Closure Planning and Reclamation: 24-MMP-A6 Mining and the Environment, May 2016, Q3

Question 3: Closure Planning and Reclamation (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. Controlling water-based erosion on impervious-capped waste dumps (5 marks)

Graded surface contouring and terracing (benching). Shaping the cap into a series of gently-graded benches with a shallow overall slope (rather than one long uninterrupted slope) breaks up runoff into shorter flow paths, keeping individual flow velocities and erosive energy low.
Engineered surface water diversion – interceptor ditches, berms and lined channels. Diversion ditches placed along terrace breaks and at the toe collect concentrated runoff and route it to a stable, armoured (rip-rap lined) discharge or sediment pond rather than allowing sheet flow to erode the cap surface directly.
Vegetative cover / erosion-control blankets or riprap armouring. A self-sustaining vegetative cover establishes root structure that binds the surface soil layer and slows overland flow; on steeper slopes or in the immediate post-construction period before vegetation establishes, synthetic erosion-control blankets or a riprap/rock armour layer provide the same protection mechanically.

B. Engineering investigations for an underground mine closure plan

Engineering investigations by hazard category, underground mine closure plan
Hazard category	Engineering investigations required
Mine entry points (shafts, adits, portals, raises)	Inventory and survey of every known opening (including old workings from historical records); geotechnical assessment of the surrounding rock/soil for stability of a permanent closure structure; design of physical closures – engineered concrete bulkheads/caps for shafts, backfilling or grating for adits, sized to prevent unauthorized human/wildlife access and to withstand design loading (traffic, snow, seismic); provision for continued ventilation or drainage function where an opening cannot simply be sealed.
Mine working areas (stopes, drifts, pillars)	Rock mechanics assessment of long-term pillar and stope stability, including time-dependent strength degradation and the potential for pillar failure or subsidence propagating to surface; delineation of a surface subsidence zone / setback area from stability modelling; identification and engineering treatment (backfill, controlled collapse, or monitoring) of any workings within that at-risk depth-to-span ratio of surface infrastructure or public access areas.
Mine gas	Characterization of residual gas generation potential (methane in coal-associated deposits, radon in uranium/some hard-rock settings, or oxygen-deficient atmospheres from timber decay/oxidation in old workings); design of passive or active venting systems at sealed openings to prevent gas accumulation and pressure buildup behind bulkheads; long-term gas monitoring program at closure structures and any nearby surface receptors.
Groundwater (mine flooding and geochemistry)	Hydrogeological modelling of the post-closure flooding process (rebound rate, final static water level, connection to surface water bodies and other aquifers); prediction of flooded-mine-water geochemistry (particularly ARD/ML from any sulphide-bearing zones, per Question 1's testing methods) and design of any required passive or active water treatment before discharge; assessment of whether a permanent bulkhead/seal is needed to control or direct the direction and rate of water level recovery, and design of long-term water quality and water-level monitoring wells.
Surface subsidence and ground movement	Numerical or empirical subsidence prediction (angle of draw, subsidence trough geometry) over shallow or historic workings; structural risk assessment for any surface infrastructure or public land within the predicted zone of influence; ongoing survey monument monitoring program to confirm predictions and detect any reactivation.
Waste rock and tailings facilities (linked to the underground complex)	Long-term geochemical and geotechnical stability assessment per the closure requirements common to all mine types (Questions 1, 4 and 5-6) – not unique to underground mines but must be integrated into the same closure plan and financial security estimate.

Topic. The engineering scope of an underground mine closure plan, organized by the hazard categories the exam specifically names (entry points, working areas, mine gas, groundwater).

Key relations. None quantitative; the governing framework is hazard identification → engineering investigation/characterization → physical or institutional control design → long-term monitoring, applied separately to each hazard category.

Why this works. An underground mine's closure hazards are physically hidden (behind a sealed portal, below a subsidence-prone surface, within a slowly-flooding void), so unlike an open pit's largely visible final landform, the closure plan must rely on predictive geotechnical, hydrogeological and gas-generation modelling rather than direct observation to demonstrate the site will be safe and stable in perpetuity, which is exactly why regulators require this level of engineering investigation before issuing or maintaining a mining licence.

Common pitfall. Treating "closure" as simply plugging the visible openings – a defensible closure plan must also address invisible long-term risks (gas buildup behind a sealed bulkhead, slow subsidence propagation, decades-long mine-water geochemistry evolution during and after flooding) that only manifest well after the mine has stopped operating.

Source. BC Health, Safety and Reclamation Code for Mines (closure planning requirements); INAP GARD Guide (flooded-mine water geochemistry).

Question 3 of 6: Closure Planning and Reclamation

Question 3: Closure Planning and Reclamation (20 marks)

A. Controlling water-based erosion on impervious-capped waste dumps (5 marks)

B. Engineering investigations for an underground mine closure plan