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24-MMP-A6 Mining and the Environment · December 2013

Question 4 of 6: Reclamation and Mine Closure

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Mining and Mineral Processing Engineering, 09-MMP-A6 Mining and the Environment, 2013-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. 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); 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 4: Reclamation and Mine Closure (20 marks)

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.

a) Limiting pond water and reducing pore water pressure (any three satisfy the mark scheme)

Methods of controlling pond water and pore water pressure in a tailings impoundment
MethodHow it works
Internal (finger/chimney) drainsFree-draining sand or gravel drains placed within the tailings mass intercept infiltrating and pore water and route it to a collection sump, actively lowering the phreatic surface and accelerating consolidation of the underlying fines.
Basal underdrainsA drainage blanket or pipe network installed at the tailings/foundation interface before or during deposition removes pore water from the base upward, giving the deepest, slowest-draining material a shorter drainage path and speeding strength gain.
Engineered surface grading and progressive cappingSloping the reclaimed surface to shed precipitation as runoff rather than letting it pond and infiltrate, combined with progressive placement of a low-permeability cover as areas are decommissioned, minimizes the water actively being added to the impoundment.
Prefabricated vertical (wick) drainsWick drains installed on a grid through soft, slow-draining tailings shorten the drainage path for consolidation dramatically, allowing pore pressures generated by the tailings' own weight (or by a reclamation surcharge) to dissipate in months rather than years.

b) Open pit closure hazards and engineering investigations

Open pit mine closure – hazards and the engineering investigations that address them
Closure hazardEngineering investigation(s) required
Pit slope instability / rockfallGeotechnical slope-stability analysis (limit-equilibrium and, for complex structure, numerical/finite-element modelling) to confirm the final, unmaintained pit walls are stable long-term at the as-mined angle, informed by structural mapping and, where warranted, remedial scaling, bolting or slope flattening.
Pit lake water quality (ARD-impacted lake)Geochemical characterization (ABA/kinetic testing, Question 3a) of the exposed pit-wall rock, combined with a pit-lake filling and water-balance/quality prediction model, to determine whether the lake will meet discharge or beneficial-use criteria or require a permanent treatment/management strategy.
Open-pit fall/drowning hazard to public and wildlifePhysical-hazard risk assessment to size fencing, berms, signage and wildlife-exclusion measures around the final pit crest and any residual pit lake.
Regional groundwater drawdown/recoveryHydrogeological modelling of post-closure water-table recovery around the dewatered pit, to predict effects on nearby wells, wetlands and surface water features and to design any needed mitigation (e.g. alternate water supply).
Waste rock dump stability and ARDGeotechnical stability analysis of the reclaimed dump slopes and, in parallel, ABA/kinetic geochemical testing of the waste rock to confirm whether cover, blending or drainage controls (Question 3c) are needed.
Erosion and sediment transport from reclaimed surfacesErosion-potential and sediment-yield modelling to design final surface grading, vegetative cover and sediment-control structures capable of surviving design storm events without maintenance.
Loss of vegetation / land capabilitySoil-cover design and revegetation/land-capability studies (species selection, growth-medium depth and quality) to establish a self-sustaining ecosystem matching the approved post-closure land use.
Long-term monitoring and verificationDesign of a monitoring-well and surface-water station network sized to confirm, over a multi-year post-closure period, that predicted water quality, slope performance and revegetation success are actually being achieved before financial surety (Question 1) is released.