156 TUNNELING 



CHAPTER XV. 



THE FULL SECTION METHOD CF TUNNELING; 

 ENGLISH METHOD ; AUSTRIAN METHOD. 



ENGLISH METHOD. 



THE English method of tunneling through soft ground, as 

 its name implies, originated in England, where, owing to the 

 general prevalence of comparatively firm chalks, clays, shales, 

 and sandstones, it has gained unusual popularity. The dis- 

 tinctive characteristics of the method are the excavation of the 

 full section of the tunnel at once, the use of longitudinal strut- 

 ting, and the alternate execution of the masonry work and 

 excavation. In America the method is generally designated as 

 the longitudinal bar method, owing to the mode of strutting, 

 which has gained particular favor in America, and is commonly 

 employed here even when the mode of excavation is distinc- 

 tively German or Belgian in other respects. 



Excavation. Although, as stated above, the distinctive 

 characteristic of the English method js the excavation of the 

 full section at once, the digging is usually started by driving 

 a small heading or drift to locate and establish the axis of the 

 tunnel, and to facilitate drainage in wet ground. These ad- 

 vance galleries may be driven either in the upper or in the 

 lower part of the section, as the local conditions and choice 

 of the engineer dictate. Whether the advance gallery is located 

 at the top or at the bottom of the section makes no difference in 

 the mode of enlarging the profile. This work always begins 

 at the upper part of the section. A center top heading is 

 driven and strutted by erecting posts carrying longitudinal bars 

 supporting transverse poling-boards. This heading is imme- 



