24-MMP-A2 Underground Mining Methods and Design · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Mining and Mineral Processing Engineering, 09-Mmp-A2 Underground Mining Methods and Design, 2013-May. 3 hours duration, closed book; only a Casio or Sharp approved calculator permitted. Question 1 is compulsory (40 marks, all seven parts 1.1–1.7); a candidate then selects FOUR of Questions 2–7 (each worth 15 marks).
Reference texts: Hartman & Mutmansky (eds.), SME Mining Engineering Handbook, 3rd ed. (underground mining methods, mine ventilation, shaft hoisting systems, backfill practice — the primary reference throughout this paper); BC Ministry of Energy, Mines and Low Carbon Innovation, Health, Safety and Reclamation Code for Mines in British Columbia (Canadian regulatory context for mine ventilation and hoisting-plant safety); Wills & Finch, Wills' Mineral Processing Technology, 8th ed. (tailings thickening/filtration and paste preparation for backfill).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Shrinkage and sub-level caving both mine without filling the void, but for opposite reasons: shrinkage retains the broken ore itself as a temporary working platform in an otherwise competent orebody, while sub-level caving deliberately induces the overlying rock to cave and fill the void, which demands the opposite rock mass character — cavable, not self-supporting.
| Criterion | Shrinkage stoping | Sub-level caving |
|---|---|---|
| Geology, orebody shape/size/orientation | Narrow to moderate width, steeply dipping (>50°, so ore flows by gravity), regular | Large, steeply dipping to massive orebodies suited to induced caving of both ore and overlying waste |
| Host and ore rock properties | Competent walls needed (stand unsupported for the whole stope life); ore must not degrade/oxidize while stored | Ore and cap rock must be cavable (moderate to weak, well-jointed); overly competent rock resists caving and hangs up |
| Development size/amount/time to extraction | Moderate: access at top and bottom of each stope; relatively quick to first ore | Extensive: closely spaced sublevel drifts and ring-drill development throughout the orebody height; slower to reach full production |
| Ground support | Minimal standing support beyond stope access; walls rely on inherent competence | Support concentrated in sublevel drifts, which are progressively consumed by the advancing cave |
| Mining sequence & mill feed rate | Slow, incremental slice-and-partial-draw cycle; feed rate low and tied up in standing muck | Continuous top-down sublevel retreat; feed rate can be high and sustained across many active drawpoints |
| Personnel numbers & skills | Small crews, generalist stope miners | Larger crews across many simultaneous sublevels; specialised ring-drillers and draw-control personnel |
| Mechanized equipment types/numbers | Light drilling and mucking equipment; minimal mechanization | Heavy ring-drill rigs and large LHD fleets across multiple sublevels simultaneously |
| Cost of mining | Moderate unit cost — low mechanization but slow rate ties up capital in standing ore | Low unit cost at scale — highly mechanized, continuous, high-tonnage bulk method |
| Mine life | Suited to smaller, narrow deposits; often a legacy/lower-capacity method | Suited to large orebodies mined over a long life at sustained high tonnage |
| Mining rate | Low (most broken ore sits idle as platform) | Very high (among the highest of underground methods) |
| Dilution & recovery | Moderate dilution risk in the final full-scale draw; recovery good if walls hold | Dilution rises progressively through each draw as caved waste works down; overall recovery lower than selective methods |
| Pillar recovery methods/cost | Sill/crown pillars sometimes left; recovered by separate blasting at added risk once surrounding stopes are mined out | Largely avoided by design — the method is inherently pillarless, continuous retreat |
| Post-mining stabilization | Residual voids from incomplete draw may need monitoring or backfilling | Surface subsidence is expected and must be planned for (exclusion zones, monitoring); the mined-out zone is not stabilized, it is allowed to cave |
The practical takeaway is that shrinkage suits a modest, narrow, competent orebody where tying up ore as platform is an acceptable price for simple, low-development mining, whereas sub-level caving suits a large, cavable orebody where sheer scale and mechanization outweigh the dilution and subsidence it deliberately accepts.
