24-MMP-A2 Underground Mining Methods and Design · December 2014
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, 2014-Dec. 3 hours duration, closed book; only an approved Sharp or Casio calculator permitted, one hand-written 8.5×11 in. reference sheet allowed. Question 1 is compulsory (40 marks, all seven parts 1.1–1.7); a candidate then selects THREE of Questions 2–7 (each nominally 20 marks, Question 7 sub-totalling higher).
Reference texts: Hartman & Mutmansky (eds.), SME Mining Engineering Handbook, 3rd ed. (underground mining methods, rock support, mine ventilation, shaft hoisting design, headframes, backfill practice, mine cost estimation — the primary reference throughout this paper); Hustrulid & Bullock, Underground Mining Methods: Engineering Fundamentals and International Case Studies (room-and-pillar, vertical crater retreat and trackless mechanized stoping practice); O'Hara, T.P., "Quick Guides to the Evaluation of Orebodies," CIM Bulletin, February 1980, and Mular, A.L. & Poulin, R., CapCost – CIM Special Volume 47, 1998 (parametric underground mine capital-cost models used in Question 7); 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, overwind protection and shaft ventilation).
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.
① Initial cross-cuts and drifts (3.0.1). A bottom-sill drift is driven from the shaft into the orebody at the base of the intended stope, and a top-sill (drilling) drift is driven at the planned top of the stope — both are permanent development, unlike the room-and-pillar case, because they carry the full production drilling and mucking cycle.
② Initial stope access — the raise (3.0.2). A raise is driven (drilled/blasted upward, or by raise-boring) from the bottom sill to the top sill, establishing vertical access/ventilation between the two levels and the starting point for the undercut.
③ Starter stope — the slot (3.0.3). The raise is slashed out into a full-width slot at the bottom of the stope, giving the first free face the crater blasting needs — without this initial void, the first ring of long blastholes has no relief to break to.
④ Stope at peak production (3.0.4). Long, large-diameter blastholes drilled downward from the top sill are fired in successive horizontal crater slices, each slice breaking upward against the void left by the slice before it — the stope grows progressively taller as production proceeds, its side walls and back exposed but the muck pile itself supporting the broken ore below each freshly fired slice until it is drawn.
⑤ Support for stability, ore losses (3.0.5). The open hanging wall and footwall rely on rock-mass competence rather than installed support (Question 1.3); permanent ore loss is any crown or rib pillar left unrecovered at the stope boundary, and temporary loss is broken ore left choking the stope awaiting mucking capacity or a favourable draw sequence.
⑥ Mining sequence (3.0.6). A set of adjacent VCR stopes is typically mined in a primary/secondary sequence — primary stopes are blasted and drawn first, leaving rib pillars between them; once the primary stopes are backfilled (3.1), the secondary rib pillars are recovered against the now-supported fill walls, sustaining continuous production while never leaving two adjacent unfilled voids open at once.
⑦ Pillar reclamation and mining (3.0.7). Secondary (rib) pillars are recovered by the same crater-blasting technique once flanked on both sides by cemented fill, which both confines the blast and supports the newly exposed fill faces.
⑧ Permanent stabilization (3.0.8). The mined-out stope is backfilled (rock or cemented tailings fill, Question 3.1) once drawn out, or, where fill is not used, a permanent crown pillar is left at the top sill to protect the drilling drift and any workings above.
A low-cement-ratio tailings (hydraulic or paste) fill is placed in each stope as soon as it is mucked out, at just enough cement content to let the fill stand as a self-supported, vertical exposed face once the adjacent stope is subsequently blasted and drawn. This does two things directly: it lets a rib or secondary pillar between two primary stopes be fully recovered rather than left as a permanent loss, because the pillar's function (holding back the void on either side) is now performed by the fill instead — directly reducing the permanent ore loss counted in 3.0.5; and it confines dilution, because the fill face, not a raw rock wall, is what the secondary blast breaks against, so waste sloughing from an unsupported hanging wall or footwall is eliminated at the fill/ore contact. Cement content is kept to the minimum that achieves standing capability (rather than a stronger, more expensive mix) because the fill's only structural job here is short-term self-support during the adjacent draw, not permanent load-bearing.
C.W. Livingston's crater-blasting theory shows that the ideal geometry for breaking maximum rock per unit of explosive is a spherical (point-source) charge fired at its own "critical depth" of burial below a free face — a concentrated, roughly equidimensional charge, not the long, distributed column typical of conventional bench blasting. Practical VCR blasting approximates this by loading only a short, compact length of each long, large-diameter blasthole with explosive (leaving the rest stemmed/air-decked), so that each charge behaves, to the free face above it, like a concentrated spherical source at a controlled burden distance rather than a long cylindrical column.
Drilling. Long, large-diameter (150–250 mm) holes are drilled from the top-sill drift with down-the-hole (DTH) percussion or rotary-percussion drills capable of holding a straight, accurate hole over the full stope height — hole deviation directly changes the effective burden and ruins the spherical-charge approximation.
Explosive loading. A compact charge of ANFO or a bulk emulsion is pumped or poured into the bottom of the hole at the calculated critical-depth location, with the remainder of the hole above the charge stemmed (or deliberately air-decked with an inert spacer) so the charge's energy concentrates locally rather than distributing along the hole length.
Initiation. Each charge is initiated from a single primer/detonator placed at (or near) the charge's own centre, so the detonation propagates outward from one point — again approximating the theoretical point-source spherical charge rather than the multi-point initiation of a long column charge.
Delay timing to avoid "frozen" rock. Successive rings/slices are fired on sequential delays (typically tens to a few hundred milliseconds apart) rather than simultaneously, so each slice always breaks toward the free face and void left by the slice fired immediately before it. Rock fired with no adjacent relief (all charges confined, no free face to break toward) is "frozen" — it receives the blast energy but cannot move or fragment properly, wasting explosive and leaving poorly fragmented, hard-to-muck rock; correctly sequenced delays guarantee every slice always has a just-created free face to break into.
Load-haul-dump (LHD) from drawpoints. A rubber-tired LHD mucks broken ore directly from stope drawpoints to an ore pass or truck. Flexible and mobile between multiple active drawpoints, but the diesel engine adds particulate and heat ventilation load, and the operator working directly under a freshly blasted, unsupported VCR back/hanging wall is a real exposure risk.
Teleremote (remote-controlled) LHD. The same machine operated from a safe, shielded remote station via line-of-sight or camera control, specifically to remove the operator from under the unsupported back while mucking the highest-risk drawpoints closest to a freshly fired slice. Advantage: removes personnel from the most hazardous exposure; disadvantage: line-of-sight/camera and radio-control range limitations, and typically slower cycle times than a manned LHD.
Gravity draw through ore passes/finger raises. Broken ore flows by gravity from the stope directly into ore passes or finger-raise drawpoints feeding the haulage level below, with no mucking machine required at the draw point itself. Advantage: no moving equipment (and hence no diesel/ventilation load) at the draw; disadvantage: relies entirely on good, uniform fragmentation — oversize or interlocked blocks cause hang-ups that require secondary breakage (and re-expose workers) to clear.
Ventilation for VCR ore handling. Because the stope itself is a large, open, irregular void rather than a fixed-cross-section drift, auxiliary ventilation (flexible ducting fed from the bottom-sill drift) is run directly to the active drawpoint level, with brattice or regulators used to direct primary airflow past the open stope mouth rather than letting it short-circuit through the void; diesel-equipment drawpoints need proportionally more auxiliary air than gravity/teleremote draw for the same production rate, given the added engine exhaust load.
