The Status Of Rock Mechanics In Blasting

DRILLING CONSIDERATIONS In considering blasting it is necessary to consider the drilling and explosives since one is generally concerned with the total cost of muck of a certain size in the pit or through the crusher. To get a current idea of the drilling situation in open pit mines plotted in Fig. 1 are the drilling costs per ton of material vs. hole diameter for rotary drill holes in large limestone, iron ore and taconite operations. The exception is the 2-in. diam hole which was drilled with a percussion drill in limestone. Each curve in Fig. 1 represents a different operation and the data cover many months of production drilling. The costs are total operating and maintenance and do not include machine depreciation. As the hole size is increased so are the machine size and the investment. Providing the tonnage is available to be drilled, then as the machine size goes up so does the productivity-markedly-and this same shape of curve applies. In some instances it is even steeper. The interesting thing to note is that in each case studied there is a substantial drop-off in cost/ton as the hole size is increased. This is true for the softest material considered, limestone, through to the hardest, taconite. How has this come about? In going from 10-in. to 123-in. diam holes, the powder factors have generally stayed about the same and the same type of muck has been produced. To date this has been mainly in the softer materials but it has also been steadily taking hold in the harder ones. On this basis, then, patterns are generally expanded as the ratio of the hole diameters or close to it, if the explosive type is kept the same. In the harder rocks the cost per foot of hole has increased somewhat so that the overall reduction in drilling cost per ton is about 25 pct. In somewhat weaker rocks of about 25,000 psi compressional strength the drilling cost per