300 TUNNELING 



CHAPTER XXV. 



THE COST OF TUNNEL EXCAVATION AND 

 THE TIME REQUIRED FOR THE WORK. 



Cost. THE cost of a tunnel will depend upon the cost of 

 the two principal operations required in its construction, viz., 

 the excavation of the cross section and the lining of the exca- 

 vation with masonry, metal, or timber. These two operations 

 may in turn be subdivided, in respect to expense, into cost of 

 labor and cost of materials. It is a comparatively simple mat- 

 ter to calculate the cost of the building materials required to 

 construct a tunnel ; but it is very difficult to estimate with 

 accuracy what the cost of labor will be. The reason for this is 

 that it is impossible to foresee exactly what the conditions will 

 be ; the character of the material may change greatly as the 

 work proceeds, increasing or decreasing the cost of excavation ; 

 water may be encountered in quantities which will materially 

 increase the difficulties of the work, etc. Nevertheless, while 

 accurate preliminary estimates of cost are not practicable, it is 

 always desirable to attempt to obtain some idea of the probable 

 expense of the work before beginning it, and the more usual 

 means of getting at this point will be discussed here. 



Two methods of estimating the cost of tunnel work are em- 

 ployed. The first is to calculate the probable expense of the 

 various items of work, based upon the available data, per unit 

 of length, and then add to this a margin of at least 10% to allow 

 for contingencies ; the second is to apply to the new work the 

 unit cost of some previous tunnel built under substantially the 

 same conditions. In the first method it is usual to consider 

 the strutting and hauling as constituting a part of the work of 



