Optimal geometry of excavation faces is important in open-pit mining to guarantee economic success and safety. The key geometrical parameters of an open pit mine are bench height and berm width. Bench height refers to the vertical distance between adjacent benches (horizontal levels), while berm width is the horizontal distance between the top of one bench and the bottom of another bench lying above it. As the depth of an open-pit mine increases, the search for the proper ratio of these two parameters is becoming more complicated.
The choice of bench height and berm width is dependent on the detailed geomechanical assessment of the rock mass. The engineer has to determine the strength of rocks and their intact properties, as well as geological discontinuities, such as faults, bedding planes, and joints. These discontinuities control the maximum local inclination and bench face angle that can be cut without causing slope failure in the whole area. When the anisotropy of jointed rock mass is high, proper geometries of benches are included as constraints to control local inclination of pit walls (Agosti et al., 2024).
Moreover, operational efficiency significantly impacts the decision about bench height. The selected height should accurately match the reach of the digging and the volume of the bucket used by the loading machinery, like hydraulic excavators and rope shovels, to avoid ore dilution and increase the effectiveness of blasting holes. Otherwise, in case of excessive height, there will be a danger of falling rocks, while in case of insufficient height, repeated equipment movements will lower its efficiency and prolong extraction time.
On the contrary, the role of berm width is to intercept falling rocks and stop them from sliding down along the pit wall to the working area. The minimum berm width is calculated based on rockfall trajectories and back-break studies performed during the blasting process. In the right proportions, the berm acts as an absorber of kinetic energy released by the rocks. As the mine becomes deeper, it is vital to maintain the required berm width to ensure safety of people and equipment.
Economically speaking, the shape and size of the benches and berms have significant impacts on the slope angles of the whole pit. More inclined slope angles would reduce the amount of waste rock material to be stripped from the top of the orebody; thus, it reduces mining expenses. Today’s advanced mathematical models consider bench heights, bench face angles, and minimum widths of berms in order to increase the slope angles of the pitwall. It is evident that using this method results in increasing the net present value (NPV) of the mining project more than 50% and significantly reduces the amount of waste rocks and the carbon footprint (Agosti et al., 2021).
To summarize, finding the optimal bench height and berm width of a deepened open-pit mine is an iterative process that requires balancing different engineering and geological factors. Combining geotechnical rock mass conditions, equipment properties, rock fall capture facilities, and economic optimization, engineers could design open pit geometries that optimize the profits without risking anything. Eventually, mastering this interplay between the different geometric factors is crucial for the development of modern open-pit mines.
References
Agosti, A., Utili, S., Gregory, D., Lapworth, A., Samardzic, J., & Prawasono, A. (2021). Design of an open-pit gold mine by optimal pitwall profiles. CIM Journal, 12, 149–168. https://doi.org/10.1080/19236026.2021.1979382
Agosti, A., Cylwik, S. D., & Utili, S. (2024). Optimal mine pitwall profiles in jointed anisotropic rock masses. International Journal of Mining, Reclamation and Environment, 39, 210–234. https://doi.org/10.1080/17480930.2024.2387988
