Bench heights and widths in open pit mines are determined based on a combination of geological, geotechnical, operational, and economic factors:
Bench height determination
- Geotechnical and rock properties: bench height depends largely on the strength and stability of the rock mass. The bench face angle and height must be designed to avoid instability and collapse risks. Typically, bench heights range from about 5 to 20 meters, with common values being 5, 8, 10, 12, 15, and 20 meters.
- Equipment capabilities: the height is often synchronized with the technical parameters of the mining equipment, such as the maximum digging height of excavators and the bucket capacity. For example, a 10-meter bench height is common when using hydraulic excavators with bucket volumes around 7.6 to 10 m³ and trucks with capacities around 77 tons.
- Mining method and production scheduling: bench height affects dilution, selectivity, and production rates. Higher benches may increase dilution and reduce ore grade selectivity, especially in vein-type deposits, while also influencing drilling, blasting, and loading operations. Multi-criteria decision-making approaches weigh factors like safety, cost, equipment availability, and production scheduling to select optimal bench heights.
- Safety and stability: bench height must ensure physical stability of the pit walls and safe working conditions, considering factors like rock discontinuities and groundwater presence.
Bench width determination
- Haulage and equipment size: bench width must accommodate the largest haul trucks plus clearance. A general guideline is that bench width should be at least twice the width of the largest hauling vehicle plus clearance to allow safe manoeuvring and operation.
- Berms/catch benches: width also includes berms or catch benches, which are horizontal ledges designed to catch falling rock and prevent it from rolling down the pit wall, enhancing safety. A rule of thumb for berm width is approximately 4.5 meters plus 0.2 times the bench height (e.g., for a 10 m bench height, berm width would be about 6.5 m).
- Slope stability: bench width, together with bench face angle and height, is chosen to fit the overall pit slope design criteria, balancing the inter-ramp angle (the angle between toes of successive benches) to maintain slope stability.
In essence, bench heights and widths are optimized to balance operational efficiency, equipment capabilities, safety, and geological conditions, often using multi-criteria decision-making tools to achieve the best compromise.
