24-MMP-A6 Mining and the Environment · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Mining and Mineral Processing Engineering, 09-MMP-A6 Mining and the Environment, 2014-May. 3 hours duration, open book, no calculator permitted. Six questions of equal value (20 marks each) constitute the bank; a candidate answers any FIVE. Most questions require an essay-format answer.
Reference texts: BC Ministry of Energy, Mines and Low Carbon Innovation, Health, Safety and Reclamation Code for Mines in British Columbia (current edition) (permitting, reclamation and closure planning requirements); Canadian Dam Association (CDA), Dam Safety Guidelines (2013/2019 update) (dam classification, embankment raising methods, design flood/PMF); Global Industry Standard on Tailings Management (GISTM, 2020) (tailings governance, upstream-construction restrictions); Price, W.A., MEND Report 1.20.1, Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials (1997/2009) (acid-base accounting, NPR screening criteria); International Network for Acid Prevention (INAP), Global Acid Rock Drainage (GARD) Guide (ARD prediction, prevention and treatment); Vick, S.G., Planning, Design, and Analysis of Tailings Dams, 2nd ed. (embankment construction methods, seepage control); Government of Canada, Metal and Diamond Mining Effluent Regulations (MDMER) under the Fisheries Act; Government of Canada, Impact Assessment Act (successor to the 2012 Canadian Environmental Assessment Act).
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.
| Criterion | Upstream | Centerline | Downstream |
|---|---|---|---|
| Embankment fill / construction material requirements | Lowest – only a starter dyke plus thin lifts, largely built from the tailings themselves | Moderate – compacted engineered fill placed vertically over both tailings and prior fill each raise | Highest – a full engineered rockfill/earthfill volume is added with every raise |
| Mill tailings requirements | Requires a coarse, free-draining (typically cycloned sand) fraction to build each raise – not usable with all-slimes, thickened or paste tailings | Benefits from a coarser fraction but is more tolerant of finer tailings than upstream | None – raises are built entirely from external fill/rockfill regardless of tailings gradation |
| Suitability for water storage | Poor – pond must be kept small and set back from the crest, since each dyke is partly founded on saturated tailings | Moderate – some water storage is feasible with adequate freeboard and internal drainage | Good – a robust, fully engineered structure comparable to a conventional water-retention dam |
| Seismic resistance | Poor – foundation includes loose, saturated, potentially liquefiable tailings; implicated in most historic catastrophic tailings dam failures | Moderate – partial tailings foundation but a stiffer engineered core | Good – entirely engineered fill foundation, can be designed to conventional dam seismic standards |
| Typical failure mode | Liquefaction / static or seismic slope instability of the tailings foundation; overtopping | Slope instability; internal erosion (piping) along the tailings–fill interface | Overtopping; piping/internal erosion through the engineered fill (liquefaction risk much lower) |
| Rising-rate restrictions | Strict – vertical rise rate is limited to allow pore pressure in the tailings foundation to dissipate between raises | Moderate restriction, governed by the tailings-supported portion of each raise | Minimal – rise rate is governed by fill placement/compaction logistics rather than pore-pressure dissipation |
| Relative cost | Lowest – least fill volume, smallest footprint | Intermediate | Highest – largest fill volume and footprint added at every raise |
Submerging PAG tailings beneath a permanent water cover excludes atmospheric oxygen far more effectively than any dry soil cover, which is the technical benefit that drives interest in this option: it suppresses sulphide oxidation and ARD generation at the source (Question 3c) more reliably and with less ongoing maintenance than an engineered cover system, while also controlling dust and wind erosion and, where a natural basin is used, potentially reducing the height and footprint of new embankment construction. Set against this are significant issues on both the technical and social sides. Technically, the water cover itself becomes a permanent dam-safety and closure liability equivalent to (or exceeding) a conventional impoundment – it must be maintained in perpetuity, and the failure modes of Question 6a (overtopping, piping, seismic instability) all still apply to whatever structure retains it. Socially, using a natural lake or marine embayment as a Tailings Impoundment Area (Question 1a) permanently destroys that water body as fish and aquatic habitat, which triggers a full federal impact assessment, mandatory fish-habitat compensation and MDMER Schedule 2 listing, and increasingly draws strong opposition from Indigenous nations and coastal/lakeside communities on cultural, traditional-use and precautionary grounds – new natural-water-body TIA approvals in Canada have become rare for exactly this reason, even where the technical ARD-suppression case is strong. An engineered (constructed) subaqueous impoundment avoids the natural-habitat loss but still carries the permanent water-cover dam-safety liability and is markedly more costly to build than simply using an existing lake or embayment.
