24-MMP-A2 Underground Mining Methods and Design · December 2015
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, 2015-Dec. Closed book exam, Casio/Sharp approved calculator plus one aid sheet permitted. Question 1 is compulsory (40 marks, all six parts 1.1–1.6); a candidate then selects THREE optional questions following the group rules (one or both of Questions 2/3; one or two of Questions 4/5/6).
Reference texts: Hartman & Mutmansky (eds.), SME Mining Engineering Handbook, 3rd ed. (underground mining methods, mine ventilation, shaft hoisting design, mine economics — the primary reference throughout this paper); Hustrulid & Bullock, Underground Mining Methods: Engineering Fundamentals and International Case Studies (cut-and-fill, longhole/sublevel open stoping, VCR practice); 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 and ventilation practice); Mutmansky & Wang, "A Review of the Vertical Crater Retreat (VCR) Mining Method," and the original crater-blasting theory of C.W. Livingston, Trans. AIME/CIM (Question 6).
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.
The development and production cycle begins with top- and bottom-sublevel access drives run the length of the planned panel, from which a slot raise is developed at one end to provide the initial free face; long, near-vertical fan or parallel blastholes are then ring-drilled downward from the top sublevel drill drift across the full panel width. Production blasting proceeds ring by ring, retreating away from the slot along the panel, with each ring's broken ore falling to the bottom sublevel and drawn out through drawpoints by LHD, typically to an ore pass or truck haulage; the open void left above the drawn ore is either left open for the panel's life or, if the mine plan requires later pillar recovery or stability, backfilled once the panel is exhausted.
| Criterion | Longhole (sublevel open) stoping |
|---|---|
| Geology, orebody shape/size/orientation | Wide (commonly >6–10 m), regular, steeply dipping (>50°) tabular or massive orebody with well-defined, planar contacts |
| Host and ore rock properties | Competent, self-supporting host and ore rock able to stand a large open span for the full panel life without support |
| Development size/amount/cost/time to extraction | Top and bottom sublevel drifts along the full panel length plus a slot raise; concentrated development gives a lower development-cost-per-tonne than cut-and-fill, but a full panel's development (both sublevels, slot) typically takes many months before first production blast |
| Ground support | Support concentrated in the permanent sublevel access/drill drifts (bolts/mesh/shotcrete); the open stope walls themselves are essentially unsupported |
| Mining sequence & mill feed rate | Ring-by-ring retreat from the slot; mill feed is high but somewhat lumpy (large ring blasts) rather than continuous |
| Personnel numbers & skills | Small crew per stope — specialised longhole drillers, blast crew, LHD operators; far fewer personnel per tonne than cut-and-fill |
| Mechanized equipment types/numbers | Large-diameter longhole/ITH production drill rigs (one or two per active panel), LHDs for mucking, minimal or no fill equipment unless post-mining backfill is planned |
| Cost of mining | Low unit cost per tonne — bulk mechanized drill/blast/muck cycle with minimal support and no fill cycle |
| Mine life | Set by the total panel reserve and sequencing of panels across the orebody; individual panels are relatively short-lived, mine life comes from many sequential panels |
| Mining rate | High — hundreds to low thousands of t/day per active stope |
| Dilution and recovery | Higher dilution than a filled method, from wall sloughing/blast overbreak into the open void over the panel's exposure time; recovery good but reduced by rib/sill pillars left for stability |
| Pillar recovery methods/costs | Rib and sill pillars left between panels are recovered later by secondary longhole/VCR-type blasts, at extra drilling/blasting cost and higher risk (working near an already-destressed, partially open void) |
| Post-mining stabilization | The void is backfilled if later pillar recovery or surface subsidence control requires it, or left open/allowed to relax and eventually cave if it does not; larger unfilled voids need ongoing monitoring |
5.2.1 Avoca (longitudinal retreat). A variant in which the stope is mined longitudinally (along strike, rather than across the full panel width in one pass) and backfilled concurrently — ore is blasted and mucked from one end of a narrow panel while cemented rock fill is placed immediately behind the retreating face, so the fill itself becomes the working platform and the exposed span is kept small at any one time. Compared to conventional longhole (which leaves the whole panel open until it is finished), Avoca trades some of longhole's low unit cost for much better ground control in narrower or less competent orebodies, since the concurrent fill limits the unsupported span to a fraction of the panel length rather than the whole panel.
5.2.2 Viscaria (Alimak). Named for the Alimak raise-climber platform used to work the stope, this variant mines a narrow, steeply dipping orebody upward in successive slices using an Alimak (self-climbing) platform running on a fixed guide rail up the stope, drilling and blasting from the platform itself rather than from ring-drilled longholes — effectively an overhand cut-and-fill-like advance but without needing full lift-by-lift fill placement in every case, since ore can be mucked from the bottom via a drawpoint while the platform advances upward. Compared to conventional longhole, Viscaria/Alimak trades longhole's bulk-blasting productivity for much greater selectivity and applicability to narrower, more irregular orebodies than a longhole ring pattern can economically follow.
5.2.3 VCR (vertical crater retreat). Large-diameter vertical blastholes are drilled downward from a single top sublevel only (no bottom-sublevel ring drilling is needed) and blasted in horizontal crater-shaped slices retreating upward, one slice at a time, using concentrated ("spherical") charges rather than a full-column charge — broken ore is drawn from bottom drawpoints as in conventional longhole, but the drilling and blasting geometry (horizontal slice-by-slice crater blasting from one sublevel) is fundamentally different from longhole's ring/fan pattern blasted from top to bottom sublevel in one campaign. Compared to conventional longhole, VCR needs only one drilling sublevel (lower development cost per tonne of the panel) and gives finer control over each blast's fragmentation and the advance rate (one crater slice at a time), at the cost of a slower overall production rate per panel than a full-ring longhole blast delivers, and a more demanding blast-design/timing discipline (Question 6 develops VCR's practical crater-blasting cycle in full).
| Variant | Key difference from conventional longhole |
|---|---|
| 5.2.1 Avoca | Longitudinal retreat with concurrent backfill; small exposed span instead of the whole open panel |
| 5.2.2 Viscaria (Alimak) | Raise-climber platform mining upward in slices; far more selective, no ring-drilling required |
| 5.2.3 VCR | Single top sublevel only; horizontal crater slices retreat upward instead of a full top-to-bottom ring pattern |
