136 TUNNELING 



they have to be excavated in such a way as not to endanger it. 

 At intervals along the central trench No. 4, transverse or side 

 trenches about 2 ft. wide are excavated on both sides, and 

 struts are inserted to support the masonry previously supported 

 by the earth which has been removed. The next step is to 

 widen these side trenches, and insert struts until all of the 

 material in parts No. 5 is taken out. 



When the material penetrated is firm enough to permit, the 

 plan of excavation illustrated by the diagram, Fig. 62, is substi- 

 tuted for the more typical one just described. The only differ- 

 ence in the two methods consists in the plan of excavating the 

 upper part of the profile, which in the second method consists 

 in driving first the center top heading No. 1, and then in tak- 

 ing out the remainder of the section above the springing lines 

 of the arch in one operation, while in the first method it is done 

 in two operations. The distance ahead of the masonry to 

 which the various parts can be driven varies from 10 ft. to, in 

 some cases, 100 ft., being very short in treacherous ground, and 

 longer the more stable the material is. 



Strutting. The longitudinal method of strutting, with the 

 poling-boards running transversely of the tunnel, is always 

 employed in the Belgian method of tunneling. In driving the 

 first center top heading, pairs of vertical posts carrying a trans- 

 verse cap-piece are erected at intervals. On these cap-pieces 

 are carried two longitudinal bars, which in turn support the 

 saddle planks. As fast as part No. 2, Fig. 61, is excavated, 

 the vertical posts are replaced by the batter posts A and B, 

 Fig. 63. The excavation of parts No. 3 is begun at the top, 

 the poling-boards a and I being inserted as the work pro- 

 gresses. To support the outer ends of these poling-boards, the 

 longitudinals X and Y are inserted and supported by the batter 

 posts (7 and D. In exactly the same way the poling-boards c 

 and cZ, the longitudinals F"and IF", and the struts E and F, are 

 placed in position ; and this procedure is repeated until the 

 whole top part of the section is strutted, as shown by Fig. 63, 



