Question 5 of 6: Tailings Disposal Options

Question 5: Tailings Disposal Options (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) The upstream method of raised embankment construction

All three raised-embankment methods start the same way: a starter dyke, typically compacted borrow fill or waste rock, is built on the natural foundation, and tailings are then discharged from spigots or a perimeter pipeline around the crest, allowing the coarser sand fraction to settle out and form a beach near the point of discharge while fines and water flow toward the pond centre. The methods differ only in where each successive raise is placed relative to the one before it. In the upstream method, each new raise is constructed further toward the pond – on top of and partly over the previously deposited, still-consolidating tailings beach – so the dam crest migrates progressively inward (upstream, against the direction of tailings deposition) over the life of the facility. The structural fill for each raise is often the tailings' own coarse (cycloned sand) fraction, so very little imported borrow material is needed beyond the initial starter dyke.

The upstream method's benefits follow directly from this geometry: it is the cheapest of the three by a wide margin, because it uses the tailings themselves as structural fill instead of imported borrow, and construction is fast and simple, making it attractive for high-tonnage, low-margin operations. Its limitations follow from the same geometry. Because each raise sits partly on saturated, still-consolidating fine tailings, the method depends on the tailings having an adequate coarse-sand fraction to provide bearing capacity and shear strength (fine, low-permeability tailings cannot form a stable beach/foundation for the next raise); the crest advancing over the pond shortens the seepage path with every raise, making the phreatic surface difficult to keep low without extensive internal drainage; and the reliance on loose, saturated, often poorly compacted tailings as structural fill makes the upstream method the most susceptible of the three to static or seismic liquefaction – the failure mechanism behind the Mount Polley (2014), Fundão/Samarco (2015) and Brumadinho (2019) dam failures. This construction approach is not appropriate in high-seismicity regions, on sites where the tailings gradation cannot reliably produce a firm beach, for high-consequence-classification facilities under CDA/GISTM, or wherever the facility must retain a significant volume of free water – upstream construction is fundamentally unsuited to storing large ponded water volumes, since that is precisely the condition (a high phreatic surface, a large driving head) it controls worst. The 2020 Global Industry Standard on Tailings Management, adopted directly in response to Brumadinho, now effectively rules out new upstream construction for "Extreme" and "Very High" potential-consequence facilities among signatory companies, pushing new high-risk designs toward centerline or downstream construction despite the higher capital cost.

b) Subaqueous tailings disposal in natural water bodies

Placing tailings permanently under a water cover – a lake or, historically, the deep ocean via submarine tailings disposal – excludes atmospheric oxygen from sulphide surfaces, which is the same mechanism as the elevated-water-table prevention method in Question 3c and is the primary technical benefit where the tailings are potentially acid-generating: as long as the water cover is maintained permanently, sulphide oxidation and ARD generation are effectively suppressed at the source, avoiding the perpetual active-treatment liability discussed in Question 3b. It also avoids the need to design, build and maintain an engineered surface dam and a dry cover system in perpetuity, and there is established Canadian and international precedent (Island Copper Mine's submarine tailings disposal, several BC and international lake TIAs). The technical issues are the reverse side of the same coin: the receiving water body must remain permanently submerged for the benefit to hold, so any future dewatering (climate change, water-balance error, or an unforeseen future demand for the water) risks exposing previously suppressed sulphides to oxidation all at once; discharge can generate turbidity plumes and density currents that transport fine tailings beyond the intended footprint; and benthic habitat under and immediately around the discharge point is smothered, essentially permanently. Socially, subaqueous disposal in a natural water body faces strong public, Indigenous and ENGO opposition because it is, by definition, a permanent alteration of fish habitat under the Fisheries Act, even where MDMER Schedule 2 listing makes it legally permitted (Question 1a); it typically requires extensive fish-habitat compensation, a lengthy federal environmental assessment and public consultation process, and can conflict directly with Indigenous rights, title and traditional use of the water body – social and regulatory hurdles that, in practice, now weigh at least as heavily on the choice as the technical ARD-suppression benefit.