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.
| Failure mode | Typical causes | Physical failure process |
|---|---|---|
| Erosion | Inadequate vegetative/riprap protection, excessive slope length or steepness, concentrated flow at a poorly armoured discharge point | Rainfall/runoff or wind progressively strips material from the downstream face, crest or spillway channel, incising rills and gullies that thin the effective cross-section and can expose or undermine internal drains, potentially initiating piping or a slope failure |
| Foundation failure | An unidentified weak/compressible foundation layer (soft clay, buried organics) or karst/solution features missed in the geotechnical investigation | The dam and its foundation fail together along a slip surface passing beneath the structure, rather than through the fill alone; unexplained differential settlement, toe bulging or cracking are typical warning signs before a full failure |
| Liquefaction | Loose, saturated, cohesionless tailings (especially an upstream-raised foundation) subjected to cyclic (seismic) or rapid static loading | Rising pore-water pressure approaches the total confining stress, causing the material to lose shear strength almost entirely and behave as a fluid; can trigger a rapid, near-total flow failure with little warning – the mechanism behind the worst historic tailings dam disasters (e.g. Fundao/Samarco 2015, Brumadinho 2019) |
| Overtopping | Undersized or blocked spillway/decant system, or an inflow design flood underestimated relative to actual site hydrology | Flood inflow or wave run-up exceeds available freeboard and spillway capacity, so water flows uncontrolled over the crest; the unprotected downstream face erodes rapidly under this flow and can breach within hours – historically one of the most common embankment dam failure modes worldwide |
| Piping | Seepage exit gradients too high, or a drainage/filter zone that lets fines migrate rather than retaining them | Internal erosion progressively enlarges a concentrated seepage path into an open conduit that can propagate backward from the downstream exit toward the reservoir; often shows warning signs (boils, cloudy seepage, crest sinkholes) before a full breach, unlike liquefaction |
| Rotational sliding | Rapid rise of the phreatic surface (heavy rainfall, fast pond rise), rapid drawdown of the pond, or over-steepened slopes | A mass of the embankment (and sometimes foundation) moves along a roughly circular slip surface once destabilizing forces exceed the available shear resistance (factor of safety falls below 1.0 in a slope-stability analysis) |
Each term in the diagram is measured or estimated by a different method. Direct precipitation on the pond and beach is estimated from an on-site or regional rain gauge multiplied by the wetted surface area. The mill process water / tailings slurry discharge, usually the dominant inflow, is measured directly with a flow meter on the tailings delivery line. Upstream catchment surface runoff is estimated with a rainfall–runoff model (e.g. the Rational Method or a curve-number method) applied to the contributing catchment area and its land cover. Groundwater inflow, where the impoundment is hydraulically connected to a permeable aquifer, is estimated from piezometer gradients and a site hydrogeological model, and is usually a minor term. On the outflow side, evaporation is estimated from pan-evaporation data (with a pan coefficient) or a Penman-type equation applied to the pond surface area; reclaim/decant water pumped back to the mill – typically the single largest managed outflow, since most operations recycle the bulk of their process water – is measured directly by a flow meter on the reclaim line; seepage through or beneath the embankment is measured from underdrain/toe-ditch collection flow, or calculated from Darcy's law (Question 1) given the phreatic surface position and material permeability; and controlled discharge at the spillway or MDMER compliance point is measured with a calibrated weir or flow meter and sampled under the mine's Environmental Effects Monitoring program.
