Digging deeper into cave mining
Dr Fernando Vieira, Senior Principal Consulting Engineer, Cartledge Mining and Geotechnics
With renewable energy and technology driving increased demand for ore and minerals at a time when extensive orebodies have become harder to access – block caving is more frequently hyped as the future of mining. But is this technique as efficient, safe, or as well understood as it should be?
Dr Fernando Vieira argues the industry does not yet have a sufficient level of understanding of the ground responses induced by cave mining, and that not enough research has gone into the geomechanics phenomena associated with caving methods. As cave mining progresses to deeper or wider horizons, geotechnical practitioners need more advanced knowledge and certainly a lot more information on the behaviour of caving rock masses to mitigate or avoid catastrophic failure and substantial financial losses in these mines.
A worker in part of the 90kms of tunnels that make up Oyu Tolgoi in Mongolia. Photographer: Munkh-Erdene Eenee
Block Caving is an underground mining method that creates a self-propagating mine that allows for the bulk extraction of large, relatively lower-grade ore deposits. A section of the mineralised rock mass is initially undercut over a large area to induce a self-propagating void that fills with the material from the top (back) of the cave as it collapses under its own weight. The ore is intermittently extracted from draw bells on an extraction level below the void volume.
The entire mine will progress upward (lift) until the mineralised volume is exhausted within the delineated bock.
Block Caving is an attractive method for mining companies planning to develop a new mine or extend the life of the asset by switching from an open pit operation, because it
allows extraction of very large orebodies resulting in mobilising less waste rock material than open pits;
requires minimal change to the mine layout design once established; and
costs significantly less to operate than other underground extraction methods.
The design also makes the block caving orebody extraction option highly suitable for implementing autonomous production systems for ore hauling. It is often argued block caving production methods mimick manufacturing industry processes that are highly efficient and reduce operating costs. But this comparison is false, as many uncertainties and dependences intrinsic to mineral deposits and mining render the concept of “mining as a factory” unbearably idealistic.
Of course, cave mining is not a new ore extraction technique. It has been around since the 1960s and went through large-scale application in the late 1970s and 80s. Many underground mines have some elements of cave mining, and the value and scale of block caving has been tested and proven earlier on in Chile’s copper mines and South Africa’s diamond mines. What has changed in recent years is that the caves are getting much larger and much deeper, in complex geology, and this should be a huge concern. The greatest concern, beyond technical, is the ridiculously insufficient number of engineers developing as cave mining specialists. This matter warrants a whole separate, very serious discussion.
Balancing low operational costs with large upfront expenditure
The major justification for block caving applications is undoubtedly the sheer volume of ore that can be extracted relatively easily once the caving process becomes operational. It may take more than a decade before the first ore can be drawn from the cave. The dimensions of block cave voids can be massive. We’re talking up to 250m to over one kilometre wide and varying depths, typically lifts ranging from 250 to 500m in extent these days. Yet these excavations are commonly planned on a rudimentary understanding of the ground responses in varying geological, geotechnical, and hydrogeological environments.
Countering the low operational costs of block caving is the enormous upfront capital expenditure that is required to establish the mine. Block caving requires deep extraction shafts, horizontal and vertical developments, access tunnels for workers and of course, further tunnels for conveyer belts to extract the ore and ventilation. Implementing these changes to switch Chuquicamata, one of the world’s largest copper mines, from an open pit to a block cave mine has cost Codelco US$5.5 billion as of 2022. Whereas, establishing the newly operational Oyu Tolgoi in Mongolia required excavation of more than 90kms of tunnels over 11 years. It has cost $7.06 billion – more than $1.75 billion over the initial May 2016 budget.
This large upfront capital expenditure creates demand for a high rate of production and tonnage once the mine is operational. This in turn places more importance on engineering designs and production systems to create profitable block caving operations. The delays and cost blow outs on Oyu Tolgoi were blamed on challenging geology. Even if the orebody is a good size and grade, it doesn’t mean it is always easily economically feasible to extract.
Digging deeper into the science
While we have some good engineering principles of design, the science of block caving optimisation hasn’t really progressed much. The methodologies used in appraising cave mining were initially conceived for relatively shallow orebodies, generally occurring at depths between 200 and 800 metres below the surface. Even at these depths many mines do report major ground instability that presents engineering challenges, safety concerns, and stops or slows of production output.
These same techniques are now being used to design caving operations sited at much greater depths, at a much larger scale. While current best practice methodologies draw on empirical schemes, untested assumptions, chronically insufficient rock mass characterisation data, and extrapolations, the ‘super caves’ they are applied to can be as much as 1500 to 2000 metres below the surface. Obviously, these caves are subject to a whole range of different geological, geotechnical, ventilation, economic, environmental, mine and infrastructure design challenges that should be understood in better, and in far more in-depth ways than before.
When we are designing very large excavations underground, we do not do enough and appropriate research on the responses of the rock mass to varying conditions of stress at increasing depths, of groundwater flow, volume and pressure, of the geothermal factors and its effects on ground response, and of course on the propensity for caving-induced seismicity. All of this information combined is fundamental for us to understand how the rock mass behaves and how we can design engineering systems around that. The industry has relied on best practices which are based on multiple empirical rules and, lately, on a few schemes involving numerical modelling-centred analysis. It all appears sophisticated, yet quite insufficiently complete.
Practically this means we don’t really know how productive or successful a block cave mine might be when planning its development, or even when already operating it. There is a lot at risk. A major seismic event can cause an uncontrollable fall in the cave. Suddenly rocks the size of a house can fall and block the draw point. You can’t get in there to engineer it out. This can, and has, caused operating blocks and even entire cave mines to be abandoned. Seismicity, ground water impacts and stiff rock mass that causes hang ups are all major challenges – made worse by the lack of scientific understanding of the pressures caused by block caving. On the other hand, relatively soft or less cohesive rock masses, poorly appraised at the onset, some being highly structurally complex, cause unplanned and uncontrol caving (piping) of the host lithologies inducing abnormal diluting leading to abandonment of draw bells, or a premature closure of the extraction altogether.
A need for more research
As the industry turns towards block caving as a solution to meet future demands for minerals and ore, it is vital more research is done so we can understand and potentially anticipate the ground phenomena in deeper and larger caves. This research ought not be centred on enhancing numerical simulation schemes as been the focus of cave mining research of past decades; but on physical investigation and experimentation (in situ), physical measurements and observations of real (not simulated) phenomena, through more comprehensive and targeted monitoring of ground responses to mining, and a lot more, plenty in fact, of domain-specialist data analytics and phenomena interpretation, to postulate or more credibly explain (hoping to predict) the mechanisms of rock mass response in caving.
We need this research to help the industry develop the knowledge and design guidelines required to operate safely, productively, and profitably, the cave mines of the future. We need such research, more importantly, to develop the future practitioners, and to advance the technology required. We are considerably behind when we consider that cave lifts are currently planned to operate at 2000 metres below surface, while we can only speculate what to expect down there.
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