Here is a question posed of me in a recent email:
It is not unusual for earthen tailings dams to use a Factor of Safety of 30%. There are many more unknowns in constructing an earthen dam than in constructing a building or concrete water dam. The hundreds of failed tailings dams demonstrate this. It seems this Factor of Safety is significantly line with the risks. Buildings, made of predictably engineered steel and factory made concrete use a Factor of Safety of 200%. An appropriate number for earthen dams would be much larger than that, without taking into consideration the hazardous contents of tailings dams.
My reply: The Factor of Safety (FOS) is at best a fudge-factor. Acceptable FOSs vary from industry to industry, from situation to situation. Most FOSs are based on practical experience and observation of what stood at what FOS and what failed at what FOS.
Some think of the FOS as the Ignorace Coefficient. The more ignorant you are of a parameter, the higher the FOS you adopt. It all boils down to judgment.
Many other approaches have been tried. Risk of failure is but one of them. Think of probability of malfuntion, then consider consequence, then formulate a risk factor. They all work and they all fail if mis-applied.
The point is that regardless of FOS, Risk, Probability, etc., in the absence of sound engineering, tailings dams will fail. There is no point in upping the FOS if you do not implement good engineering practice.
But then maybe this reply is too simplistic. Let us have your opinion.
Jack, I think your general comment on FOS is right. There is another significant point in the question that bears addressing. It implies that engineers are much more comfortable with the risk of failure in slimes dams than they are in water dams and buildings. Is that true? If so, why, and is that acceptable or not?
My impression is that this is true. I believe it is because mining is cavalier about environmental impact. What is not acceptable is that the failure trend data that I have seen show the trend of failures is steady, not decreasing. That implies that the engineers are not adopting lessons learned.