40 TUNNKLIM; 



(3) This method, as its name implies, consists in taking 

 out .short lengths of tli-' whole sectional profile before begin- 

 ning the buil.ling of Uie masonry. In tin* Knglish method 

 tin- lengths of section excavated vary from 10 ft. to 25 ft. 

 The masonry invert is luiilt first, then the side walls, and 

 finally the arch. The excavators and the masons work alter- 

 nately, the excavation being stopped while the masonry is 

 U-in^ built, and vice versa. The Austrian method differs in 

 two particulars from the Knglish: the length of section opened 

 is made great enough to allow the excavators to continue work 

 ahead of the masons, and the side walls and roof are built 

 U-fore the invert. 



(4) The Italian method is very seldom employed on account 

 of its e \pensiveuess, but it can often be used where the other 

 methods fail. It consists in excavating the lower half of the 

 section, and building the invert and side walls, and then filling 

 the space between the walls in again except for a narrow 

 -- _>\\..\ for the cars ; next the upper pert of the section is 



\ated.as iii the Belgian method, and the arch is built; and 

 finally the soil in the lower part is permanently removed. 



Tunnels in Quicksand. Tu:i:iels through quicksand are 

 driven ly one of the ordinary soft-ground methods after drain- 

 ing auay the water, or else as submarine tunnels. 



Open-Cut Tunnels. Open-cut tunnels are those driven at 

 such a small depth under the surface that it is more convenient 

 to excavate an ojien cut, build the tunnel masonry inside it, 

 and then refill the open spaces, than it is to carry on the work 

 entirely underground. In firm soils the usual mode of opera- 

 tion is to excavate first two parallel trenches for the side walls, 

 then remove the core, and build the arch and the invert. In 

 unstable soils, since the invert must be built first, it is usual 

 to open up a single wide trench. In infrequent cases where 

 a tunnel is desired in a place which is to be filled in, the 

 masonry is built as a surface structure, which in due time is 

 covered. 



