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TUNNKUM. 



tween the sills are all set vertically, but those at the extreme 

 sides of the roof section are inclined. In loose soils, however, 

 where the sides of the excavation must be shored, the V- 

 bracing shown by Fig. 82 is employed between one or more 

 pairs of sills as the conditions necessitate. The manner of 

 holding the transverse framework upright is explained quite 

 clearly by Fig. 83 ; inclined props extending from the com- 

 pleted masonry to the sills of the framework being employed. 

 Two props are used to each sill. Sometimes, in addition to the 



FiOS. 82 and 83. Sketches Showing Construction of Strutting, English Method. 



props shown, another nearly horizontal prop extends from the 

 en \vn of the arch masonry to the middle piece of the strutting. 

 Referring to Fig. 83, it will be observed that the longitudinal 

 crown bars are above the extrados of the roof arch. When, 

 therefore, the lining masonry has been completed close up to 

 the transverse framework, the latter is removed, leaving the 

 crown bars resting on the arch masonry ; and excavation, which 

 has been stopped while the masonry was being laid, is continued 

 for another 12 ft. to 20 ft., and the transverse framework is 

 erected at the face, and braced or propped against the completed 

 lining as shown by Fig. 83. The next step is to place the 



