At this link is a great Master thesis recently completed by Genki Taguchi, a student of the University of British Columbia. He is now back in Japan working in the coal mining industry. Dirk Van Zyl supervised the thesis. And if that link does not work, here is a second link. A great job by both. And a document which should become required reading by all in tailings.
The thesis is call Fault Tree Analysis of Slurry and Dewatered Tailings Management – A Framework. I helped him a little by suggesting that he go back to the Bafokeng tailings failure and analyze the failure using the methods he writes of. He does a superb job and to my mind sheds new light on what may have happened.
He has prepared fault trees to quantify the probability of the main culprits that may have caused the failure: piping, static liquefaction, bulldozer error; penstock failure. Then he does some simple slope stability analyses and shows that the factor of safety at the time of failure was just about unity. He concludes:
It turns out that the likelihood of static slope instability is very high as a result of fault tree analysis. Although the static slope instability would be the main failure of Bafokeng TMF, it seems to be reasonable to think that not only one failure but also a string of failures triggering each other causes this disastrous failure.
I concur. After the Bafokeng failure, I was involved with three other similar dams, of similar construction, similar tailings, and identical foundation soils. They all failed by slope instability. Luckily we had arranged to have the water off the top as a precautionary measure and so there was no off-site flow.
To give you added reasons to download the thesis, here is the abstract.
Fault trees are used in reliability and risk analysis to develop the probability of occurrence of the top event, or failure mode. The top event results from a logical sequence, or combination, of lower level events using “and” and “or” logic.
Probabilities for the basic events, i.e. the lowest level events identified, are calculated or estimated in order to calculate the probability for the top event.
This thesis develops a framework for fault tree analysis for failure of alternative tailings depositional schemes (slurry, thickened, paste and filtered). Failure is narrowly defined as the release of tailings to the environment. The following failure modes are evaluated for each of the depositional schemes: overtopping, static liquefaction, internal erosion, static slope instability and seismic slope instability.
The fault trees are representative of potential failure sequences in the industry as a whole and not on site-specific conditions. Expert elicitation methods are used to select the likelihoods of the basic events.
Not all events in the fault trees are applicable to the range of depositional schemes, e.g. overtopping as a result of a large pool on slurry deposited tailings management facilities is not an event that will occur for filtered tailings. The outcome is that some of the events and parts of fault trees “fall away” as the tailings solids content increases. Apart from providing a visualization of the reduction in probability of occurrence of the top events for the failure modes, the results also provide a range of probabilities for the overall probability of failure for the range of tailings management options.
The framework is used to develop a site-specific likelihood of failure of the Bafokeng tailings facility. The result demonstrates that the fault tree framework can provide useful insights in both industry-wide and site-specific tailings management facility failure likelihoods.
I wonder if it will take as long for us to compile the fault tree of the Mt Polley tailings failure?