TABLE 13. Data for a large blast in a limestone quarry-Continued. The 12 snake holes required 548 pounds of "60 per cent" gelatin dynamite. A total of 7,130 feet of cordeau was used, the cost of which was $304.95. The following summary indicates the high efficiency of the shot: Results of shot. Total weight of explosives, pounds_-_ Total cost of explosives, including dynamite, fuse, and caps Weight of rock shot down, tons__ Cost of explosive, cents per ton of rock moved: 41, 525 $5, 568.37 242, 096 Primary shot__ Secondary shots Total cost of explosives. Quantity of rock moved per pound of explosive, tons___ 2.30 1.37 3. 67 5.83 Plate IV shows the quarry face prior to the blast, and Plate V, the new face and the mass of shattered rock resulting from the blast. Plate VI illustrates the blast itself, photographed at the moment of detonation. GENERAL CONSIDERATIONS IN BLASTING. PROPER BALANCE BETWEEN EXPLOSIVES COST AND DRILLING COST. A given quantity of rock may be broken with less explosive by drilling a large number of small holes closely spaced than with a smaller number of larger and more widely spaced holes. Therefore, within certain limits the cost of explosives evidently will decrease as the cost of drilling increases, and vice versa. If either factor is overemphasized, blasting efficiency is decreased. It is important, therefore, that the quarryman attain a proper balance between these two costs. The soundness of the rock, the rate of drilling, the ease or difficulty of shattering, the method of loading rock, and other considerations must all be taken into account. A ratio of drilling costs to explosives costs that might prove the cheapest in one quarry might not be suited to another quarry. The quarryman must determine by experiment the relative costs of drilling and explosive that will give the lowest aggregate cost of the two. PROPER BALANCE BETWEEN BLASTING EFFICIENCY AND ROCK-LOADING EFFICIENCY. All quarrymen know that the size of a charge must be so regulated that the rock will not be thrown too far. Aside from obstructing and damaging of tracks, a wide, thin mass of rock is more expensive to load with a steam shovel than a thicker mass which covers a smaller area. It may be assumed that loading can be done most efficiently when the face of the rock pile stands at its maximum angle of repose, that is, as steep as the fragments will lie without rolling down. This condition is most easily attained in comparatively low benches where "blanket" blasting may be employed. The buffer of shattered rock prevents fragments scattering and thus aids in maintaining a steep face. With a high face the proper adjustment of the charge, both as to spacing of drill holes and quantity of explosive used, is a more difficult matter. Moreover, there is an intimate relation between rate of loading and efficiency in blasting. If light charges are employed the explosives costs may be low, but the rock may be so imperfectly shattered that loading is repeatedly delayed while blocks ahead of the steam shovel are being broken by secondary blasting. Owing to such delays, the daily tonnage loaded with each shovel may be small. PROPER BALANCE BETWEEN SIZE OF BLAST AND SIZE OF STEAM SHOVEL, In some quarries where "blanket" blasting is employed on a low face, blasting may precede steam-shovel loading by a long period of time. Under such conditions the two operations are independent of each other insofar as size of blast and width of steam-shovel cut are concerned. In other quarries steam-shovel loading follows blasting closely. In blasting on a high face the rock brought down by one blast is all removed before another mass is shot down, but in "blanket " blasting on a low face, a definite width of shattered rock may be left for a buffer. In either case the position of the steamshovel tracks for the final cut is guided by the position of the face. As pointed out on a subsequent page (p. 123), one of the necessary |