168 



TUNNELING 



FIG. 92. Diagram Show- 

 ing Sequence of Excava- 

 tion in Italian Method of 

 Tunneling. 



the center bottom heading No. 1, and this is widened by taking 

 out parts No. 2. Finally part No. 3 is removed, and the lower 

 half of the section is open. As soon as the invert and side 

 wall masonry has been built in this excavation, parts No. 2 

 are filled in again with earth. The exca- 

 vation of the center top heading No. 4 is 

 then begun, and is enlarged by removing 

 the earth of part No. 5. The faces of this 

 last part are inclined so as to reduce their 

 tendency to slide, and to permit of a 

 greater number of radial struts to be 

 placed. Next, parts No. 6 are excavated, 

 and when this is done the entire section, 

 except for the thin strip No. 7, has been 

 opened. At the ends of part No. 7 nar- 

 row trenches are sunk to reach the tops of the side walls 

 already constructed in the lower half of the section. The 

 masonry is then completed for the upper half of the section, 

 and part No. 7 and the filling in parts No. 2 are removed. 



The various drifts and headings and ^_ -._^ 



the parts excavated to enlarge them 



are seldom excavated more than from / N \ 



/ 



6 ft. to 10 ft. ahead of the lining. 



Strutting. - - The bottom center 

 drift, which is first driven, is strutted 

 by means of frames consisting of side 

 posts resting on floor blocks and car- 

 rying a cap-piece. Poling-boards are 

 placed around the walls, stretching 

 from one frame to the next. As 

 soon as the invert is sufficiently completed to permit it, the 

 side posts of the strutting frames are replaced by short struts 

 resting on the invert masonry as shown by Fig. 93. To permit 

 the old side posts to be removed and the new shorter ones to 

 be inserted, the cap-piece of the frame is temporarily supported 



FIG. 93. Sketch Showing Strut- 

 ting for Lower Part of Section. 



