164 



TUNNELING 



of the top heading frame and the other beveled and resting on 

 the top of the sill a near the end. The timber b is laid tangent 

 to the curve of the roof arch, and to support it against flexure 

 the strut c is inserted as shown. To support the thrust of this 



I : 



strut the additional post d is 

 inserted and the original bot- 

 tom heading frame is rein- 

 forced as shown. The next 

 step is to insert the strut e, 

 and when this and the previ- 

 ous construction are dupli- 

 cated on the opposite side of 

 the tunnel section we have 

 the strutting of the parts Nos. 

 1 to 5, inclusive, complete. 

 Part No. 6 is then removed 

 and strutted by extending the 

 bottom drift cap-piece by a 

 timber similar to timber a above, and then by inserting a side 

 strut between the outer ends of these two timbers, as indicated 

 by Fig. 90. As the final parts, Nos. 7 and 8, are removed, the 

 inclined prop a, Fig. 90, is inserted as shown. When the soil 





PIGS. 88 to 90. Sketches Showing Construc- 

 tion of Strutting, Austrian Method. 



