Mining Methods and Mine Planning
Method selection
Surface versus underground is decided economically, not geologically. A pit is deepened only while the incremental stripping ratio (waste tonnes per ore tonne in the next pushback) stays below the break-even ratio (BER). Orebody shape, grade continuity, rock mass quality and the selectivity the grade requires then drive the underground method: room-and-pillar and longwall for tabular or stratiform deposits; sublevel stoping and sublevel caving for steep, competent orebodies; cut-and-fill and shrinkage where walls are poor and selectivity high; block caving where the mass caves readily and capital is available.
Planning sequence
Resource model → pit or stope optimisation → reserves → phase and pushback design → schedule → equipment selection. Cut-off grade is a decision variable: raise it and reserves shrink while grade rises; lower it and the reverse happens. Haul roads are commonly designed at 8–10% maximum grade, with running surfaces three to four vehicle widths wide.
Traps
- Mixing strip ratio (total waste:ore) with the incremental ratio; only the incremental ratio governs the next pushback.
- Forgetting that dilution and ore loss reduce effective grade and belong in reserve calculations.
- Assuming block caving suits strong rock; it needs a caveable mass and a subsidence allowance.
- Treating a phase and a pushback as the same design unit.
Worked example
Net ore value $44/t, ore mining plus processing cost $32/t, waste mining cost $6/t. BER = (44 − 32)/6 ≈ 2.0. A pushback with an incremental ratio of 1.6 is worth mining; one at 2.4 is not.