TIMBERING OR STRUTTING; Tt'NNELS 



FIG. 31. Rziha's Combined Strutting and Centering 

 of Cut Iron. 



the poling-boards, running longitudinally, and spanning the 

 distance between consecutive arch ribs. By removing the l>ent 

 iron frames the cast-iron rib forms a center upon which to con- 

 struct the masonry. Fi- 

 nally, to ivniovr the cast- 

 iron rib itself, the sand 

 is drawn out of the mor- 

 tise and tenon joint in 

 the crown segment, which 

 allows the joint to close, 

 and loosen the segments 

 so that they are easily 

 unbutted. 



The illustration, Fig. 

 31, shows longitudinal 

 poling-boards; more often 

 longitudinal crown bars 

 of railway rails span the space between connective arch ribs, 

 and support transverse poling-boards. In building the masonry, 

 work is begun at the bottom on each side, the bent iron frames 

 being removed one after another to give room for the masonry. 



As each frame is removed, it is 

 replaced with a sort of screw- 

 jack to support the poling-boards 

 until the masonry is sufficiently 

 completed to allow their removal. 

 The interior bracing of the arch 

 rib shown at a a and b b consists 

 of railway rails carried by brack- 

 ets cast on to the segments. A 

 similar bracing of rails connects the successive arch ribs. These 

 lines of bracing serve to carry the scaffolding upon which the 

 masons work in building the lining. 



Iron Shaft Strutting. In soft-ground shaft work, tho use of 

 an iron strutting, consisting of consecutive cast-iron rings, has 



I -TnM-Iron Segment of Rziha's 



Strutting and Centering. 



