148 



TUNNELING 



the method of strutting the side trenches when the excavation 

 is carried downward from the center top heading instead of 

 upward from bottom side drifts. 



Masonry. Whatever plan of excavation or strutting is 

 employed, the construction of the masonry lining in the German 

 method of tunneling begins at the foundations of the side walls 

 and is carried upward to the roof arch. The invert, if one is 

 required, is built after the center core of earth is removed. 



Centering. Tunnel centers are generally employed in the 

 German method of tunneling, a common construction being 



shown by Fig. 77. It is essen- 

 tially a queen-post truss, the tie 

 beam of which rests on a transverse 

 sill as shown by the illustration. 

 The transverse sill is supported 

 along its central portion by the 

 unexcavated center core of earth, 

 and at its ends either directly on 

 the vertical posts or on longitudi- 

 nal beams resting on these posts. 

 The diagonal members of the 

 queen-post truss form the bottom 

 chords of small king-post trusses 



which are employed to build out the exterior member of the 

 center to a closer approximation to the curve of the arch. 



Hauling. When the bottom side drift plan of excavation 

 is employed, the spoil from the front of the drift is removed in 

 narrow-gauge cars running on a track laid as close as practicable 

 to the center core. These same cars are also employed to take 

 the spoil from the drifts above, through holes left in the ceiling 

 strutting of the bottom drifts. The spoil from the soffit sec- 

 tions may be removed by the same car lines used in excavating 

 the drifts, or a narrow-gauge track may be laid on the top of the 

 center core for this special purpose. In the latter case the soffit 

 tracks are usually connected by means of inclined planes witli 



PIG. 77. Sketch Showing Roof Arch 

 Centers and Arch Construction. 



