24-MMP-A6 Mining and the Environment · May 2013
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, 2013-May. 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. 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, tailings dam consequence categories, freeboard/PMF design basis); 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) under the Fisheries Act; Government of Canada, Impact Assessment Act (successor to the 2012 Canadian Environmental Assessment Act); Government of Ontario, Mining Act (progressive rehabilitation requirements).
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 | Starter dyke only; every subsequent raise is built from cycloned coarse tailings itself – minimal imported borrow. | Cycloned coarse tailings on the downstream face plus imported compacted fill on the crest, raised with the centerline held fixed over the starter dyke. | Fully engineered, zoned/filtered embankment (compacted earthfill/rockfill) built entirely on new ground beyond the previous raise – largest imported-fill requirement of the three. |
| Mill tailings requirements | Needs a coarse (sand) fraction, typically ≥40–50% by cycloning, to build a stable beach and dyke – unsuitable for fine, high-clay tailings. | Benefits from a moderate coarse fraction for the beach/downstream face, but is less strict than upstream. | No specific gradation requirement – works with any tailings, including fine or slimy material, because the embankment itself is not built from tailings. |
| Suitability for water storage | Poor – only a small, controlled decant pond kept well away from the crest is compatible. | Moderate – a somewhat larger, better-controlled pond is possible. | Best – can safely retain a large permanent pond, similar to a conventional water-retention dam. |
| Seismic resistance | Poorest – loose, saturated beach sands beneath each new raise are susceptible to liquefaction/flow failure; several major historic tailings dam failures were upstream-constructed. | Intermediate – the vertical raise geometry provides more confinement than upstream, reducing but not eliminating liquefaction susceptibility. | Best – a fully engineered, filtered/drained embankment provides the highest resistance to seismic loading and liquefaction. |
| Rising rate restrictions | Most restrictive – each raise must wait for pore-pressure dissipation/consolidation of the underlying tailings, typically limiting rise to a few metres per year. | Moderate – faster than upstream since less of each raise sits directly over loose saturated tailings, but consolidation must still be monitored. | Least restrictive – governed by conventional embankment construction and fill supply rather than tailings strength gain. |
| Methods for phreatic surface control | Largely passive and difficult – relies on beach permeability and keeping the pond small and distant from the crest; internal drains are hard to retrofit. | Internal chimney/blanket drains can be built into each raise, actively lowering the phreatic surface. | Internal filter/drain zones (chimney drain, drainage blanket, toe drain) give the most reliable and easily engineered phreatic control of the three. |
| Relative cost | Lowest – minimal borrow material and construction effort, offset by the highest failure-consequence risk. | Intermediate – more imported fill and drainage works than upstream, less than downstream. | Highest – greatest volume of imported/engineered fill and the largest downstream footprint growth over the facility's life. |
| Sketch | See figure below (left panel). | See figure below (centre panel). | See figure below (right panel). |
The beach above water typically slopes on the order of 1–4% away from the spigot, while the submerged (below-water) beach within the pond is markedly flatter, generally well under 1% and close to horizontal, because turbulent sheet flow on the exposed beach loses competence to carry coarse particles quickly, while the quiescent, ponded water beyond the waterline allows only the very finest particles still in suspension to settle, over a much longer travel distance and gentler resulting slope. Horizontally, grain size therefore fines with increasing distance from the discharge point (coarse sand near the crest → medium/fine sand at mid-beach → silt at the waterline → clay-sized slimes at the pond centre). Vertically, because the discharge point and rate are moved periodically (cyclic spigotting, seasonal changes in tonnage or the operator's own beach-management practice), a single vertical profile at a fixed location commonly shows interbedded coarse-fine couplets rather than one smooth fining-upward sequence.
