100 TUNNELING 



is used only where necessary, and in that case is so built as to 

 form part of the lining of the tunnel when eventually com- 

 pleted. Concrete is put in to form a foundation for the side 

 wall, and a water channel. The cross-headings, connecting the 

 two parallel headings, occur every 220 yds., and are placed at 

 an angle of 56 to the axis of the tunnel, to avoid sharp curves 

 in the contractors' railway lines. They will eventually be used 

 as much as possible for refuges, chambers for storing the tools 

 and equipment of the platelayers, and signal-cabins. The ref- 

 uges, 6 ft. 7 ins. wide by 6 ft. 7 ins. high and 3 ft. 3 ins. deep, 

 occur every 110 yards, every tenth being enlarged to 9 ft. 10 

 ins. wide by 9 ft. 10 ins. deep and 10 ft. 2 ins. high, still larger 

 chambers being constructed at greater intervals. 



Method of Excavation. The work at each end of the tunnel 

 is carried on quite independently, consequently, though similar 

 in principle, the methods vary in detail, apart from the fact that 

 different geological strata require different treatment. Broadly 

 speaking, the two parallel headings, each 59 sq. ft. in section, 

 are first driven by means of drilling-machines and the use of 

 dynamite, this work being carried on day and night, seven days 

 in the week ; No. 1 heading is then enlarged to full size by 

 hand-drilling and dynamite. On the Italian side, where the 

 rock is hard and compact, breakups are made at intervals of 

 50 yds., and a top gallery is driven in both directions, but, for 

 ventilation reasons, is never allowed to get more than 4 yds. 

 ahead of the breakup, which is gradually lengthened and 

 widened to the required section. No timbering is required, 

 except to facilitate the excavation and the construction of the 

 side walls. Steel centers are employed for the arch ; they entail 

 fewer supports, give more room, and are capable of being used 

 over again more frequently, without damage. They consist 

 of two I-beams bent to a template and riveted together at the 

 crown, resting at either side on scaffolding at intervals of 6 ft. ; 

 longitudinals, 12 ft. by 4 ins. by 4 ins., support the roof. Hand 

 rock-drilling is carried out in the ordinary way, one man holding 



