Fig. 8

Fig. 9

Fig. 10

Figs. 8 to 10.—Typical Sectional Profiles for Tunnel.

Dimensions of Section.

—The dimensions to be given to the cross-section of a tunnel depend upon the purpose for which it is to be used. Whatever the purpose of the tunnel, the following three points have to be considered in determining the size of its cross-section: (1) The size of clear opening required; (2) the thickness of lining masonry necessary; and (3) the decrease in the clear opening from the deformation of the lining.

Railway tunnels may be built either to accommodate one or two, three and four tracks. In single-track tunnels a clear space of at least 21⁄2 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. On the three- and four-track tunnels only the width varies while the height remains almost equal to the two track. Referring to [Fig. 7], 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:—

Height, D. F.Width, G. E.Height, C. F.Height, C. H.
Feet.Feet.Feet.Feet.
Single track17.6 to 1816.5 to 186   to 7.41⁄4 to 1⁄8 AB
Double track26.6 to 2826.6 to 286.3 to 6.91⁄4 to 1⁄8 AB