GENKUAI, MKTHIS <>F KXCAVATION 39 



an arbitrary arrangement adopted for the sake of order and 

 convenience in treating the subject. 



Tunnels in Hard Rock. With the numerous labor-saving' 

 methods and maehines now available, hard rock is perhaps the 

 safest and easiest of all materials through which to drive & 

 tunnel. Tunnels through hard rock may be excavated, either 

 by a drift or by a heading. The difference depends upon 

 whether the advance gallery is located close to the floor or 

 near the soffit of the section. 



Tunnels in Loose Soils. In driving tunnels through loose 

 soils many different methods have been devised, which may be 

 grouped as follows: (1) Tunnels excavated at the soffit 

 Belgian method ; (2) tunnels excavated along the perimeter 

 German method; (3) tunnels excavated in the whole sec- 

 tion English and Austrian methods; (4) tunnels excavated 

 in t\vo halves independent of each other Italian method. 



(1) Excavating the tunnel by beginning at the soffit of 

 the section, or by the Belgian method, is the method of tunnel- 

 ing in loose soils most commonly employed in Europe at the 

 present time. It consists in excavating the soffit of tin* 

 section first; then building the arch, which is supported npo:i 

 tin- unexcavated ground; and finally in excavating the lower 

 portion of the section, and building the side walls and 

 invert 



(2) In excavating tunnels along the perimeter an annular 

 excavation is made, following closely the outline of the sec- 

 tional profile in which the lining masonry is built, after whirli 

 the center core is excavated. In the (iennan method two> 

 drifts are opened at each side of the tunnel near the l>ottom. 

 Other drifts are excavated, one above the other, on each sido 

 i" extend or heighten the first two until all the perimeter is 

 open except across the l>ottom. The masonry lining is then 

 built from the bottom upwards on each side to the crown of 

 the arch, and then the center core is removed and the invert 

 is built 



