Mining Waste and Management – Tailings: 24-MMP-A6 Mining and the Environment, Undated paper, Q4

Question 4: Mining Waste and Management – Tailings (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. General

i. Mine tailings are the finely-ground waste solids remaining after valuable minerals have been separated from ore by milling and concentration (e.g. flotation), discharged as a slurry; by weight, tailings slurry is typically on the order of 50–70% water (a wide range is acceptable, as the question notes).

ii. True – tailings water chemistry depends entirely on the ore mineralogy and the process reagents used, and can be basic (e.g. lime-dosed circuits, or carbonate-hosted ore), acidic (sulphide-rich ore generating ARD/ML), or saline (evaporative climates, or process water with high dissolved-solids reagents).

B. Tailings storage/disposal practices and site selection

Conventional slurry impoundment – tailings pumped as a dilute slurry behind an engineered raised embankment, with a supernatant pond; the most common practice but requires the largest footprint and a permanent water cover/dam.
Thickened/paste tailings – tailings dewatered to a higher solids content before deposition, reducing water use, seepage and the footprint/height of any retaining structure, and allowing a self-draining, steeper deposit.
Dry-stack (filtered) tailings – tailings filtered to a low-moisture cake and transported/compacted like an engineered earth fill without a retaining water pond, minimizing dam-breach consequence but at higher dewatering capital/operating cost.
Sub-aqueous or co-disposal – tailings deposited under a permanent water cover (e.g. in a pit lake) to exclude oxygen and control ARD/ML, or co-mingled/paste-backfilled with waste rock or underground voids to reduce the surface footprint altogether.

Site-selection considerations (environment):

Seismic setting and geotechnical foundation conditions (liquefaction potential, bearing capacity).
Proximity/hydraulic connectivity to fish-bearing water bodies and downstream water users.
Downstream population and infrastructure exposure, which sets the dam's consequence classification and design standard.
Site water balance/climate (precipitation surplus vs. deficit) driving long-term pond and water-management requirements.

C. Tailings impoundment construction – downstream, upstream and centerline

Deliverable	Upstream	Downstream	Centerline
Description/Details	Each new raised lift is built progressively toward the pond, on top of the previously deposited tailings beach itself, so the crest migrates inward over the tailings mass with each raise.	Each new raised lift is built progressively away from the pond, entirely on the original starter-dyke foundation/engineered fill mass, so the crest migrates outward and no lift is ever structurally supported by tailings.	The crest stays at a fixed horizontal position; each raise adds engineered fill vertically on the downstream (outer) face while the pond-side face continues to be supported partly by the tailings beach.
Advantage	Lowest construction material volume and fastest, lowest-cost raise rate of the three methods.	Most stable design under static and seismic loading since the whole structure sits on a designed, compacted fill mass/foundation, never on tailings; permits internal drainage/filter zones.	Better seismic/liquefaction performance than upstream (has a compacted structural fill mass) while using less fill volume and footprint than a full downstream design.
Disadvantage	Structurally relies on the tailings beach for foundation support, making it the most susceptible to static or seismic liquefaction failure (e.g. Mount Polley, 2014; Brumadinho, 2019) and the least favoured design for high-consequence facilities under GISTM (2020).	Requires by far the largest volume of borrow/fill material and the largest footprint, since the crest keeps migrating outward – highest capital cost of the three.	Still partially reliant on tailings support beneath the pond-side portion, and construction control (compaction, internal drainage) is more complex than a pure upstream raise.
Sketch	See Fig. 1	See Fig. 2	See Fig. 3

Fig. 1 – Upstream method: each lift built inward on the previous beach.

Fig. 2 – Downstream method: each lift built outward on an engineered fill mass/original foundation, never on tailings.

Fig. 3 – Centerline method: crest stays fixed; downstream face raised on fill, pond side still beach-supported.

Topic. Raised tailings embankment construction methods and the structural-support/liquefaction-risk trade-off that drives modern (GISTM-era) design preference.

Key relations. Upstream: crest migrates toward pond, fully tailings-supported. Downstream: crest migrates away from pond, fully foundation/fill-mass-supported. Centerline: crest fixed, hybrid support.

Why this works. The deciding engineering question for each method is "what is the new fill sitting on?" – loose, saturated tailings (upstream) are the most liquefaction-prone foundation a raise could have, a purpose-built compacted fill mass (downstream) is the most resistant, and centerline splits the difference by building the compacted fill mass only on the downstream half of each lift.

Common pitfall. Assuming "upstream" and "downstream" describe the direction of tailings deposition or water flow rather than the direction the embankment crest itself migrates with each raise – it is the crest migration direction relative to the pond that defines each method.

Source. Vick, S.G. (1990), Planning, Design, and Analysis of Tailings Dams; CDA Dam Safety Guidelines (2013/2019); GISTM (2020).

Question 4 of 6: Mining Waste and Management – Tailings

Question 4: Mining Waste and Management – Tailings (20 marks)

A. General

B. Tailings storage/disposal practices and site selection

C. Tailings impoundment construction – downstream, upstream and centerline