24-MMP-A6 Mining and the Environment · December 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-Dec. 3 hours duration, open book (any non-communicating calculator permitted). Six questions of equal value (20 marks each) constitute the bank; a candidate answers any FIVE, with only the first five questions appearing in the answer book marked. Most questions require an essay-format or point-form 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, tailings dam consequence categories, freeboard/PMF design basis, dam safety review and inspection intervals); Global Industry Standard on Tailings Management (GISTM, 2020) (tailings governance, design and closure principles); Price, W.A., MEND Report 1.20.1, Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials (1997/2009) (acid-base accounting, neutralizing potential ratio screening criteria); International Network for Acid Prevention (INAP), Global Acid Rock Drainage (GARD) Guide (ARD prediction, prevention and treatment across the mine lifecycle); Vick, S.G., Planning, Design, and Analysis of Tailings Dams, 2nd ed. (upstream/centreline/downstream embankment construction methods); Government of Canada, Metal and Diamond Mining Effluent Regulations (MDMER, the current name for the exam's "MMER") under the Fisheries Act; Government of Canada, Impact Assessment Act (successor to the 2012 Canadian Environmental Assessment Act); Mining Association of Canada, Towards Sustainable Mining Tailings Management Protocol; International Cyanide Management Institute, International Cyanide Management Code.
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.
An embankment dam – earthfill, rockfill, or (as in a raised tailings dam) built partly from the tailings themselves – resists the retained water or slurry primarily through the MASS and internal shear (frictional) strength of the soil/rockfill making up its broad, sloped cross-section; it tolerates some settlement and foundation deformation, is built up progressively in stages as the impoundment fills (Question 4b), and depends critically on internal zoning (core, filter and drain zones) to control seepage without piping, since the fill itself is never fully watertight. A conventional gravity dam (mass or reinforced concrete) instead resists the water load primarily by its own weight resisting sliding and overturning, and requires a strong, essentially incompressible and non-erodible foundation rock to carry that weight without differential settlement. An arch dam is thin and curved in plan, transferring most of the load horizontally into the canyon abutments via arch action rather than relying on its own weight, and therefore needs a narrow valley with sound, stiff abutment rock. A buttress dam has a relatively thin, sloping upstream face slab supported at intervals by a series of triangular concrete buttress walls that carry the water load down to individual footings on the foundation, using far less concrete volume than a solid gravity section for the same height. In short: embankment dams are staged, soil-mechanics-governed, seepage-controlled structures tolerant of a wide range of foundation conditions and commonly built at least partly from the tailings they retain, whereas conventional gravity/arch/buttress dams are essentially watertight, single-campaign, structural-concrete (or masonry) structures governed by material strength and requiring strong, sound foundation or abutment rock – which is also why gravity/arch/buttress dams are rarely used for a raised tailings facility, where the retained material accumulates over the mine's life rather than being impounded all at once.
| Criterion | Upstream | Centerline | Downstream |
|---|---|---|---|
| Embankment fill / construction material requirements | Each raise is built on the previously deposited tailings beach, so the crest moves toward the pond; lowest fill volume – only a starter dyke plus thin lifts, largely built from the tailings (cycloned sand) themselves | Crest rises vertically above the starter dyke, each raise founded partly on the beach and partly on the prior fill; moderate fill volume of compacted sand or engineered fill | Each raise is placed on the downstream slope of the previous one, so the crest moves away from the pond and never rests on tailings; highest fill volume – 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 |
