RKLINING TIMBER-LINED TUNNELS WITH MASONRY 281 



with a full center circular arch of four rings of brick laid in 

 rowlock form. When greater strength was needed the thick- 

 ness of the side walls was increased to 30 ins. and that of the 

 arch to six rings of brick. 



The first plan adopted in putting in the masonry was to 

 remove all the timbering ; but owing to the large number of 

 falls and slides this was abandoned, and the plan followed was to 

 leave in the three roof segments of the timbering with the over- 

 lying cord-wood packing and debris. In carrying on the work 

 the first step was to remove the side timbers. This was done 

 by supporting the roof timbers, as shown in Fig. 141 ; that is, 

 the first and fourth arch rib of an 8-ft. section containing four 



OOM Section. 

 KI..S. 141 and 142. Relining Timber-Lined Tunnel. 



arch ribs were supported by temporary posts. The intermedi- 

 ate an-h ribs were supported against the downward pressure by 

 6 X 6 in. timbers, extending from the side ribs near the tops 

 of the temporary posts to the opposite sides of the intermediate 

 roof segments, as shown in the longitudinal section, Fig. 142. 

 '!' resist the pressure from the sides, 4x6 in. braces were 

 placed across the tunnel from near the center of the intermedi- 

 ate segments t tin- II|.|MT ends of tin- hip sf^incnts. as >ho\\ n 

 in the cross-section, Fig. 141. The hip segments were then 

 sawed off below the notch, and the side timbering removed and 

 the masonry built 



The stone was conveyed into the tunnel on flat cars, and laid 

 by means of small derricks located on the cars. Two derricks 



