Costs of sinking a mine shaft vary with the size and depth of the shaft; nature of the rock (the ease with which it can be drilled and broken, and the nature and amount of support required); amount of water that
must be handled, speed of sinking, organization of the work, and the equipment employed; and labor and other local conditions.
The effect of most of these factors upon costs is self-evident, but effect of the rate of sinking is not so apparent. There is probably a certain rate of sinking that will vary somewhat with each job, which will result in the lowest cost per foot of shaft and below or above which this cost will increase.
If the exigencies of the case require the greatest possible speed in sinking, certain economies otherwise possible cannot be achieved. Speed work requires a high degree of organization and a routine, which, if temporarily interrupted, may disrupt the operations for some time; extra labor not normally required may be employed as an insurance against interruptions, which obviously adds to the cost. Furthermore, crowding the bottom of a shaft with a large crew does not make for economy.
A striking example of economy by slow-speed sinking is afforded by the experience at the Burra-Burra mine in Tennessee, where the three-compartment McPherson shaft was sunk 6 feet 7 inches by 16 feet inside the timbers. By sinking at a leisurely rate averaging a little over 2 feet per day, a saving of $50 per foot was made as compared to previous higher-speed sinking. During a typical 25-day working month when an advance of 57 feet was made, costs per foot were as follows:
No time was lost from powder sickness or bad air, since 8 hours elapsed between blasting and resumption of work in the bottom of the shaft. In earlier work higher costs were attributable to (a) large force of men in shaft, (b) unpleasant working conditions, (c) greater hazards, and (d) periodic upsets in organization.
Table 15 presents some typical shaft-sinking costs.
