TUNNELING 



It should be understood that the dimensions given in the 

 table are those for ordinary conditions of leakage ; where larger 

 quantities of water are met with, the size of the culverts has, 

 of course, to be enlarged. To permit the water to enter the 

 culvert, openings are provided at intervals along its side ; and 

 these openings are usually provided with screens of loose stones 

 which check the current, and cause the suspended material to 



FIG. 48. Kefuge Niche in St. Gothard Tunnel. 



be deposited before it enters the culvert. In cases where 

 springs are encountered in excavating the tunnel, it is necessary 

 to make special provisions for confining their outflow and con- 

 ducting it to the culvert. In all cases the culverts should be 

 provided with catch basins at intervals of from 150 ft. to 300 

 ft., in which such suspended matter as enters the culverts is 

 deposited, and removed through covered openings over each 

 basin. At the ends of the tunnel the culvert is usually divided 

 into two branches, one running to the drain on each side of the 

 track. 



Niches. In short tunnels niches are employed simply as 

 places of refuge for trackmen and others during the passing of 

 trains, and are of small size. In long tunnels they are made 



