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.
| Failure mode | Typical causes & physical failure process |
|---|---|
| Erosion | Caused by uncontrolled or under-designed surface water management – concentrated runoff channelling down the downstream slope, wave action against the upstream face, or inadequate spillway/diversion capacity during a large storm. The physical process is progressive removal of surface material (rilling, then gullying) that deepens over repeated events, eventually undermining the embankment's structural cross-section or creating a preferential seepage path; unlike the other modes below, erosion failure is typically slow and progressive rather than sudden, giving more opportunity for visual detection and intervention. |
| Foundation failure | Caused by an undetected or under-characterized weak layer in the natural foundation beneath the embankment (e.g. a soft glaciolacustrine clay or loose saturated sand) that cannot support the embankment and tailings mass as it is raised. The physical process is a deep-seated shear failure surface developing through the weak foundation layer rather than through the embankment fill itself, often triggered once the cumulative load from successive raises exceeds the foundation's undrained shear strength – this was the root cause of the Mount Polley (2014) failure. |
| Liquefaction | Caused by loosely-placed, saturated, low-density tailings sand or silt (characteristic of upstream-constructed embankments, Question 4A) losing shear strength when subjected to a triggering disturbance – seismic shaking or, in some documented cases, purely static loading (rising pore pressure from continued raising with no seismic trigger at all). The physical process is a near-total, sudden loss of effective stress and shear strength as excess pore pressure develops faster than it can dissipate, transforming the material from a solid to a fluid-like state and allowing rapid, large-displacement flow failure – the mechanism behind both Fundão/Samarco (2015) and Brumadinho (2019). |
| Overtopping | Caused by insufficient freeboard, an undersized or blocked spillway/decant system, or a design flood/inflow event that exceeds the structure's water-handling capacity (sometimes compounded by a failed decant pipe or pump). The physical process is water flowing directly over the embankment crest, which rapidly erodes the unprotected downstream face (since it was never designed as a spillway) and can breach the crest within hours once erosion cuts a channel deep enough to concentrate the full overtopping flow. |
| Piping | Caused by uncontrolled internal seepage finding a preferential flow path through the embankment or its foundation (often along a poorly-compacted zone, a conduit/pipe penetration, or a permeability contrast) at a hydraulic gradient exceeding the material's critical gradient for particle mobilization. The physical process is progressive internal erosion: fine particles are washed out along the seepage path, enlarging it into a continuous open "pipe" that increases flow and erosion rate in a self-accelerating feedback loop, ultimately collapsing the roof of the pipe and triggering a sudden breach – often with little external warning until very late in the process, making internal seepage/piezometer monitoring (Question 6B) especially critical for this mode. |
| Rotational sliding | Caused by the embankment or foundation slope's mobilized shear strength being exceeded by the driving shear stress along a potential slip surface – commonly from over-steepened slopes, elevated phreatic surface/pore pressure reducing effective strength, or added load from rapid raising. The physical process is a rotational (roughly circular-arc) shear failure surface developing through the slope, with the upper portion of the failure mass rotating downward and outward while the toe bulges upward – a classic slope-stability (limit equilibrium, e.g. Bishop's method) failure mechanism rather than the internal-erosion or liquefaction mechanisms of piping/liquefaction. |
Visual observation. Site personnel conduct routine (daily-to-weekly) walk-around inspections of the crest, slopes, toe and decant/spillway structures, looking specifically for new seepage or a change in seepage colour/turbidity, cracking, settlement, sinkholes, erosion gullies, or any unusual wet area on the downstream face. It is the cheapest and most frequent layer of surveillance and, critically, the only method that can catch a genuinely novel or localized problem an instrumentation network was not specifically sited to detect; observations are logged against a standard checklist and any anomaly triggers escalation to the Engineer of Record for a follow-up formal inspection, one layer within the same defence-in-depth surveillance hierarchy (daily visual → scheduled instrumentation → annual formal inspection → independent multi-year Dam Safety Review) used industry-wide for tailings facilities.
Piezometers. Vibrating-wire or standpipe piezometers installed at multiple depths and locations through the embankment and foundation directly measure pore water pressure, from which the position of the phreatic surface (Question 4A) and the factor of safety against both piping (comparing local hydraulic gradient to the critical gradient) and slope stability (via reduced effective stress) are calculated. Readings are taken on a schedule tied to the dam's consequence classification (continuously via automated data loggers for higher-consequence dams) and trended over time against predefined trigger and action levels – a piezometer reading that rises unexpectedly or fails to respond to a known change (e.g. pond drawdown) is one of the most sensitive available leading indicators of a developing internal seepage or piping problem, often providing warning well before any external visual sign appears.
Slope displacement via survey monuments or inclinometers. Surface survey monuments (or increasingly, InSAR/GPS-based automated monitoring) resurveyed on a fixed schedule track absolute crest and slope movement over time, while inclinometer casings installed vertically through the embankment measure the internal depth-distribution of any lateral deformation, together locating both the magnitude and, for the inclinometer, the depth of any developing shear surface. A steady, slow, decelerating displacement rate is generally consistent with normal consolidation, whereas an accelerating displacement trend (particularly one localized at a specific inclinometer depth, indicating a discrete developing shear plane) is a strong precursor signal for an impending rotational or foundation-mode slope failure, giving the operator time to intervene (unloading, drainage improvement or evacuation) before a sudden failure.
