7:2 TUNNELING 



yielding soils a special foundation bed has to be prepared for 

 the foundation masonry. In some instances it is found suffi- 

 cient to lay a course of planks upon \\hirh the masonry is con- 

 structed, but a more solid construction is usually preferred. 

 This is obtained by placing a concrete footing from 1 ft. to 2 

 ft. deep all along the bottom of the foundation trench, or in 

 some cases by sinking wells at intervals along the trench and 

 filling them with concrete, so as to form a series of supporting 

 pillars. 



The form given to the foundation courses and lower 

 portions of the side walls varies. Where 

 a large bearing area is required, the back 

 of the wall is carried up vertically as 

 shown by the line AB, Fig. 45, otherwise 

 the rear face of the wall follows the line of 

 excavation AC. For similar reasons the front 

 _____^^ face of the wall may be made vertical, as at 

 " G Fa, or inclined, as at FH. The line FE 



FIG. 45. Diagram 



showing Forms indicates the shelf construction designed 



Adopted for Side- .1 < r .\_ i , 



wan Foundations. to support the feet of the posts used to 

 carry the arch centers during the construc- 

 tion of the roof arch. 



Side Walls. The construction of the side walls above the 

 foundation courses is carried out as any similar piece of 

 masonry elsewhere would be built. To direct the work and 

 insure that the inner faces of the walls follow accurately the 

 curve of the chosen profile, leading frames previously described 

 are employed. 



Roof Arch. For the construction of the roof arch, the 

 centers previously described are employed. Beginning at the 

 edges of the center on each side, the masonry is carried up a 

 course at a time, care being taken to have it progress at the 

 same rate on both sides, so that the load brought onto the 

 centering is symmetrical. As soon as the centers are erected, 

 the roof strutting is removed, and replaced by short props 



