152 



TUNNELING 



of square timbers, as shown by Fig. 79. This construction of 

 the iron centers is shown by Fig. 80. Each of the iron centers 

 consisted of two 6 x 6 in. angles butted together, and bent into 

 the form of an arch rib. Six of these ribs were set up 4 ft. 

 apart. They were made of two half ribs butted together at the 

 crown, and were held erect and the proper distance apart by 



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Fro. 78. Sketch Showing Method of Excavating and Strutting Baltimore Belt 

 Line Tunnel. 



spacing rods. The rearmost rib was held fust to the completed 

 arch masonry, and in turn supported Uie forward ribs while the 

 lagging was being placed. 



Masonry. The side walls of the lining were built first in 

 the bottom side drifts, as shown by Fig. 78. They were gen- 

 erally placed on a foundation of concrete, from 1 ft. to 2 ft. 

 thick. As a rule the side walls were not built more than 20 

 ft. in advance of the arch, but occasionally this distance was 

 increased to as much as 90 ft. The roof arch consisted ordina- 



