SPECIAL TREACHEROUS GROUND METHOD 169 



by inclined props arranged as shown by Fig. 97. When parts 

 No. 2 are excavated the roof is strutted by inserting the trans- 

 verse caps a, Fig. 98, the outer ends of which are carried by the 

 system of struts ft, e, rf, and e. The longitudinal poling-boards 

 supporting the ceiling and walls are held in place by the cap 

 a and the side timber e. To stiffen the frames longitudinally 

 of the tunnel, horizontal longitudinal struts are inserted between 

 them. 



The excavation of the upper half of the tunnel section is 

 strutted as in the Belgian method, with radial struts carrying 

 longitudinal roof bars and transverse poling-boards. On ac- 

 rmmt of the enormous pressures developed by the treacherous 

 soils in which only is the Italian method employed, the radial 

 strutting frames and crown bars must be of great strength, 



Pios. 94 and 06. Sketches Showing Construction of Centers, Italian Method. 



while the successive frames must be placed at frequent intervals, 

 usually not more than 3 ft. After the masonry side walls have 

 been built in the lower part of the excavation, longitudinal 

 planks are laid against the si<le posts of tin- renter bottom 

 drift frames, to form an enclosure for the filling-in of parts 

 No. 2. The object of this filling is principally to prevent 

 the squeezing-in of the side walls. 



Centers. Owing to the great pressures to l*> resisted in the 

 treacherous soils in which the Italian method is used, the con- 

 struction of the centers has to be very strong and rigid. Figs. 

 94 and 95 show two common ty|>08 of center construction MM-.I 

 with this method. The construction shown in Fig. 94 is a 

 strong one where only pressures normal to the axis of the 

 tunnel have to be withstood, but it is likely to twist under 



