18 



TUNNELING 



two tracks. In single-track tunnels a clear space of at least 2 

 ft. on each side should be allowed for between the tunnel wall 

 and the side of the largest standard locomotive or car, and a 

 clear space of at least 3 ft. should be allowed for between the 

 roof and the top of the same locomotive or car. Since the roof 

 of the tunnel is arch-shaped, to secure a clearance of 3 ft. at 

 every point will necessitate making the clearance at the center 

 greater than this amount. In double-track tunnels the same 

 amounts of side and roof clearances have to be provided for, 

 and, in addition, there has to be a clearance of at least 2 ft. 

 between trains passing on the two tracks. Referring to Fig. 8, 

 and assuming the line AB to represent the level of the tracks, 

 then the ordinary dimensions in feet required for both single- 

 and double-track tunnels are as follows : 



The thickness of the masonry lining to be allowed for varies 

 "with the material penetrated, as will be explained in a succeed- 

 ing chapter where the dimensions for various ordinary condi- 

 tions are given in tabular form. The lining masonry is subject 

 to deformation in three ways : by the sinking of the whole 

 masonry structure, by the squeezing together of the side walls 

 by the lateral pressures, and by the settling of the roof-arch. 

 The whole masonry structure never sinks more than three or 

 four inches, and merits little attention. The movement of the 

 side walls towards each other, which may amount to three 01 

 four inches for each wall without endangering their stability, 

 lias, however, to be allowed for ; and similar allowance must be 

 made for the settling of the roof-arch, which may amount to 

 from nine inches to two feet. 



