172 



TUNNKUNU 



a temporary masonry arch sprung between the side walls 

 with its feet near the bottom of the walls, and its crown, 

 just below the level of their tops, as shown by Fig. 101. 

 This construction was employed in the 

 Stazza tunnel in Italy. In this tunnel 

 the excavation was begun by driving the 

 center drift, No. 1, Fig. 98, and immedi- 

 ately strutting it as shown by Fig. 99. 

 The other parts, Nos. 2 and 3, completing 

 the lower portion of the section, were then 

 taken out and strutted. While part No. 2 

 FIG. 99. Sketch showing was being excavated at the bottom, and 



Methed of Strutting First ,v r ,1 L i_ 'ij. j.1. 



Drift, stazza Tunnel. the center part of the invert built, the 

 longitudinal crown bars carrying the roof 

 of the excavation were carried temporarily by the inclined 

 props shown by Fig. 100. After completing the invert and 

 the side walls to a height of 2 or 3 ft., a thick masonry arch 

 was sprung between the side walls, as shown in transverse 

 section by Fig. 101, and in longitudinal section by Fig. 100. 

 This arch braced the side walls against tipping inward, and 



FIGS. 100 and 101. Sketches Showing Temporary Strutting Arch Construction, 

 Stazza Tunnel. 



carried short struts to support the crown bars. The haunches 

 of the arch were also filled in with rammed earth. The upper 

 half of the section was excavated, strutted, and lined as in 

 the standard Italian method previously described. When the 

 lining was completed, the arch inserted between the side walls 

 was broken down and removed. 



