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 (2.0.1). A main haulage drift is driven from the shaft station into the orebody, with cross-cuts branching off at regular intervals to give access across the deposit's width — these become the permanent haulage and ventilation backbone of the panel.
② Initial stope access (2.0.2). A short raise or ramp is driven from the cross-cut up into the orebody, establishing the first working face and connecting the stope level to the haulage drift below (or, in an up-dip layout, alongside).
③ Starter stope (2.0.3). The first room is opened from the access point — drilled, blasted and mucked on a conservative round to establish a stable initial void and confirm ground conditions before the pattern is extended.
④ Stope at peak production (2.0.4). Multiple rooms are worked in parallel on the design pillar grid, each advancing on its own drill-blast-muck cycle, giving the panel its maximum simultaneous production rate.
⑤ Support for stability, ore losses (2.0.5). Roof bolting (Question 1.1) supports the room backs; pillars are sized by the tributary-area design of Question 1.2. Permanent ore loss is the ore left in the pillars themselves (and any barrier/crown pillars); temporary loss is ore left in rib pillars intended for later recovery, plus fragmented ore left on the floor as a working muck pile/roadbed.
⑥ Mining sequence (2.0.6). Rooms are typically advanced outward from the access point in a staggered or checkerboard sequence, so that each newly started room has at least one already-mined, already-supported neighbour — this limits the number of simultaneously "green" (unassessed) roof spans and lets ground conditions in each new room be judged against its neighbour before committing the full round.
⑦ Pillar reclamation and mining (2.0.7). Once the primary rooms are exhausted, selected pillars are recovered by secondary mining (slabbing or full pillar pull), usually retreating toward the shaft/access so that reclaim always advances away from ground already destressed and toward the exit — the classic "retreat" sequencing that keeps escape routes through already-completed, backfilled or naturally caved ground.
⑧ Permanent stabilization (2.0.8). The mined-out panel is left with its remaining (barrier/crown) pillars in place, or is backfilled, or is allowed to controllably subside/cave — the choice depends on whether the ground above must be protected (surface infrastructure, adjacent workings) and on the value of ore left in unrecovered pillars versus the cost of backfilling.
Services. Conventional hard-rock room and pillar runs drill jumbos, LHDs and haul trucks/shuttle cars needing compressed air (legacy pneumatic drills), diesel fuel and exhaust handling, and blasting-agent supply/storage. Continuous mining (a rotating-drum continuous miner, as used in coal/potash/trona) is typically electric or electro-hydraulic, eliminating blasting-agent logistics entirely and reducing diesel-exhaust ventilation demand, but it needs a reliable, close-following power-trailing-cable supply and a continuous haulage interface (shuttle car or conveyor) matched to the miner's continuous cut rate rather than a cyclic blast-muck rate.
Ventilation. Hard-rock blasting produces a short, intense post-blast gas/dust peak requiring a clearance period before re-entry, so ventilation is sized and scheduled around the blast cycle. Continuous mining produces dust continuously rather than in blast pulses (no post-blast re-entry delay, but persistent respirable dust at the cutting head demanding water sprays and scrubber/venturi dust collection) and, being electric, has far lower diesel-particulate/CO ventilation loading than a trackless hard-rock fleet of equivalent tonnage.
Roof support. Hard-rock room and pillar backs are bolted after each blast round on a cycle that follows advance; continuous-miner operations in soft, often weaker/more friable ground typically bolt immediately behind (or, with a bolter-miner, simultaneously with) the cutting head, since the softer roof cannot tolerate the same unsupported exposure time a competent hard-rock back can.
Where seam/orebody thickness exceeds the height a single mining pass can safely and productively extract (roughly 6 m, beyond which roof bolting, ventilation and equipment reach in a single lift become impractical), stope-and-pillar mining extracts the seam in two or more vertical lifts (benches) rather than the single-pass rooms of 2.0/2.1. A lower bench is typically mined first (or a top bench, depending on backfill/support strategy), each requiring its own sub-level access, ventilation and roof-bolting pass, with the pillars themselves correspondingly taller and more massive to carry the greater vertical extraction. This multi-lift geometry also changes the pillar-recovery sequence: with two exposed benches, pillar reclamation (2.0.7) must be sequenced bench-by-bench, and permanent stabilization more often relies on deliberate crown-pillar retention or backfill rather than controlled caving, since a >6 m open span is harder to manage safely on collapse.
Advantages. Rubber-tired horizontal/vertical drill jumbos, cable shovels and 30 t trucks let Gaspe drive larger openings and move far more rock per machine-hour than track-bound or hand-held equipment, sharply raising productivity (t/man-shift) and lowering unit mining cost; trackless equipment is also far more flexible — it can be redeployed to a different heading or level without relaying track, shortening development lead time and letting the mine react quickly to changing ground/ore conditions; and eliminating fixed rail infrastructure removed a significant capital and maintenance burden.
Disadvantages. The larger openings needed for jumbo/shovel/truck access require larger pillars (or more ground support) for the same stability margin, reducing the achievable extraction ratio versus a hand-held/track operation sized to smaller openings; diesel-powered equipment (jumbos, shovels, trucks) adds a substantial diesel-particulate and heat ventilation load that a compressed-air/electric-rail operation did not carry; capital cost of the trackless fleet (and its maintenance shop/parts inventory) is higher than legacy rail equipment; and wider drifts/larger drawpoints increase the dilution potential at contacts if drilling/blasting control does not keep pace with the larger opening size.
