF LINING TUNNELS 



77 



larger, and are also employed as places for storing small tools 

 and supplies employed in the maintenance of the tunnel. 

 Niches are simply arched recesses built into the sides of the 

 tunnel, and lined with masonry ; Fig. 48 shows this construc- 

 tion quite clearly. Small refuge niches are usually built from 

 6 ft. to 9 ft high, from 3 ft. to 6 ft. wide, and from 2 ft 

 to 3 ft deep. Large niches designed for storing tools and 

 supplies are made from 10 ft to 12 ft. high, from 8 ft to 10 ft. 

 wide, and from 18 ft to 24 ft deep, and are provided with 



Fio. 49. Kant Portal of Iloosac Tunnel. 



doors. Refuge niches are usually spaced from 60 ft. to 100 ft. 

 apart, while the larger storage niches may be located as far as 

 8000 ft. apart. The niche construe (ion shown by Fig. 47 is 

 that employed ( ,n the St Gothard tunnel. 



Entrances. The entrances, or portals, of tunnels usually 

 consist of more or less elaborate masonry structures, depending 

 upon the nature of the material penetrated. In soft-ground 

 tunnels extensive wing walls are often required to sup]Hrt the 

 earth above and at the sides of the entrance; while in tunnels 



