3 important questions to ask when making a mining method change
January 25, 2021
Thinking of transitioning from open pit to underground, or vice versa? Here are some ideas to consider when making the switch
Mining is in my blood. I’m a fourth generation mining professional. A childhood interest in minerals, heavy equipment, and exploration camp stories led me down the path of becoming a professional mining engineer. I’ve worked in some fascinating operational and technical roles at underground and open pit mines throughout North America.
To me, the idea of changing the mining method at a mine is intriguing. A mine’s technical team may evaluate a method change—where you transition from an open pit mine to underground, or vice versa—as the mine progresses through its life. Since orebodies come in many different shapes and sizes, mine operators may need to change their method to reach a different part of the ore body. For example, an open pit may need to transition to underground if the surface operation can’t go deeper to access the bottom of an orebody. You may also consider a method change for financial reasons—to make sure the highest-value ore can be mined.
Regardless of the drivers for the shift you’re planning, a change to an operating mine’s method is a challenging and complex undertaking that requires careful technical assessment. Let’s ‘dig’ into some of the questions to ask when transitioning from open pit to underground, or vice versa.
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Here’s an example of an open pit mine that has switched to underground. This transition is more common than the reverse—changing from underground to open pit.
1. What are some unique design considerations in this evaluation?
When pondering a transition, it’s important to start your design by determining where surface operations and the underground mine will physically intersect. When considering access, the portal or shaft will have to be located somewhere it won’t be impacted by pit operations, waste piles, or tailings facilities.
Geotechnical considerations are also a key aspect of the evaluation. Depending on the locations of the excavations, they might impact each other. This can happen when mines require a crown pillar (a rock mass that creates a barrier and protects the mine and workers). It can also happen when underground excavations daylight into the pit (when you mine a tunnel underground and design it to break through to the wall of the pit). You might also encounter this situation when underground access is planned from the highwall—also known as the vertical wall—of the pit.
Ventilation must also be considered in your planning. Where applicable, the location of raises—vertical or inclined openings that help with ventilation, among other things—should avoid the footprint of pit operations, as well as waste rock storage locations or tailings facilities. If you’re planning an open pit mine and contemplating the idea of eventually switching to underground, leave a few spots where raises and shafts can be installed. It can be challenging to put them in afterwards if there’s no room on the surface, so you’ll be happy you planned ahead. If there will be any interaction between underground openings and a legacy pit, you may need additional design to prevent ventilation short circuiting.
Here’s another design consideration: How will the open pit and underground dewatering systems interact? These systems will likely have very different design characteristics, so you’ll need to think about how they work together. This consideration is especially important for orebodies that are susceptible to mud rushes, which are sudden, uncontrolled flows of wet ore in underground mines that can hurt people, destroy equipment, and lead to production delays.
If you’re considering a mining method change, you need to figure out if the underground and open pit will operate at the same time or sequentially.
2. What is the timing of the transition?
Will the underground and open pit operate at the same time or sequentially? That’s another an important consideration. The equipment that you use for a surface mine won’t be the same equipment you need for underground mining. Given the differences in technical and operational skill-sets of the two operating environments, you may even need two of everything—management structures, fleets, incentive schemes, technical support, and more. This adds management complexity and cost to an operation.
You also need to consider the impact on processing, both in terms of capacity and changing ore characteristics. Examine the differences. A bulk open pit may produce more tonnes of a lower grade compared to an underground mine with lower throughput (tonnes of ore mined per day) at a higher grade. The treatment processes and costs will be different for both and may require careful consideration when completing the detailed engineering.
You should also think about the surface footprint from the larger tailings facility that comes with open pit, because that may present challenges, such as permitting. The timeline for permitting an open pit tends to be longer than for an underground mine.
A change to an operating mine’s method is a challenging and complex undertaking that requires careful technical assessment.
3. What about transitioning from underground to open pit?
A transition from underground to open pit happens less frequently than open pit to underground, and sometimes that shift produces unique challenges. In addition to the above points, it’s important to understand what needs to be done to decommission the underground mine prior to making the switch. The transition could result in a loss of access to the underground. If the mine has legacy liabilities, such as long-term dewatering requirements, they must be addressed prior to completing the transition.
You also must consider planned and unplanned breakthroughs. Mitigate safety risks by having procedures and remote equipment operation capabilities in place. You may need to look at some less obvious considerations as well, such as mitigation strategies to prevent freshly blasted ore from falling down a void and not being recovered for months.
The Super Pit in Kalgoorlie, Western Australia, is an example of an underground mine that transitioned to open pit.
Important considerations before making a change
Mining provides metals and minerals that make life as we know it possible. I’m proud of the mining projects that I’ve contributed to so far, and I enjoy solving problems for mining operations as they transition their mining methods. Making a method change can be tricky, and the points listed above are a few of the most important things to consider when changing your approach.