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The influence of geologic logs of drill holes on tunnel projects is considerable. Based on the geological logs, the rock is classified and engineers then use formulas by Terzaghi, Deere, Wickham, Barton, or others as a means of obtaining a "rock load" on the tunnel. For this paper, the "rock load" is considered to be supported by steel ribs. Engineering calculations for the steel ribs required to support a given rock load result in given rib sizes and weight and spacing, which can then be cost estimated by the design engineer to get an Engineer's Estimate. The Owner's budget is usually based on the design engineer's estimate of tunnel cost.
Additional factors influencing tunnel cost, based on the geologic logs, are the timber lagging required and the overbreak concrete required in increasingly bad ground. The somewhat slower rate of advance in increasingly bad ground also affects the tunnel cost. All of these factors have been parametrically evaluated by Tudor Engineering Company. For all the rock load formulas studied, rock loads become severe (i.e., "expensive") only when the rock core was classified as "VERY BLOCKY AND SEAMY" or equivalent. Terzaghi's method, being simpler than Wickham's or Barton's, is used by Tudor Engineering Company.
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Tunnel designs in rock can proceed logically as shown below:
Obtain representative rock samples.
Classify rock, based on jointing, weathering, etc.
Calculate the load this rock will exert on tunnel.
Design a support for this rock load.
Make a cost estimate of this tunnel support system.
Karl Terzaghi in 1946 (4) published his rock classification scheme with formulas for calculating the rock load for each classification (Fig. 1). His formulas assumed the tunnels were below the water table and stated that if they were not, rock loads could be reduced by half. Put in another way, Terzaghi doubled the rock load due to the effect of ground water. Tor Brekke in 1968 (2) pointed out that real tunnels below the water table behave quite well and that Terzaghi's assumption that the water acted to increase rock loads was conservative.
Deere in 1969 (3) published a large number of field measurements indicating that Terzaghi's formulas were conservative, and recommended reducing Terzaghi's computed rock loads by 20 percent for steel ribs in drilland-blast-tunnels.
Wickham, et al., in 1974 (6) and Barton in 1974 (1) devised new formulas for computing rock loads. Each has a large number of parameters.
Tudor Engineering Company made parametric studies of all of the above methods, using Terzaghi's classification as a basis (as though NX cores had been obtained for tunnels for each class of rock). It was found that a reasonable difference of opinion among qualified geologists using the Wickham and Barton methods could lead to wider extremes of rock load than by using the Terzaghi system, due to the complexity and large number of parameters in Wickham and Barton. See Fig. 1 and 2.
