24-MMP-A2 Underground Mining Methods and Design · May 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-May. 3 hours duration, closed book; only a Casio or Sharp approved calculator permitted. Question 1 is compulsory (40 marks, all seven parts 1.1–1.7); a candidate then selects FOUR of Questions 2–7 (each worth 15 marks).
Reference texts: Hartman & Mutmansky (eds.), SME Mining Engineering Handbook, 3rd ed. (underground mining methods, 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 (narrow-vein longitudinal-retreat/Avoca-family stoping, cut-and-fill variants); 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 shaft ventilation); Camm, T.W. (1991), Simplified Cost Models for Prefeasibility Mineral Evaluations, U.S. Bureau of Mines IC 9298 (source of the Question 4 parametric cost models).
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.
3.1 Vein and wall-rock characteristics for Avoca. True Avoca mining is applied to narrow (generally <4 m) veins that are steeply dipping and reasonably continuous along both strike and dip, with grade good enough to justify the extra unit cost of a fill-supported narrow-vein method over a cheaper bulk method. Critically, the wall rocks must be of an intermediate, "fair" competence: strong enough to stand the open span of one retreating slice for the days-to-weeks it takes to drill, blast and muck it, but not so competent that a simpler, unsupported open-stoping method would do — Avoca specifically trades the cost of scheduled backfilling for the ability to mine ground that cannot be left permanently open.
3.2 Discussion by dip, extent, continuity, grade and rock quality. A steep dip (well past the angle of repose) is essential so broken ore flows freely to drawpoints and the exposed back area at any instant stays small. Good lateral (strike) continuity lets the retreat sequence progress through a series of panels while adjacent panels are simultaneously mined and filled, which is what sustains continuous mill feed (3.4). Sufficient vertical extent, developed as multiple sub-levels, allows several panels at different elevations to be worked concurrently, again spreading production risk across panels. Grade continuity along strike matters because Avoca is less selective than pure cut-and-fill — some wall-rock dilution is normal, so an erratic, patchy grade distribution erodes the method's economics faster than it would a highly selective method. Wall-rock quality of the hanging wall, footwall and ore itself must be internally consistent with the "fair" characterisation above: an unusually weak zone anywhere along the retreat forces either accelerated filling (shorter open spans) or additional ground support, both of which raise unit cost.
3.3 Development and production sequence.
Development first drives a footwall (or hanging-wall) access drift and sub-level drifts at the planned spacing, then a slot/undercut raise at the far end of each panel to create a free face for the first retreat slice. Production then proceeds slice by slice: longhole rings are drilled from the sub-level drift into the vein, blasted, and the broken ore is mucked by LHD to a drawpoint or ore-pass, retreating the face back toward the access drift. As each slice is cleared, backfill (rock or cemented tailings) is placed behind the retreating face before the next slice is blasted, restoring wall support progressively rather than leaving the whole panel open. Production and filling therefore alternate along the same panel, and, at the mine level, several panels are kept at different points in this cycle simultaneously.
3.4 Production fluctuations and continuous mill feed. A single Avoca stope inherently produces in a stop-start rhythm: ore comes only from the drill–blast–muck portion of the cycle, and output from that panel falls to zero while it is being filled and while the fill cures enough to support the next slice. Relying on one panel therefore gives a highly cyclical mill feed. Continuous feed is achieved by deliberately staggering several panels — on different sub-levels or at different points along strike — so their production/fill cycles are out of phase: while one panel is filling, one or more others are actively drilling, blasting or mucking. A surface or underground ore stockpile provides an additional short-term buffer against any residual mismatch between the combined panel output and the mill's steady feed-rate requirement.
