24-MMP-A2 Underground Mining Methods and Design · December 2016
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, 2016-Dec. 3 hours duration, closed book; only a Casio or Sharp approved calculator permitted. Question 1 is compulsory (40 marks, all six parts 1.1–1.6); a candidate then selects TWO of Questions 2–4 (Section B) and ONE of Questions 5–6 (Section C), each worth 20 marks.
Reference texts: Hartman & Mutmansky (eds.), SME Mining Engineering Handbook, 3rd ed. (underground mining methods, backfill systems, mine hoisting design, mine ventilation, mine cost estimation — the primary reference throughout this paper); Hustrulid & Bullock, Underground Mining Methods: Engineering Fundamentals and International Case Studies (room-and-pillar, VCR, cut-and-fill, longhole, shrinkage and sub-level caving practice); ASHRAE, ASHRAE Handbook — Fundamentals (psychrometric relations, humidity ratio and enthalpy of moist air); BC Ministry of Energy, Mines and Low Carbon Innovation, Health, Safety and Reclamation Code for Mines in British Columbia (Canadian regulatory context for hoisting-rope safety factors); Camm, T.W. (1989), Simplified Cost Models for Prefeasibility Mineral Evaluations, U.S. Bureau of Mines IC 9298 (Question 4 parametric cost models); O'Hara, T.A. (1980), "Quick Guides to the Evaluation of Orebodies," CIM Bulletin, February 1980 (Question 1.4.3).
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 stoping mines a steeply dipping orebody upward in horizontal slices, leaving roughly 60% of the broken ore in the stope as a working platform/temporary wall support and drawing off only enough to keep a safe working headroom until the stope is complete, when the full pile is finally drawn. Sub-level caving drives a grid of parallel sub-level drifts through the orebody, drilling and blasting each ring so the ore caves and is drawn from the drift face while the overlying waste is deliberately allowed to cave and follow the ore down, with no stope left open at any point. Both are largely non-selective bulk methods for steep, competent-to-moderate orebodies, but they differ sharply in how ground control and dilution are managed.
| Criterion | Shrinkage stoping | Sub-level caving |
|---|---|---|
| Geology, orebody shape/size/orientation | Steeply dipping (≥50°, ideally near-vertical so broken ore gravity-flows to drawpoints), narrow to moderate width, regular walls | Steeply dipping, larger/thicker orebodies where the overlying rock mass is suitable (or can be induced) to cave uniformly |
| Host and ore rock properties | Ore and walls both need to be reasonably competent — walls must stand unsupported against the temporarily-retained broken-ore pile | Ore should be moderately competent (fragments well on blasting) while the overlying waste/host rock must be capable of caving readily and continuously as ore is withdrawn |
| Development size/amount/cost/time to start | Moderate: a single set of sub-level drifts (top for drilling, bottom for drawpoints) per stope block; relatively fast to bring into production | Extensive: a full grid of closely spaced parallel sub-level drifts and crosscuts driven through the ENTIRE orebody height, repeated at every sub-level; high up-front development cost and time |
| Ground support | Rib/wall support as needed on the standing walls; minimal support inside the stope itself (the broken-ore pile is the working floor) | Significant support in the closely spaced production drifts, which must remain serviceable while overlying waste caves close behind the retreating face |
| Mining sequence and mill feed rate | Slice-by-slice upward blast with partial draw between slices, full draw only at completion; moderate, somewhat batch-like mill feed rate | Continuous ring drilling/blasting and drawing, retreating along each sub-level drift; high, continuous mill feed rate once the method is established |
| Personnel (number/skills) | Moderate crew, skilled in slice drilling/blasting and safe partial-draw practice under a broken-ore back | Larger crew given the extent of sub-level development, with skilled ring-drilling/blasting and LHD draw-control (avoiding waste dilution) personnel |
| Mechanized equipment | Smaller-scale drilling equipment (often hand-held or light jumbo, since headroom in the stope is limited); minimal LHD use until final draw | Ring/fan drill jumbos and a large fleet of LHDs working continuously along the production drifts |
| Cost of mining | Moderate unit cost — less bulk than caving but more than a fully mechanised bulk method, owing to the slower slice cycle | Low unit cost once established — high mechanisation and continuous production offset the heavy up-front development cost |
| Mine life | Suited to moderate-tonnage, narrower deposits over a moderate mine life | Suited to large-tonnage deposits, sustaining production over a long mine life once the sub-level grid is developed |
| Mining rate | Low to moderate | High, among the highest of underground bulk methods |
| Dilution and recovery | Low-to-moderate dilution if walls are competent; recovery limited by the roughly 40% of broken ore that must be drawn early to keep working headroom, plus ore left against irregular walls | Inherently high dilution — waste rock mixes with ore as both cave and are drawn together through the same drawpoint, and the draw must be stopped once waste content rises, capping recovery per ring |
| Pillar recovery method/cost | Sill pillars at the stope base are recovered as a final, higher-risk draw once the stope is otherwise exhausted | No conventional pillars are left (the method is inherently non-pillared); crown remnants above the uppermost sub-level are the main un-recovered loss instead |
| Post-mining stabilization | Remaining voids after final draw typically require backfilling or are allowed to collapse in a controlled manner | Surface subsidence is an EXPECTED, planned consequence of caving and must be managed by a pre-planned subsidence/exclusion zone rather than treated as an incidental stabilization afterthought |
