I have written extensively in this blog about the Alberta Directive 74. Generally I have been derogatory about the Directive. For example at this link I write the following:
That incredibly silly Alberta Directive 74 says that 5 kPa is the desired strength of oil sands tailings after one year. Where they got his number from is a mystery shrouded in their ignorance of the reality of soil and tailings strengths. 5 kPa is still a fluid and not a solid.
Directive 74 says 10 kPa is the needed strength of oil sand tailings after five years. This is based on the mistake assumption that tailings of 10 kPa strength will be trafficable.
Fact is that you cannot stand on 10 kPa tailings. You still sink in or get stuck in messy goo. No equipment I have ever seen can traffic this strength material unless it is made to float, as some equipment is indeed made.
Now once the tailings get to a strength of 15 kPa, it begins to act as a solid. Not a solid solid, but at least not a flowable material. From lots of observations I can assure you that once the tailings is 15 kPa and has a 50 cm upper crust of dry and cracked material you can begin to move on it. The plate effect of the upper crust of dried tailings helps distribute the load from your feet, so if you walk slowly and carefully you may traverse the mass. But be careful: if you break through the crust, you will sink into the 15 kPa tailings. It may be a soil, but it is still not walk-on-able.
I have always been of the impression that the 5 and 10 kPa requirements were intended to facilitate access so that you can close the facility.
Recently I have begun to wonder if there was not another purpose behind the regulation, namely that in the event of a dam breach, the tailings will not flow out and down to the Athabasca River.
You will have to search with Google for the document, DE-LICENSING OF OIL SANDS TAILINGS DAMS-Technical Guidance Document put out by the OIL SANDS TAILINGS DAM COMMITTEE. In that document they write:
De-licensing process: During the de-licensing process, a tailings facility transitions from a licensed dam to a de-licensed structure. The decommissioning and the initial period of active care are part of the de-licensing process. The process includes preparation of a design plan for the measures to be implemented for the transition, submittal of this design to the regulator, authorization from the regulator to install the measures and approval by the regulator, at which point the structure is de-licensed as a dam and becomes a solid earthen structure. During this process, it is necessary to demonstrate that the transition of a tailings facility into a solid earthen structure has achieved the objectives of the de-licensing process, that the tailings dam no longer meets the definition of a dam, and that it will not revert to a dam in the future.
Note the emphasis on solid earthern structure and will not revert to a dam, i.e., a fluid containing facility. The idea is that in the goodness of time, perimeter dike or dams will surely breach, but that does not matter, for the solid tailings will stay put, or at worst erode at the same rate as the surrounding landscape.
At this link is the document Guidelines for Performance Management of Oil Sands Fluid Fine Tailings Deposits to Meet Closure Commitments. It purports to tell you how to manage a tailings facility to achieve a solid mass at closure. Certainly interesting reading and a good guide to the many other documents pertaining to operation and closure of oil sands tailings facilities.
We know that the oil sands industry is trying many things to get 5 and 10 kPa. The easiest way is to add polymer into the tailings no more than 100 m from the discharge point. I know this works. In fact with polymer amendment it is very easy to get 5 kPa in far less than a year. And 10 kPa in less than five years. See my paper at this link for a much more detailed exposition of this idea.
Of course many other approaches besides polymer amendment are being tested. Cyclones, centrifuges, direct mixing with overburden waste, and freeze/thaw are also being investigated.
But the fact remains that Alberta, by plan or inadvertently, demands that the tailings be placed so that they are not a fluid for very long and that in the event of a dam breach the tailings will not flow to the river.
We have heard, and I have written about filter press tailings recently in the light of the Mt Polley failure. Filter pressing is held out as one way to preclude tailings flow through a dam breach. Commenters have noted shortcomings with filtered tailings. One example is Greens Creek in Alaska where they have used filter tailings management for more than thirty years. We recommended that because the site is in a very high seismic zone and because it was impossible to manage water in the very rainy environment. The tailings stack is lined, seepage is collected, and treated prior to discharge.
Another example is the Escobal Mine in Guatemala. We have recently installed wick drains to speed up excess pore pressure dissipation. We now add some half a percent lime to the filtered tailings to get moisture contents suitable for compaction. None of this is cheap.
I propose that we take up the contentious discussion anew of how to dispose of tailings so that in the event of a dam breach they will not flow out. Consider filter pressing—it works. Consider polymer amendment—it works. Consider paste with a bit of lime or cement—it works. Consider wick drains—they work.
Of course all these approaches are more expensive than hydraulic fill—at least until failure & flow.
The issue is should BC go the way Alberta has gone? Should BC regulate a minimum tailings strength one year and five years after deposition? Should we leave it to the individual mining company to decide on the basis of risk tolerability? Or should we just demand safer dams to impound fluid tailings?
Let us hear your opinion.