24-MMP-A2 Underground Mining Methods and Design · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
09-MMP-A2 Underground Mining Methods and Design — National Exam, December 2017. Compulsory Question 1 (Section A, 40 marks) plus three optional questions (two from Section B, one from Section C) constitute a graded 100-mark paper; every optional question (2–6) is answered in full below as a complete study resource.
Reference texts: Hartman, H. & Mutmansky, J., Introductory Mining Engineering, 2nd ed., Wiley (2002); Hartman, H. (ed.), SME Mining Engineering Handbook, 2nd/3rd ed., SME; Hartman, H., Mutmansky, J., Ramani, R. & Yang, Y., Mine Ventilation and Air Conditioning, 3rd ed., Wiley (1991) — the three texts named on the exam's own reference line.
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 breaks ore upward in horizontal slices from a bottom-sill drift, deliberately leaving roughly 35–40% of the broken muck in the stope at all times (only enough is drawn off after each blast to re-establish a safe working clearance beneath the back) so the standing muck pile both serves as the miners' working platform for the next slice and passively supports the stope walls until final, total draw-down once the block is finished. Two well-known methods are essentially engineered variants of this same "break upward, retain broken muck as floor/support" principle. Vertical Crater Retreat (VCR) mining resembles overhand shrinkage closely: large-diameter, deep blastholes are drilled UP (or down) from a top or bottom access into the full stope height, and the stope is retreated by firing a horizontal SLICE of these holes at a time with a spherical (crater) charge, drawing off only enough broken ore after each firing to expose the next slice for firing — exactly shrinkage's "blast a slice, partially draw, blast the next" sequence, but with the slice broken all at once by a single ring of long blastholes rather than face-drilled and blasted lift by lift by miners working on the muck pile itself, and the muck retained in the stope performs the same wall-support role as in conventional shrinkage. The Avoca method (a shrinkage/cut-and-fill hybrid developed at the Avoca mine, Ireland) resembles underhand-toward-overhand shrinkage in its retreat logic: ore is broken upward as in shrinkage, but instead of retaining the full 35–40% muck pile for the whole block, the stope is periodically drawn down further and backfilled, so the method behaves like a sequence of short shrinkage lifts each followed by a partial fill cycle rather than one continuous shrinkage draw — it resembles shrinkage in its upward-breaking, muck-supported working sequence while borrowing cut-and-fill's periodic fill cycle to permit better ground control in weaker walls than pure shrinkage could stand.
| Criterion | Shrinkage stoping | Vertical Crater Retreat (VCR) | Avoca method |
|---|---|---|---|
| 6.2.1 Geology, orebody shape/size/orientation | Steeply dipping (>50–55°, at or above the angle of repose of broken ore so muck slides clear of the working face), narrow-to-moderate width, regular tabular veins | Steep to sub-vertical, can tolerate somewhat wider/more massive orebodies than hand-shrinkage because slice height is set by blasthole design, not manual reach | Steep veins similar to shrinkage, typically where walls are too weak for pure shrinkage to stand unsupported for the full block life |
| 6.2.2 Host/ore rock properties, ground support | Requires COMPETENT ore and walls (must stand with only the muck pile for support, no formal ground support installed mid-block) | Requires competent ore/walls similar to shrinkage, though the large-diameter ring-blast approach tolerates slightly more massive, moderately jointed ground | Suited to WEAKER walls than shrinkage or VCR can tolerate — the periodic backfill cycle actively substitutes for wall competence the other two methods must have naturally |
| 6.2.3 Mining sequence, mill feed rate | Continuous upward slice sequence, partial draw after each blast; final total draw-down delivers a large tonnage surge to the mill at block completion | Slice-and-partial-draw sequence like shrinkage but on a larger-tonnage-per-blast basis (each ring breaks a full stope-width slice at once), giving a more concentrated, higher-rate mill feed per blast event | Broken-lift-then-fill cycle gives a steadier, more even mill feed rate than either shrinkage or VCR, since tonnage is delivered in smaller, more regular increments between fill cycles |
| 6.2.4 Development size/cost/time to extraction | Modest development (bottom sill only, plus raises); relatively FAST time to first extraction among sub-level-scale methods | Requires more extensive top/bottom drilling access and larger-diameter longhole drilling infrastructure, somewhat higher development cost/time than plain shrinkage | Development similar to shrinkage but with the ADDED fill-system development/infrastructure (fill raises, barricades) built in from the start, raising both cost and time to steady-state production |
| 6.2.5 Personnel numbers/skills | Labour-intensive, skilled stope miners working directly on the muck pile face; relatively small crew per stope but high skill/experience requirement | Fewer face workers (drilling/loading crew, not a continuous face crew), but requires specialized longhole-drilling and precision ring-blast-design skill | Similar crew size to shrinkage plus the additional fill-crew personnel (barricade construction, fill-line operation) needed for the periodic backfill cycle |
| 6.2.6 Mechanized equipment types/numbers | Minimal mechanization — hand/stoper drills, slushers/scrapers for interim draw; the method predates and resists heavy mechanization by its very geometry (miners on a moving muck floor) | Higher mechanization — production longhole drill rigs, mechanized loading of large-diameter holes, LHDs for draw at the drawpoints | Similar equipment to shrinkage for the breaking cycle, PLUS fill-delivery equipment (pumps, pipeline, barricade-building equipment) for the backfill cycle |
| 6.2.7 Cost of mining | Low-to-moderate direct mining cost per tonne (simple method, little mechanization) but limited by low overall productivity | Higher unit drilling/blasting cost (large-diameter longhole drilling, precision charge design) offset by significantly higher productivity per stope | Highest per-tonne cost of the three, driven by the added fill-cycle capital and operating cost layered onto a shrinkage-like breaking cost |
| 6.2.8 Mine life | Suits smaller, narrower deposits mined over a longer relative period per stope given its lower productivity | Suits similar deposit sizes to shrinkage but extracted over a SHORTER period per stope given VCR's higher productivity | Comparable mine life to shrinkage, potentially extended somewhat by the ability to mine in weaker ground that would otherwise be sterilised or require a slower, more conservative method |
| 6.2.9 Mining rate | Lowest of the three — limited by manual face-drilling/blasting cycle time and interim-draw handling | Highest of the three — large-tonnage ring blasts and mechanized drawpoint mucking give substantially higher stope production rates | Intermediate — the fill cycle interrupts the breaking cycle's rate compared with continuous shrinkage or VCR retreat |
| 6.2.10 Dilution and recovery | Moderate dilution (wall sloughing into the standing muck pile over the block's life) and good recovery at final total draw, though final draw-down itself is a known point of ore loss/dilution mixing | Generally LOWER dilution than shrinkage (shorter total exposure time per slice before draw, precision-designed charge geometry limits overbreak) with comparably good recovery | Lowest dilution of the three — the periodic fill actively confines the walls and limits the exposure time/area over which sloughing can occur, at some cost to recovery where fill/ore boundaries are imprecisely drawn |
| 6.2.11 Methods/costs of pillar recovery | Sill/rib pillars typically recovered as a separate, higher-risk final phase once the main block is drawn, at meaningful additional cost and risk (working beneath a large, now-unsupported void) | Similar sill-pillar recovery challenge to shrinkage, though VCR's own ring-blast technique is sometimes used to recover the pillar itself (VCR pillar recovery is a recognised sub-application) | Pillar recovery is comparatively straightforward because the fill mass already in place provides a structural substitute, letting pillars be recovered with the fill standing in for the void that would otherwise open up |
| 6.2.12 Post-mining stabilization | Stope typically left open/caved after final draw (unfilled void) unless a separate post-mining fill campaign is undertaken | Similar post-mining condition to shrinkage — an open/caved final void unless separately backfilled | Inherently self-stabilizing — the block is left substantially backfilled as a direct product of the mining sequence itself, requiring little to no separate post-mining stabilization campaign |
Room-and-pillar mining bears essentially no resemblance to shrinkage, VCR or Avoca and could not be substituted for either comparison example. Where shrinkage/VCR/Avoca all rely on breaking ore UPWARD from a bottom access in a steeply dipping orebody and depend on gravity to move broken muck down to a draw point, room-and-pillar is fundamentally a FLAT-LYING, single-pass extraction method: ore is removed from a horizontal or gently dipping, typically tabular deposit by driving a regular grid of rooms (openings) while leaving a systematic pattern of pillars to carry the overburden load, with the rooms and pillars all developed and mucked at essentially the SAME horizon rather than in vertically retreating slices. It requires none of shrinkage's muck-pile working-platform concept (mechanized equipment works on a solid, permanent floor throughout), gives up entirely on gravity-assisted ore flow to a draw point (loading is by trackless mobile equipment moving laterally to a fixed haulage level, not vertical draw), and is applicable only to flat-lying, laterally extensive orebodies (coal, potash, many flat-bedded industrial-mineral and some stratabound base-metal deposits) — a geometry in which shrinkage, VCR and Avoca (all inherently steep-dip methods relying on gravity flow within the stope) simply cannot be used at all.
