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Author: Admin | 2025-04-28
RecoveryNote thatThe cutoff grade will increase as the costs increase is shownin Fig. 8.The difference between the mining cutoff grade and themilling cutoff grade is shown in Fig.9.2 February 2016 Prof. Dr. H.Z. Harraz PresentationMining Methods, Surface mining 28. 2 February 2016 Prof. Dr. H.Z. Harraz PresentationMining Methods, Surface mining 29. Geometry of a Working BenchShovelTruck WidthBack-upTruckTruckTurningradiusBerm widthShovelLengthA bench big enoughTo accommodateEquipment working isMuch wider than oneOnly intended to catchRolling rocks. 30. Impact of a Working BenchOver-allSlopeThe over-all slope of the pit is drasticallyReduced if one must accommodate wideWorking benches. 31. 1.7) Open Pit StabilityThe following are the key items affecting theOpen Pit Stability:i) Pit slopeii) Pit wall stabilityiii)Rock strengthiv)Pit Depthv) Pit diametervi)Water Damagevii)Strip Ratio (SR)February 2, 2016Prof. Dr. H.Z. Harraz Presentation 32. i) Pit Slopes The slope of the pit wall is one of the major elements affecting the size andshape of the pit. The pit slope helps determine the amount of waste that must be moved tomine ore. The pit wall needs to remain stable as long as mining activity is in that area. The stability of the pit walls should be analyzed as carefully as possible. Rock strength, faults, joints, presence of water, and other geologicinformation are key factors in the evaluation of the proper slope angle. The physical characteristics of the deposit cause the pit slope to changewith rock type, sector location, elevation, or orientation within the pit. Pit slopes are cut into benches to aid stability and contain any slopefailures. Rock most be stronger than sand so the angle of repose can be larger. 45° is usually the maximum slope. Pit slopes are benched.• The revenue from ore must pay for the cost of excavating waste from thepushback and for excavating the ore.• The slope cannot exceed 45° and remain stable, so at some point itbecomes impossible and/or uneconomic to continue mining.2 February 2016 Prof. Dr. H.Z. Harraz PresentationMining Methods, Surface mining 33. Fig. 2 illustrates how the pit slopes may vary in the deposit. A proper slope evaluation will give the slope that allows the pit walls to remain stable. The pit walls should be set as steep as possible to minimize the strip ratio. The pit slope analysis determines the angle to be used between the roads in the pit. The overall pit slope used for design must be flatter to allow for the road system in the ultimate pit. Fig 3 and Fig 4 show the need to design the pit with a lesser slope to allow for roads: Fig. 3 has been designed with a 450 angle for the pit walls. The pit in Fig. 4 uses the same pit bottom and the 450 inter-ramp slope between the roads, but, aroad has been added. So the overall pit slope is lesser the inter-ramp slope. In the example, almost 50% more tonnage must be moved to mine the same pit bottom. In the early design of a pit a lesser pit slope can be used to allow for the road system.
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