How to Prepare for an Open Pit to Underground Mining Transition

When is the right time to transition a mine from open pit to underground? 

Despite a lack of industry standards, there are two schools of thought. One is to initiate the transition once the open pit has reached its full potential to mitigate production rates, and operating and capital costs. The second is to approach this strictly as a cost analysis exercise to determine the depth that triggers the transition.

AMC believes the optimal solution can be found somewhere between each limit whilst also considering many technical and non-technical factors. At the heart of this lies the need for exacting, thorough mining feasibility studies and constant mining data collection and review. 

We recently met with Andrew Hall, Director of AMC Consultants and Executive Lead – Advisory, who shared his expertise on this topic. 

Mark Chesher, Executive Leader Business Development and Principal Mining Engineer at AMC Consultants, explains the likely long-term consequences of the invasion of Ukraine on global electric car battery raw material production and prices. 

Chesher believes the war will contribute to a short-term deficit in copper supply. As for nickel, Norilsk is one of the major mining houses in the world, with approximately 10% of the world's nickel supply. “Supply contracts take a long time to work through the system. But if the sanctions are applied strictly, it will bite hard in the metals market across the world.” 

Metallurgist and Technical Manager - Business Development, Rob Chesher, says that Russian nickel projects are well-developed for growth and taking them out of play for the foreseeable future will have consequences. “We talk a lot about ‘prospectivity’ – projects that are being developed further into the future, but they are currently in the study or early study phase. There are many targets following these trends throughout Eastern Siberia.”

If your mine is considering transitioning from open pit to underground operations, you cannot afford to miss these insights.

There are pros and cons for both.

Underground mining tends to leave a lesser ESG (Environmental, Social & Governance) footprint, which continue to reduce in line with technological advances. Indeed, if all ESG aspects were considered in detail, most mines would probably transition sooner. 

However, the upfront costs are typically higher, often delaying a transition. Mining underground is also more technically challenging. Open pit mining is preferable for recovering weathered and more shallow resources, and it can lead to access points to underground mines.

Begin by establishing the upper and lower limits of the transition. The minimum depth determines the upper limit for mining the ore body using underground methods. The lower limit is determined by the Lerchs-Grossman ultimate pit, which is to establish the deepest possible limit of economically mining the ore body in an open mining pit. 

When these limits are confirmed, the Lerchs-Grossman pit can be restricted by applying the opportunity cost of each block to establish the limit (as opposed to the profit), where the opportunity cost is the difference in the value of the block when mined by open pit or by underground. Here we assume that if a block can be mined by either method, if it is not an open pit mine, then it will be an underground operation.

The restricted (opportunity cost) pit is typically smaller than the ultimate Lerchs-Grossman pit shell. Notably, the basis for calculating the block values is quite different, and as a result, the restricted pit does not necessarily align with a particular phase in the ultimate pit.



So the upper and lower limits to positioning the base of the pit are now established in theory. However, targeting the precise “sweet spot” within this transition zone depends on a number of technical and non-technical influences.

As we’ve begun to mention, numerous factors will affect your ultimate decision. These include:

• The level of knowledge regarding the deeper areas of the ore body
• Access to personnel with the right skills and experience
• The method and underground mining equipment available 
• Capital cost to establish underground access 
• The time frame to move to full ore underground production 
• Whether ore can be accessed from alternative sources 
• The existing process plant’s capacity to treat what might be different ore types at a lower feed rate 
• The time it takes to establish an underground mine 
• Environmental, safety, and social considerations of open pit mining as opposed to underground mining
  

As we can see, this is an extensive number of influences. Each must be understood in detail so that management and key stakeholders can make the most considered, informed, and accurate decision possible.

In an ideal scenario, planning should commence in the project’s early concept to feasibility stages. If at this stage going underground is an option only, planning is often postponed. This is especially the case if knowledge of the ore body at depth is limited or there is a lack of in-house skills to plan accordingly. 

AMC’s mining services can provide the level of knowledge and planning skills your project requires. Contact our team to find out more. 

This is where mines can be caught out, as it is common to underestimate the amount of time a transition requires. On average, the answer lies somewhere around five years minimum. Underestimating can lead to shortcuts, which in turn can compromise the mine’s long-term future. 

Why this amount of time? It’s not surprising when you start to consider everything involved: 

• Drilling programmes 
• Test work 
• Technical studies 
• Approval processes 
• Actual construction 
• Commissioning
  

• Geology
• Mineral resource estimates
• Ground conditions
• Selection of the mining method
• Capital and operating costs
• Potential production rates
• Mined grade, metallurgical recovery and costs
  

There are two particularly common challenges a mine may encounter:

Providing appropriate feed volumes and material types to a plant that favours milling qualities over underground feed material

Strategic metallurgical plant reviews are required to balance typically reduced underground feed volumes (granted with higher grades to realize acceptable project economic outcomes). This could require:

• Plant throughput reductions (turning off a parallel stream, for example) 
• Plant reduced utilization (campaigned milling time)
• Other geometallurgy factors such as increased hardness with less throughput (generally counter-intuitive for pit operations whereby unit volume is t

Separating pit and underground activity during the transition phase

This challenge is exacerbated if underground access must come from within the pit itself. Some of the issues you might face in this instance include mismatched material movement, the operation’s access to materials, and blasting in underground mine restrictions (to cancel out underground intake airway issues or vibration problems for stope voids and/or pit walls).

Are you in the early stages of mine planning and need to factor in a transition phase? Is your open pit underway, but you will eventually need to go underground? Whatever your situation, contact our mining method transition experts. We have global experience in managing and supporting this transition for some of the biggest names in worldwide mining. 

We’re here to help make your transition smart and successful.