2 TUNNELING 



of maintenance, owing to the protection of the track from snow 

 and rain and other natural influences causing deterioration ; 

 and the decreased cost of hauling due to the lighter grades. 

 Against the tunnel, there are its enormous cost as compared 

 with an open road and the great length of. time required to 

 construct it. 



To determine in any particular case whether a tunnel or an 

 open road is best, requires a careful integration of all the factors 

 mentioned. It may be asserted in a general way, however, that 

 the enormous advance made in the art of tunnel building has 

 done much to lessen the strength of the principal objections to 

 tunnels, namely, their great cost and the length of time required 

 for their construction. Where the choice lies between a tunnel 

 or a long detour with heavy grades it is sooner or later almost 

 always decided in favor of a tunnel. When, however, the con- 

 ditions are such that the choice lies between a tunnel or a 

 heavy open cut with the same grades the problem of deciding 

 between the two solutions is a more difficult one. 



It is generally assumed that when the cut required will have 

 a vertical depth exceeding 60 ft. it is less expensive to build 

 a tunnel unless the excavated material is needed for a nearby 

 embankment or fill. This rule' is not absolute, but varies 

 according to local conditions. For instance, in materials of 

 rigid and unyielding character, such as rock, the practical limit 

 to the depth of a cut goes far beyond that point at which a 

 tunnel would be more economical according to the above rule. 

 In soils of a yielding character, on the other hand, the very 

 flat slope required for stability adds greatly to the cost of 

 making a cut. 



It may be noted in closing that the same rule may be em- 

 ployed in determining the location of the ends of the tunnel, 

 for assuming that it is more convenient to excavate a tun- 

 nel than an open cut when the depth exceeds 60 ft., then 

 the open cut approaches should extend into the mountain- or 

 hill-sides only to the points where the surface is 60 ft. above 



