166 TUNNELING 



the lengths of the consecutive openings of the full section, or 

 from 12 ft. to 20 ft. long. Except in infrequent cases in very 

 loose materials the invert is the last part of the masonry to be 

 built, since to build it first requires the removal of the strutting 

 which cannot easily or safely be accomplished until the side walls 

 and roof arch are completed. As the side wall foundations are 

 built, however, their interior faces are left inclined, as shown 

 by Figs. 90 and 91, ready for the insertion of the invert, and 

 are meanwhile kept from sliding inward by the insertion of 

 blocking between them and the bottom of the strutting. Fig. 

 91 shows the nature of this blocking, and also the manner in 

 which the side wall and roof arch mason ly is earned upward. 

 Finally when the roof arch is keyed and the centers are struck, 

 the strutting is taken down and the invert is built 



Advantages and Disadvantages. The principal advantages 

 claimed for the Austrian method of tunneling are : (1) The 

 excavation being conducted by driving a large number of con- 

 secutive small galleries, which are immediately strutted, there 

 is little disturbance of the surrounding material; (2) the 

 polygonal type of strutting adopted .is easily erected and of 

 great strength against symmetrical pressures ; (3) the masonry, 

 being built from the foundations up, is a single homogeneous 

 structure, and is thus better able to withstand dangerous pres- 

 sures ; (4) the excavation is so conducted that the masons 

 and excavators do not interfere, and both can work at the same 

 time. The disadvantages which the method possesses are : ( 1) 

 The strutting, while very strong under symmetrical pressures, 

 either vertical or lateral, is distorted easily by unsymmetrical 

 vertical or lateral pressures, and by pressure in the direction of 

 the axis of the tunnel ; (2) the construction of the invert last 

 exposes the side walls to the danger of being squeezed together, 

 causing a rotation of the arch of the nature discussed in de- 

 scribing the Belgian method of tunneling. 



