Collection of ore at El Teniente’s Sewell crushing plant. (Photo: Codelco)

Allan Moss and Rob Bewick join E&MJ to discuss the challenges and opportunities cave mining offers, both today and tomorrow

By Carly Leonida, European Editor

For the green energy transition to be a success, the production of certain metals will need to rise substantially over the next 30 years. According to the paper, “Copper demand, supply and associated energy use to 2050” by Elshkaki et al., copper demand could increase up to 350% by 2050. However, as demand soars, globally, copper grades are declining. The richest, shallowest deposits are all but gone, leaving mining companies with little choice but to exploit lower grade and more disseminated reserves at greater depths.

As production across the globe transitions from a base of aging, mainly open-pit assets to deeper underground ones, the only way to generate the tonnages required at an acceptable cost will be through mass mining techniques like caving.

No other method comes close to block or panel caving in terms of tonnages moved. For example, the world’s largest open stoping operation, Olympic Dam in Australia, produces in the region of 20,000-30,000 metric tons per day (mt/d), while Mount Isa produced around 20,000 mt/d from each of its two orebodies. But stoping is an expensive method compared to caving in terms of operating costs, and the tonnages produced are an order of magnitude less.

In comparison, Grasberg in Indonesia, one of the world’s largest mining complexes, is operating the DOZ cave and developing the DMLZ and GBC panel caves. Target production from the complex is in excess of 200,000 mt/d. Chuquicamata in Chile, which is transitioning from open-pit to underground operations, is targeted to produce around 200,000 mt/d, as will El Teniente. Even slightly smaller caving operations like Oyu Tolgoi in Mongolia and Cadia East in Australia, both of which are scheduled to produce around 100,000 mt/d, are of a completely different scale to stoping operations.

While the mining industry has many of the tools and skills needed to build and operate the cave mines of tomorrow, the challenges are mounting. Given the technical, economic and social risks operators face going forward, it’s a good time to take stock of current practices, see which ones still apply and where the industry can improve.

To generate maximum productivity at relatively low costs and with safety and precision at the fore, caving operations need to be designed and run smarter; they need to become “ore factories.” This requires a holistic approach with greater emphasis placed on new technology adoption, alongside better knowledge transfer and implementation, and optimization of the human element.

Fostering Knowledge Transfer

“By 2030, 25% of all copper produced is forecast to be from underground operations, and a substantial portion of that will be from caving,” Allan Moss, president of Sonal Mining Technology, said. “With copper, we have to go deeper. The only mining method that can produce the tonnages we require is caving. Large-scale in situ leaching is still years, perhaps even decades away.”

Moss has more than 40 years of experience in underground mining with the last 20 being in the design, construction and operations of cave mines. Through Sonal, he provides independent technical advice to underground operations worldwide and, prior to this, he was general manager for underground technology in Rio Tinto’s Copper and Diamond Group for a number of years.

Moss recently co-authored a series of articles published on Linkedin with Dr. Rob Bewick, senior principal at Golder (a member of WSP), another prominent expert in the global caving community. Together, they explored the copper mining transition from surface to underground, discussing best practice and offering advice for improvements. E&MJ followed the series with interest and quizzed the pair separately in August to find out more.

“There has been a transition in caving occurring over the last decade where deeper deposits are being designed as block or panel caves,” Bewick explained. “At the start of the transition in the late 2000s early 2010s, these deeper deposits were designed using conventional rock engineering practices that were applied successfully to previous shallower mining operations.

“When the first deep caves came online, unforeseen operating challenges arose. The challenges, in many instances, were found to be due to ground conditions and rock mass behavior differing from what was expected resulting in delays and value loss. Since then, new understanding related to ground conditions and brittle rock mass design approaches have been shown to be relevant and needed to generate reliably producing cave designs.

“This new understanding is not well distributed yet in the industry. Allan and I thought that a good platform for knowledge dissemination to the masses would be through social media where it would be open, accessible and easily shared.”

People: A Vital Puzzle Piece

A key difficulty with caving is the timeframes involved; it can take up to 20 years to develop a large mine from the portal or the headframe collar to full production. Mining companies must balance this against a huge amount of risk and uncertainty in the business environment to ensure they provide the returns that investors and stakeholders have come to expect.

“It’s a huge challenge,” Moss said. “It comes down to making the right or wrong decisions and having the right or wrong priorities. Lord Robens ran the U.K. National Coal Board during the 1960s and 70s wrote a book called Human Engineering. In it, he said that mining is 10% physics, 90% people. That was in the days when mines had a large workforce underground. That comment is still true, except the ratios have changed. I’d say, today, it’s 30% physics and 70% people.”

The “people” Moss refers to are the decision makers in middle management roles whose impact can sometimes be overlooked, particularly on operations of the block/panel cave scale and duration.

“We focus on technology as a silver bullet that will solve our problems,” Moss said. “But people are a big part of both the challenge and opportunity. These are massive, complex and very expensive projects, and we need systems in place to support decision makers. We also need the right organizational structures.”

While working at Grasberg as part of the Rio Tinto team, Moss helped to establish an internal committee that met weekly to discuss and, potentially, solve operational problems.

“We were actually talking about the lessons we had learnt,” he said. “As an industry we don’t capture our lessons, particularly those related to failures. This is really important in caving because it’s a very unforgiving mining method. When things go wrong, operators have little choice but to carry on. You can’t stop and go off to mine elsewhere in the orebody like you can in stoping or buy five new trucks and a shovel to dig your way out of a problem. You’ve got to face up to it.”

The complexities of caving mean it’s a difficult method to master. It takes years of academic study followed by learning through professional practice and mentorship to gain the necessary skills and experience for successful outcomes.

Bewick discussed the looming skills shortage with E&MJ in another recent article on ground support (see Finessing Ground Support for Deeper Mines, July 2021). In it, he cited the lack of appropriate training and as one of the main concerns with today’s acceleration of deep mining as well as the trend toward caving.

“There are very few universities and colleges that teach either deep mining or caving engineering design today,” he said. “In general, human capital is limited in the mining industry and the field of caving is no different (if not worse due to it being more niche), making skilled and experienced personnel hard to find. As a result, there is a potential for knowledge to be lost, similar to the case of deep mining experience.”

Many open-pit copper mines are transitioning to underground operations as reserves deplete. Chuquicamata is shown here. (Photo: Codelco)

Moss is an adjunct professor at the University of British Columbia. He teaches the cave mining program there and has also noticed that student intakes are dwindling.

“In many universities, the mining department doesn’t get the support it needs,” he said. “If there’s a high ratio of professors to students because of a low intake, that costs money. Universities are commercial businesses these days. There’s very little government support for mining education either.

“It takes four or five years at university to get an engineering degree, and then another 10 years on site to gain the required experience. So, let’s say, 15 years before somebody becomes very useful, but that’s an investment we really have to make.”

What’s Changed?

Though cave mining methods were originally developed in the U.S., the design approaches in use today were developed to extract relatively shallow ore deposits in South Africa and Zimbabwe. The past 20-30 years, however, have seen changes in designs as the depth of caving increased and grades reduced, resulting in greater productivity and reduced capital intensity. For example, caving lifts are now higher, and larger pillars are used along with preconditioning of the orebody. But, with greater depths and larger excavations come more challenging ground conditions.

“We’re now getting beyond the applicability of design approaches that have served the industry well as we mine deeper and production rates go up,” Moss explained. “The lessons and experiences learned from those early types of caves are no longer appropriate for big modern caves.”

Greater emphasis must be placed on new technology adoption, alongside knowledge transfer and implementation, and optimization of the human element if greater efficiencies are to be realized in caving. (Photo: Codelco)

The advantage that we have today is a greater geotechnical and geochemical understanding of orebodies and the way in which they respond to caving techniques. This has been enabled, primarily, through digital sensing and modelling technologies.

“There have been tremendous advances in modelling over the last 20 years,” Moss said. “Models are now being calibrated against accurate observations and they’re becoming quite sophisticated. The use of seismic monitoring for rock fracturing has increased and that’s providing a far better understanding of the rock mass too.

“Ore and cave tracking systems are still relatively new. We’re installing them, but we haven’t got the results yet that we can analyze to develop models of fragmentation and ore flow within the cave. But it’s coming.

“On the other hand, scanning using technologies like LiDAR has been a very significant enabler. Today, we’re using a variety of scanner types, with some mounted on vehicles for greater speed. The accuracy is much better, and we get the full 3D convergence. That is helping an awful lot.”

Today, most caving operations use large-scale, highly mechanized equipment, and automation, particularly for material handling, is the next logical step given the increasingly difficult conditions underground. But, according to Moss, the anticipated efficiencies are not quite there yet.

“For example, the efficiency of semiautonomous loaders is still 20% lower than manual loaders,” he explained. “Most mines use automation for safety reasons. Caving operations like Grasberg and Northparkes have operators sitting in an operations room at the surface running their loaders. Step one is safety and, by doing that, we’re putting the operators in a far better environment and reducing their exposure to danger. Step two is getting the system architecture right and increasing reliability that will give us greater productivity.”

Moss said Northparkes has been very successful at this, but the operation only has four automated loaders. In contrast, Grasberg plans to run 40 units.

“Automation doesn’t scale up well at this stage,” he added. “Although I expect that will change in the next few years. We’re on the road to automation, and development is starting to accelerate.”

Hard rock cutting is another technology that holds much promise in cave mining, both for development and production, again, if the creases can be ironed out.

“The trouble we’re finding is that, as we mine deeper, the rock is getting stronger,” Moss said. “It’s out of the range of roadheaders, so we need to have disk cutters. Again, we’re not quite there yet. Going back in history, Mount Isa tried mechanical cutting using a Mobile Miner. It worked, but it was very costly. Then Rio Tinto tried it with their Tunnel Boring System. Again, it wasn’t quite there. But there are new machines coming on to the market and we’re keeping a close eye on this.”

A large cave mine can have 300 km or more of tunnels including the main access drive. Development of these takes a very long time, so any technique that can speed the time to ore is worth consideration.

Bewick added: “For mine access tunnels that are long and straight, there is interest [in hard rock cutting], but this is because long straight tunnels are similar to civil tunnelling where these machines originated. For more complex mining layouts, the new machines are not currently in wide use.

“In weaker rock masses, there are applications for roadheaders and other similar cutting machines. Many large caving operations though are in strong rock where cutting is not always the best option. Yes, cutting machines are being better designed to handle stronger rocks, but this, coupled with the complex geometries in mining, are proving to be a challenge for machines. I am sure these issues will be resolved in the future, and I am aware that OEMs are working on addressing the challenges.”

It’s still hard to beat the flexibility offered by drill and blast. Moss believes there is yet more we can squeeze out of this technique, but the industry must continue to move forward with mechanical cutting, because optionality is important.

“Today, the biggest impediment to machine mining is getting the ground support in, and mechanical cutting machines are not as agile as drill rigs,” he said. “Drill and blast is going to be with us for quite a while but, what I suspect we’ll see in the not-too-distant future, are main accesses created with cutting and then the footprint development, which is much more complex, will still be in the realm of drill and blast.

“Around 60% of the capital required in caving is tied up in construction before a mine can generate revenue. If we can get to production faster, then it makes a project much more viable.”

From Mines to Ore Factories

Technological innovations like these are taking caving operations one step closer to becoming “ore factories.” However, the goal of consistent inputs and outputs, not just in terms of quantity but also quality of product, will require more than technology alone.

The ‘ore factory’ concept is about measuring and managing mine performance to ensure the mill receives a known product. This, in turn, will make the mill more efficient and, further downstream, the concentrator. (Photo: Newcrest)

“The concept of the ore factory has been around for a while,” Moss said. “The first part of it is about understanding the quality of the ore coming out of the mine. What does a factory do? It produces at a consistent rate and to a consistent quality. Underground mining produces at a consistent rate, but the quality can be all over the place; that quality is the grade of the ore.

“In mining we typically think in terms of tons, but tons don’t pay the bills. It’s copper, gold or zinc that pay the bills. So, we’ve got to think much more in terms of the quality of the product, and how that metal is held in the ore, the fragment size etc., all the stuff the team in the concentrator need to know to make their system efficient and effective too.”

The ore factory is about measuring and managing mine performance to ensure the mill receives a known product. This, in turn, will make the mill more efficient and, further downstream, the concentrator. It’s about reliability versus uncertainty, and systems rather than siloed thinking.

“Underground mining is still very much at the reactive stage,” Moss said. “When something breaks, they fix it. Underground miners are some of the best ‘firefighters’ in the world. But, to get to the factory concept, we’ve got to make our systems more proactive and reliable.

“For example, Rob and I explained in one of our Linkedin articles that, at Grasberg, a planned maintenance system for ground support was developed. This recognized that, as a tunnel deforms, it diminishes the capacity of the support elements. At a certain level of deformation, new support must be installed to maintain the same factor of safety. Normally, mines wait until the support is damaged before stripping it down and reinstalling certain elements, but that costs a lot of time and money.

“Whereas if we continually measure deformation, we can add additional support during planned maintenance before damage occurs. The work can be scheduled between shifts. It’s much faster, less costly and the drift maintains the necessary factor of safety.”

Fragmentation is another good example. Some mines will say that, once the ore is out of the ground and on its way to the crusher, it’s the mill’s problem. But, if the mill receives a different material to what they’re expecting, then the team must react and adjust the system, and that creates efficiency losses.

Semiautonomous grinding (SAG) mills can achieve higher throughputs with finer feeds. Caves tend to produce coarser material at the beginning of their life, and finer material as they mature. If fragmentation is properly measured, this can help inform the mill to adjust its parameters in advance.

Boxhole boring in progress. (Photo: Allan Moss)

“Surface mines are way ahead of underground ones on this, with technologies like bulk sorting,” Moss said. “Bulk sorting is predicated on the ability to measure grade. Technologies like these could be very valuable to caves operations too; if we could bulk sort, the vision is that we could upgrade the run-of-mine ore to the mill, then feed low-grade stockpiles for processing later and, create a feed to waste piles. That will help optimization tremendously.”

A better understanding of the generation and movement of fragments in the cave column through the application of scanning technologies, ore trackers and seismic systems is useful for both up and downstream processes. In a cave, fines at one level can cause issues like fines rushes, which can impact both safety and production. That information is also directly relatable to seismic performance and mill performance, so sensing and monitoring is a trend that will likely receive further investment and attention over the next decade.

Preconditioning of the orebody also holds much potential. Newcrest and Codelco are leading the way in this area; Newcrest is using hydraulic fracturing (fracking) techniques originally conceived in the oil and gas industry to precondition the rock mass at Cadia East. And Codelco plans to use fracking to precondition a significant percentage of the orebody at El Teniente’s New Mine Level prior to caving.

“We know preconditioning works, but we need to get a better understanding of why it works,” Moss said. “Another area we need to look at is construction. We use advanced simulations in production, but we rarely use them to simulate mine construction. At one caving project I worked on, we estimated that, through sheer congestion of vehicles on site, delays during construction amounted to around six months.

“The tools are available; we just need to get that focus going. If we can cut a month off the construction schedule through
good planning, modelling and simulation, then we can add significant value.”

Ready for the Future

The future will bring larger, deeper caves, but Moss believes that many operators, particularly those that have experienced difficulties building and operating them in the past, will be much more cautious.

“I think we’re going to see some differentiation between the experienced cavers and the less experienced cavers,” he said. “We’re also going to see more smaller projects, which are easier to handle, like New Afton in British Columbia or Northparkes in Australia. We’ll see two separate groups of mines emerge, the 10,000-30,000 mt/d ones, and the 100,000 mt/d+ mines. The 100,000 mt/d+ can only be taken on by tier one mining companies.”

Although the size of a cave is primarily dictated by the size and shape of the orebody, a bigger orebody doesn’t necessarily require a large cave. It is possible to mine large orebodies by breaking the footprint down into multiple smaller caves, or macroblocks, separated by pillars that can be caved later. This approach can help to derisk a project somewhat and is used by a number of miners, including Newcrest, Codelco (at Chuqui) and Rio Tinto, which is considering it in plans for Oyu Tolgoi.

“I think there will be a reduction in mega projects,” Bewick said. “Mining companies may have jumped too far down the mega project road before establishing reliable design and construction management practices. Currently, mega caves [those that produce 30 million mt/y+ or more] take too long to develop, and their risk profile is high relative to the capital expenditure.

“In the future, I see more orebodies being divided into smaller panels that can be developed faster and with less capital at risk and less complexity. Smaller panels will be possible in more massive, strong and deep rock masses, due to progressive improvements in preconditioning technology. Personnel exposure will also be better managed with remote and autonomous equipment operated in highly stressed rock masses that may be prone to rockbursting and other stressed ground hazards.”

Moss summarized the discussion well…

“‘Burning platforms’ are what often drive innovations,” he said. “For example, Grasberg had a problem in the DMLZ project with rockbursts, so they developed deformation-based designs for their ground support.

“We’re now moving on to different burning platforms, like dealing with wet muck and fines. I see the big innovations in caving are going to come from cheaper, better sensing technologies. These could be used in multiple applications throughout the mine, from measuring airflow and quality so that we can manage that in real time, through to measuring the grade and fragmentation of ore as it comes out of the draw points and sending that information through the system to the mill.

“We’ve now got technologies that can tell where a person or machine is within 50 m. That helps with workforce planning and construction scheduling. It’s these types of tools that are going to allow us to build mines quicker and safer.

“The next question is: why do we need the workforce where it is? Fly-in, fly-out operations are expensive and they’re not good for quality of life. If we can run an operation from a remote-control center in a town or a city, then so much the better.

Secondary breaking at a block cave. (Photo: Allan Moss)

“The end goal is to create an off-site center, which monitors ‘mine health.’ This would collect and analyze data on machine locations, fragmentation, and grade to inform the short and medium-term plan. It would then pass that information on to the control center, which executes the plan.

“This remote approach would give a substantial portion of the workforce a better quality of life which, ultimately, would make them more productive. In 20- or 30-years’ time, caving operations could look completely different.”

It sounds like there’s a lot of opportunity to improve caving operations if mining companies would be willing to take a step back and invest more time and effort into systems optimization, rather than rushing into development E&MJ commented.

“I agree,” Moss said. “It all comes back to the industry being willing to learn lessons, and to share them.”