The cost of tunneling is highly sensitive to the rock characteristics through which the tunnel is to be driven. These characteristics affect in a major way the rates at which tunnels and shafts can be driven and the degree to which the openings must be supported to maintain them in a permanent stable condition. Tunnel and shaft design and cost criteria have been computerized, permitting estimates of tunnel-shaft systems to be made directly from inputs of rock mechanics characteristics and other data. Curves of quantities of steel, rock bolts, and other rock support items as related to tunnel size and rock quality designation are presented. An illustrative example of the use of the program in calculating the cost of a hypothetical tunnel system is included.
The cost of tunnels is perhaps of as great interest to the designer as to the owner or financing agency. The reason for this intense interest is that the variation in cost of tunnels varies between wider limits than almost any other civil works. One does not have to look far to see costs varying from about $200 per linear foot to over ten times that amount for tunnels in rock in the 10 to 15 foot size range. When larger sizes of tunnels are considered, the range is seen to be even wider.
While the designer can not affect some tunnel costs, other costs are within his ability to change drastically, depending upon the nature of the requirements he establishes for the tunnel and upon the care with which he executes his design. To satisfy the need for obtaining tunnels costs sufficiently early in the design phase to be of value to the designer, the U.S. Department of Transportation sponsored the development of a computer program (called COHART) to yield the cost of tunnel-shaft systems (1). The present paper contains a description of the program, including an illustrative example of its use, and a review of the rock mechanics studies made during its development.
for appraisal and feasibility level studies of tunnel projects;
as a basis for the selection of a route with minimum cost, out of a number of alternative routes;
as a means of identification of the minimum-cost construction methods for a variety of tunnel designs and site conditions;
as a basis for trade-offs of alternative geometric tunnel design features (shape, depth, etc.) with end-use requirements; and
in designed tunnels, as a check of the reasonableness of the engineer's estimate, and of the design of the tunnel by comparison of the costs of the design with the costs of the standard design built into the computer program
The program is intended to be used to provide cost estimates:Though the program was originally intended for tunnels and shafts for mass transportation applications, there is no reason that it should not be used as an approximate guide to projects for other end uses.
Certain cautions should be observed in the use of the program.