24-MMP-A6 Mining and the Environment · May 2016
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, 2016-May. 3 hours duration, open book (any non-communicating calculator permitted). SIX questions are printed on the paper; FIVE questions constitute a complete exam paper, and only the first five questions as they appear in the answer book are marked. Most questions require an essay-format answer; clarity and organization are explicitly assessed.
Reference texts: International Network for Acid Prevention (INAP), Global Acid Rock Drainage (GARD) Guide (ARD prediction, static and kinetic testing, sampling programs); Price, W.A., MEND Report 1.20.1, Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials (1997/2009) (acid-base accounting, sampling protocols); 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); BC Ministry of Energy, Mines and Low Carbon Innovation, Health, Safety and Reclamation Code for Mines in British Columbia (current edition) (closure planning, reclamation, waste dump erosion control); Canadian Dam Association (CDA), Dam Safety Guidelines (2013/2019 update) (tailings embankment design, dam safety inspections, failure modes); Global Industry Standard on Tailings Management (GISTM, 2020) (tailings governance and monitoring); Vick, S.G., Planning, Design, and Analysis of Tailings Dams (1990) (upstream/centerline/downstream embankment construction methods, beach hydraulic sorting); ATSDR, Toxicological Profiles for arsenic, polycyclic aromatic hydrocarbons, molybdenum and silica.
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.
| Attribute | Upstream | Centerline | Downstream |
|---|---|---|---|
| Embankment fill/construction material requirements | Minimal engineered fill – a small starter dyke only; each raise is built mostly of cycloned/spigotted tailings sand itself, so little imported borrow material is needed. | Moderate engineered fill on the downstream face of every raise (compacted till, rockfill or cycloned sand), while the upstream side rests partly on the tailings beach – an intermediate material demand. | Full engineered fill (compacted till, rockfill, or waste rock) for the ENTIRE embankment cross-section at every raise, since none of the structural mass is ever supported by tailings – the highest fill/borrow demand of the three methods. |
| Mill tailings requirements | Requires a coarse, cyclonable sand fraction (typically >40-60% sand by cyclone underflow) to build a beach strong enough to found the next raise – not usable with a fine, high-clay/slimes tailings stream. | Needs a moderately cyclonable sand fraction, though less critical than upstream since the downstream face is independently engineered; can tolerate a somewhat finer overall tailings gradation than the upstream method. | No dependency on tailings gradation at all – works with any tailings type, including very fine, high-clay slimes, since the structural embankment never relies on the tailings for support. |
| Suitability for water storage | Poor – not suitable for significant permanent water storage or a large flood-attenuation pond, since a rising phreatic surface driven by a large stored head can saturate and destabilize the loosely-placed sand fill mass. | Moderate – can accommodate a modest operating pond but is still not preferred for large permanent water storage due to the partially tailings-supported upstream zone. | Good – the fully engineered fill mass can be designed (with a proper core/filter/drain zone) much like a conventional water-retention dam, making it the preferred method where substantial water storage is required. |
| Seismic resistance | Poorest – loosely placed, often saturated hydraulic sand fill is the most susceptible of the three to strength loss and static or seismically-triggered liquefaction (the dominant failure mechanism in several major historical upstream-dam failures). | Intermediate – the engineered downstream fill mass adds meaningful seismic resistance versus upstream, though the upstream tailings-supported zone remains a comparative weak point. | Best – a fully compacted, engineered structure can be designed and constructed to the same seismic performance standard as a conventional earth/rockfill dam. |
| Rising rate restrictions | Most restrictive – the rate of raise is limited by how quickly deposited tailings can consolidate and gain sufficient strength to support the next raise; raising too fast is the classic root cause of upstream failures. | Moderate restriction – the independently-engineered downstream fill is not consolidation-limited, but the upstream tailings zone still imposes some pacing requirement. | Least restrictive – raise rate is governed mainly by fill placement/compaction logistics and available borrow supply, not by tailings consolidation, so it can generally be raised fastest. |
| Methods for phreatic surface control | Difficult to control – limited opportunity to install internal drains/filters within the sand-fill raises, so the phreatic surface is managed mainly by controlling pond location (kept well back from the crest) and beach width. | Moderate control – internal chimney/blanket drains can be incorporated into the engineered downstream portion of each raise, improving on the upstream method. | Best control – a full internal zoning with chimney drain, filter and drainage blanket (as in a conventional embankment dam) can be engineered to keep the phreatic surface well within the downstream fill mass. |
| Relative cost | Lowest – minimal imported fill and simple construction equipment/methods. | Intermediate. | Highest – full engineered fill volume at every raise is far larger than the other two methods for the same overall height, driving up both material and construction cost. |
| Sketch | See the three cross-sectional sketches below. | ||
As tailings slurry is discharged (spigotted or via a cyclone) near the embankment crest and flows down the beach toward the pond, hydraulic sorting by settling velocity produces a systematic, predictable slope and grain-size pattern. The beach above water is comparatively steep (commonly on the order of 1-3%, though the exact value depends on slurry density, particle size distribution and discharge method) because the flow here is shallow, higher-velocity sheet flow that can only keep coarser sand-sized particles in suspension over a short travel distance, so the coarsest fraction drops out almost immediately near the discharge point and armours a relatively steep sub-aerial slope. Once the flow crosses the waterline into the beach below water, velocity drops sharply and the slope flattens markedly (typically well under 1%), because the remaining suspended load is now dominated by progressively finer sand and silt that settles out gradually over a much longer flow path under the ponded water. By the time the flow reaches the decant pond itself, only the finest silt and clay-sized "slimes" fraction remains in suspension, settling slowly to form a soft, low-permeability, low-strength sediment at the pond bottom. This produces a horizontal grain-size gradient – coarse near the embankment, progressively finer with distance toward the pond – that is a defining, predictable feature of hydraulically-deposited tailings and is exploited directly by the upstream and centerline construction methods (Question 4A), which rely on this coarse near-crest material to found the next raise.
