Published on October 5, 2018
Midas Gold Idaho wants to keep the community informed about the work we are doing at the Stibnite Gold Project site. The Ask Midas blog series gives the experts in our company a chance to answer some of the community’s most frequently asked questions and help clear up any misconceptions around the project.
WHAT DOES FACTOR OF SAFETY MEAN?
At Midas Gold, we often talk about the factor of safety for our proposed facilities – especially our tailings storage facility. This facility is where the ore will be sent after we extract the profitable metals from the ore. Our tailings facility is designed so that it will have a superior factor of safety. But what does that really mean?
Factor of safety is a rating that compares the design strength of a structure to the stresses put on it. You can determine the factor of safety by dividing the calculated design strength by the expected stresses put on the structure.
Here is an example:
Let’s say we are building a chair for our friend Tom.
Tom weighs 160 pounds and we need to make sure we design and build a chair that is safe for Tom to sit in. To build a safe chair, we should evaluate the expected stress on the chair in relationship to the amount of stress the chair can hold. This relationship between expected stress and structural capacity is called factor of safety. If we design a chair to hold 360 pounds and we only expect to put 160 pounds on the chair (360/160 = 2.25) the factor of safety would be 2.25.
However, it would be unrealistic to design a chair to be sat in by the same person the same way every single time. We should calculate the chair’s factor of safety based on a set of scenarios that might change the stress put on it. Most importantly, we should make sure we are comfortable with the factor of safety in our final design.
As we continue with our example of designing a chair, here are some factors we need to consider:
- Seasonal Fluctuation:
Example: Tom gains 20 pounds during the holidays. If this is the case, our original chair only has a factor of safety of 2 (360 / 180 = 2).
- Changes Over Time:
Example: Tom’s chair will last for ten years. In that time, Tom might weigh up to 200 pounds. In this case, the factor of safety would reduce to 1.8 (360 / 200 = 1.8).
- Improper Use:
Example: Tom changes how he is using the chair, and sits on it improperly, by leaning back and balancing on only 2 chair legs. This changes the Factor of Safety to 1.13 (360/ 160 / 2 = 1.13).
- Unusual Circumstances:
Example: Tom is in an overcrowded meeting and must share the chair with a coworker who is 250 pounds. The Factor of Safety then becomes 0.87 (360/ (160 + 250) = 0.87).
Knowing there could be multiple stresses on this chair at one time, we want a chair that is built to handle any scenario, even those where more than one negative scenario occurs at the same time. In our line of work, best practice recommends all facilities have at least a 1.5 factor of safety. Let’s pretend the same applies for chairs.
So, what if, 10 years from now, right after the winter holiday season, Tom has to share his chair with a colleague and they leaned back on to only two legs of the chair?
Expected Stress: 220 (remember Tom is a little heavier now and just enjoyed the holidays) + 250 (the weight of Tom’s colleague) = 470
Operating on 2 chair legs doubles the stress: 470 x 2 = 940
Now we need to determine how much weight the chair should hold if we want it to have a factor of safety of 1.5: 1.5 x 940 = 1,410
Now we know that if our chair can hold 1,410 pounds, it would have a 1.5 factor of safety for Tom plus his friend leaning back in the chair.
How does this apply to Midas Gold?
While we are not building chairs up at site, our team did consider what future conditions at the site might be and used that information to help design a tailings storage facility that would meet the 1.5 factor of safety required by the State of Idaho.
As part of our effort to reduce risk, we also considered the best location to place the tailings storage facility to further reduce its risk of failure. In our plans, we choose to build it at the upper end of the Meadow Creek Valley so 90 percent of the perimeter is surrounded by mountains that will not, and cannot, fail. This means, any perceived risk is on the 10 percent of the perimeter of the tailings storage facility not on the mountains. We also designed in features to reduce risks by lining the tailings storage facility with heavy duty plastic liners to prevent water seeping from the facility into the dam, constructing the dam with coarse rock (which is more stable than sand or dirt) and used the downstream construction method to provide improved geotechnical stability. We then subjected our design to a variety of different conditions, just like we did with our chair examples from up above. These scenarios included changes over time, seasonal fluctuations, high rainfall, in addition to typical operational considerations.
The bottom line is that we are confident our tailings storage facility meets the required 1.5 factor of safety.
However, we did not stop there – we went on to design our project so the factor of safety will significantly increase over time as we add 65 million tons of development rock to buttress the facility, increasing the factor of safety to more than 5 (right now our calculations show it will be a 5.9 factor of safety). In other words, we could have three overweight Toms leaning back on his chair and still be just fine.
