Figure 5-13.-Single-bench quarry.A single-bench quarry (fig. 5-13) is one having theentire floor on one level. The height of the bench willdepend on the reach of the equipment available. In theNCF, the recommended bench height is 10 feet;however, depending on the drilling equipment available,type of rock, magnitude of operation, and experience ofthe operating personnel, bench heights can range from8 to 40 feet.Military quarries are usually of the single-benchtype. This type offers greater safety and efficientoperation. All operations are on one level, a greateramount of rock is shot at one blast, and less equipmentis needed in the overall process. In addition, this typeof quarry requires less training for the operatingcrews.Blast trial shots are made in both existing and newquarries before installing equipment for two reasons.First, to avoid possible damage to installed equipmentand the second, in the case of new quarries, is that thetrial shots will provide necessary ballast to constructaccess roadsupon.and for foundations to place equipmentWARNINGBlasting must be supervised and controlleddirectly by a qualified blaster. Also, allpersonnel working around blasting should wearhard hats, safety goggles, dust respirators,earplugs, and hard toe safety shoes.A multiple-bench quarry (fig. 5-14) is one having aseries of ledges or terraces resembling steps. The highestbench is blasted and worked first. Then successive lowerlevels are simultaneously developed as the workprogresses and as each bench is required.Quarries are developed by the multiple-benchmethod when the face is too high for single shots,horizontal seams or separations are present, or deep andnarrow deposits exist. This method of developmentpermits equipment to be used simultaneously at more5-14
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