The following is an article from Professor van As which appeared in the Caving 2022 Supplement.
Challenges facing deep, high-column cave mines
By Professor Andre van As, The University of Queensland
Context: the future of underground mass mining is deep, high-column, cave mining
The resurgence in cave mining methods has been motivated by the fact that many of the large, easily accessible near-surface deposits have been mined out or cannot be mined for various reasons. The profitable extraction of ore from large open pits becomes increasingly more difficult with depth, not only due to the high strip ratios and logistical complexity, but also because of the expanding mining footprint and associated environment, social and governance (ESG) pressures.
Thus, a substantial amount of future ore extraction will occur deep underground (Figure 1), though most of these deposits are anticipated to be low grade (e.g. average grade <0.5% Cu) and their economic viability depends on extracting very large tonnages of ore at low cost, i.e. their extraction is most amenable to cave mining methods.
Despite the low operational cost of the method once commenced, cave mining is capital intensive, with most large cave mines costing between USD 1 and 10 billion to construct, over a 3–15 year period, and expended completely before a single ton of ore is hoisted. For this reason, modern miners no longer cap the height of blocks to be caved to less than 250 m (as they did pre-1990s), but instead seek to maximise the height of the cave block so as to reduce capital spend and mining costs (as illustrated in Figure 2). The popular trend to deep, high-column cave mines translates to a much higher level of risk in the form of cave performance, underground infrastructure stability, and resource recovery; all of which calls for the fundamental research to identify and mitigate the potential issues.
The problem: challenges associated with deep, high-column, cave mining
To date, the success of deep, high-column, cave mines has been mixed. Many argue that most cave mines have proved highly successful, where success is defined by the mine’s profitability and ability to yield positive shareholder returns on investment. However, technical success is far more elusive, with most modern cave mines experiencing some form of underperformance; notably attributed to unforeseen or poorly predicted geotechnical issues, culminating in development and construction delays, loss of resource and/or higher operating costs than were predicted by the feasibility study. There is, however, considerable debate over whether these geotechnical factors are truly unpredictable or are rather:
- The inevitable outcome of poor data acquisition and inadequate data analysis that inform the geo-models which form the basis for mine design.
- The inadequate incorporation of geohazards/risks into mine planning and reserve recovery predictive models.
- Induced through poor mine design and poor operational practices.
Given the anticipated large scale of future mass mines and the deep and harsh mining environments in which they will
mostly be developed, it stands to reason those unforeseen geo-risks can have catastrophic consequences. At the very
least they will translate into significant development and operating cost overruns. Simply put, the mining industry can
no longer afford to continue to ‘cut and paste’ mine-designs that were developed for a completely different mining environment. Applying past state-of-the-art, empirical relations that stretch beyond the supporting data introduces an unacceptable level of risk into mine design, which could continue to cost the industry billions of dollars in losses and ultimately attract the attention of regulatory authorities, and in some cases increase sovereign risk.
An industry perspective on priority research
In an effort to address the technical challenges and risks associated with deep mass mining methods, the WH Bryan Mining Geology Research Centre (BRC), within the Sustainable Minerals Institute at The University of Queensland, has formed a Deep Mining Geosciences (DMG) group which is tasked with developing a strong industry-collaborative program of applied mining research and education in the area of deep mining geoscience.
One of the first tasks undertaken by the DMG was to canvas the mining industry; in particular those companies with underground mass mining projects/mines, and request their technical personnel to identify the key geoscientific and mining-related technical issues that will most likely impact underground mass mining in the future. On the basis of the feedback collected from 18 underground mass mines/projects (mostly caving operations), the DMG group collated the findings and from this information identified six key areas (Figure 3) requiring focused research and education.
The six key areas include:
- Geoscience characterisation.
- Rock mass conditioning.
- Deep mass mining geo-risk management.
- Material flow in deep cave mines.
- Mining method design improvements.
Interestingly, these key areas are not new and have been the subject of studies and research for decades, which may explain the lethargy by the mining industry to continue to seriously support ongoing research in these fields. From these responses, it was very clear that the area of rock mass characterisation is unanimously the primary concern for ensuring successful deep mass mining into the future. The task of adequately characterising a massive ore deposit (and its host rock) so as to develop reliable geo-models on which mine design can be based, and mine performance predicted, is a vast undertaking that has yet to be achieved within the mining industry.
Not only is there a lack of understanding and consensus within the industry over governance requirements for geoscience inputs into resource and reserve models, but there is also still considerable debate and uncertainty over fundamental questions, such as the sufficiency and adequacy of data (what data and how much is required), followed closely by the effective utilisation of the data to develop reliable geo-models for mine design and performance predictions.
The second most important area of concern was around geohazards. This included geohazard identification, geohazard prediction and effective geohazard management. As mining progresses deeper, it is accompanied by an increase in stress conditions which tend to dominate the behaviour/response of the rock, whether it be the rock mass failure, major structure shear failure, increased seismicity, or even the material flow in the cave column. Although all deep mining shares common risks, irrespective of the mining method, one could argue that the impact is far greater in cave mining, owing to the method’s
inflexibility and, thus, inability to recover from adverse mining events.
Unsurprisingly, rock mass (pre- and post-) conditioning was the next most important area of research, as this includes a combination of rock mass characterisation and geohazard management. There is a popular sentiment among engineers that if one cannot develop a reliable rock mass model and/or reliably predict the rock mass response, then simply ‘engineer’ the rock mass response through rock mass conditioning. Unfortunately, over the past two decades little new research has been
undertaken on rock mass preconditioning; instead mining companies have relied on anecdotal evidence to justify the application of hydraulic fracture preconditioning without fully understanding how and why it is successful.
Consequently, these rock mass preconditioning programs lack a technical basis for design, and many prove suboptimal and ineffective, particularly in deep caves. New mining methods, cave material flow and ground support are recognised as important engineering solutions for mining at depth. New mining methods need to manage the tension between excavation stability and ore recoveries/productivity. The flow mechanisms of caved material under high-column
loads (vertical stress) are known to be significantly different
from low column, shallower caves and hence need to be
well understood to ensure reliable recovery predictions
(as illustrated in Figure 5).
Tertiary education in mining-related disciplines has increasingly suffered from a growing negative perception of mining which has driven a worldwide decline in the number of mining-related student enrollments in tertiary institutions and consequently,
there is a desperate shortage of skilled mining personnel in the mining industry. The lack of undergraduates and significant industry support has led to the unprofitability of mining (and related) schools at universities, thereby increasing mining school closures and a decline in experienced lecturers and researchers.
To meet the growing demand for mining professionals it will be imperative for universities (supported by industry) to tap into the pool of non-mining, engineering, and geoscience disciplines by developing and offering several postgraduate professional development programs to increase mining knowledge and skills, particularly in the area of deep mass mining.
The future of mass mining lies underground, and the future of underground mining resides predominantly in mass mining methods, particularly cave mines, as only these have the ability to yield the vast quantities of minerals required to meet the world’s increasing demand. Until more recently, mass mining methods have only been utilised to extract shallow to intermediate depth orebodies; however, the past two decades have seen mining companies push the envelope by applying these methods to much larger-scale deposits at depths of around 1,000 m.
Current planning/development is for even larger mass mines at greater depths, which are accompanied by significant technical and operational challenges. To address these challenges, the underground mass mining industry has identified five key areas that require fundamental research and education to ensure that the levels of mining risks associated with deep caving can be effectively managed.
Kesler, SE & Wilkinson, BH 2008, ‘Earth’s copper resources estimated from tectonic diffusion of porphyry copper deposits’, Geology, vol. 36, no. 3, pp. 255–258.