In this second article of my series of Sharing Critical Control Measures, I discuss the importance of three critical control measures to help prevent unexpected “Black Swan” events, including:
- Using independent experts.
- Challenging the basic assumptions of experts (wearing a “black hat).
- Sharing learnings to prevent similar events.
At 144 million tons, the Manefay Landslide at the Bingham Canyon Mine in 2013 was the largest in mining history. As noted in my article “Sharing Critical Control Measures #1 – Knowing the Greatest Risk”, the geotechnical team did a fantastic job of predicting the failure nearly two months before it happened, resulting in the planned evacuation of the mine, which prevented loss of life and injuries. However, predicting how the Manefay would fail did not go nearly as well, resulting in damaged and destroyed equipment.
Before the Manefay, all of the mine’s highwall failures followed a typical wedge or circular failure model. The mass would start to accelerate over time until it would go into a “progressive” failure state. The acceleration of the mass would increase exponentially, ending with the highwall losing its structural integrity and a frictional rock slide would take place. In a matter of hours or days the mass would move downward until it achieved the rock’s natural angle of repose. At that point the mass would be stable and stop moving.
The Manefay was different. Although it began accelerating like all the previous failures, when it went into progressive failure, it ultimately failed in a catastrophic event. At 9:30 pm on April 10, 2013, the accelerating mass literally exploded out of the highwall. Instead of taking hours or days, tens of millions of tons of rock flowed like an avalanche (volellmy properties versus frictional properties) nearly 1 ½ miles to the bottom of the pit – in just 90 seconds. To make matters worse, an hour and 35 minutes later, a second episode occurred and once again tens of millions of tons of rock exploded out of the highwall and flowed into the bottom of the pit, just like the first.
The way that the Manefay failed was a surprise as it traveled much further and faster than anticipated. Anything in the path of the enormous mass was either destroyed or was simply picked up and shoved out of the way. This resulted in the damage and destruction of three large mining shovels (including a P&H 4100C), thirteen 320 ton haultrucks, three drills as well as a multitude of spare parts, support equipment and supplies. The photo shows one of the haultrucks that was shoved by the slide, as shown in the opening photo of the article.
We knew a portion of the only haulroad going into the mine would be destroyed. Therefore, the plan was to be able to quickly resume mining ore and sending it to the inpit crusher and conveyor after the failure. To achieve the plan we stationed equipment and supplies on and behind the “Moly dome” which was 300 feet above the bottom of the pit and away from the Manefay mass. But because the failure traveled so fast and far it not only covered the Moly dome, it deposited another 300 feet of debris on top of it. The results of the failure were clearly not anticipated.
These photos show the before and after view of the bottom of the pit from nearly the same location. For scale, each one of the benches is 50 on the highwall is 50 foot tall.
So how is it that a geotechnical team that did such a great job of predicting the failure could be surprised with how it failed?
Unlike all previous highwall failures at the mine, the Manefay was an active/passive block failure instead of the typical wedge or circular failure. This, and the fact that the Manefay was so large that it built up a tremendous amount of pressure, resulting in the force needed to propel the rock out of the wall, defying expectations. The technical experts had relied on their extensive experience and understanding of the typical failures at the mine when planning for the Manefay and did not consider other failure mechanism scenarios. The Manefay could be classified as a “black swan” event, as described by Nassim Nicholas Taleb in his 2012 book, The Black Swan: The Impact of the Highly Improbable. Per Taleb, a “black swan” event is one that is unexpected, has a major impact and is the first recorded instance (but is often rationalized that it could have been expected, even if it couldn’t). The Manefay fits these criteria since the way it failed was unexpected, it had a major impact to both the Bingham Canyon operations as well as the mining industry understanding of possible failure modes, and there was no documented highwall failure avalanche events having occurred in open mines.
This leads to the next question – if the way the Manefay failed is classified as a black swan event, what critical control measures can be used to prevent or at least anticipate such events?
A black swan event is only a black swan because no one considered the real possibility of the event occuring. The first critical control measure that could have been implemented before the Manefay is the use of independent experts to review work on critical issues. We knew the Manefay was larger than the historic failures at the mine and it was acting differently than any previous failures. But we were confident in our knowledge and understanding of failures at the mine and relied on our own extensive experience to analysis the potential outcome. The use of independent experts could have increased the breadth of experience and understanding of different failure modes. In addition, independent experts may not have been blinded by the previous successes and biases of the operation. However, it is not a given that independent experts would have actually made a difference in anticipating the ultimate failure mode.
After the Manefay, the mine created and relied heavily on a team of independent experts called the Mine Technical Review Team (MTRT). This team reviewed the geotechnical analysis and remediation plans before we progressed on each phase of that work. The MTRT challenged our basic assumptions and were key to the success the mine had in quickly and effectively recovering from this massive event.
The second critical control measure is for leaders in the company to challenge the assumptions and work of technical experts (or “wear a black hat” as Edward De Bono describes in his book “Six Thinking Hats”). This can be difficult but essential to do in situations like the Manefay where the geotechnical team did such a good job of predicting the failure, but were still faced with conditions that they had never experienced before. I think about myself, coming in to lead the mine planning team, just six weeks before the failure and being so impressed that the geotechnical team had found the Manefay in the first place, that I did not question the analysis or conclusions. However, I was the perfect person to question or “wear a black hat”, since I did not have as the same history, experiences or preconceived expectations as other people in the company. Wearing the black hat should not be looked at as being judgmental or not trusting, it is just one more important critical control measure designed to keep people safe.
Although the first two methods may or may not have made a difference to the outcome, the third critical control measure, which is sharing and documenting our experiences about critical learnings, is the best way to prevent black swan events. Unfortunately, these learnings may be the result of experiencing a black swan or traumatic event, but sharing what is learned is a way prevent others from experiencing the same fate.
To the credit of Rio Tinto/Kennecott, they took the position of sharing what they experienced, to benefit the entire industry. There are other examples of companies sharing their learnings of large scale events such as the investigation reports from BHP/Vale in the Fundão Tailings Dam Investigation Report or Imperial Metal’s Mount Polley Investigation Report. However, not all parts of the industry are as willing or able to share what they have learned. If our industry is going to continue to improve its safety record and prevent large events, we must learn from each other and share both the good things we do as well as the hard lessons. That is the purpose of this series of articles. In addition of the University of Arizona, through the Mining Engineering and Public Health Departments, is evaluating ways to capture and document these critical control measures and learning to be shared throughout the industry. Please consider sharing your experience so we might all learn without going through a crisis.