Sublevel caving: unearthing new opportunities
The known resource base for base metals is increasingly comprised of large scale, low-grade deposits, situated at increasing depths where underground cave 1 mining methods are the only viable means of economic extraction.
In the spectrum of underground mining methods, caving methods are the lowest cost and most productive. The caving component is key to the productivity and cost gains as it eliminates the need for backfilling, and in the case of block caving the need for drilling and blasting. The caving of overlying rock results in subsidence of the surface topography or in the final open pit if present.
Sublevel caving is essentially a hybrid of caving and longhole stoping methods. It is flexible enough and scalable enough to enable rescoping of block caving projects into smaller projects, or traditional longhole stoping projects into larger projects, thereby modifying the investment character/proposition.
Figure 1. Sublevel caving method, transverse layout
Source: Atlas Copco/Epiroc
Historically sublevel caving has been perceived as a mining method that was prone to excessive dilution and poor recovery, and these issues negated the benefits. This negative perception, while probably not unwarranted in many cases, was rooted in practices based on early theories of rock flow which did not reflect reality (Laubscher, Guest, & Jakubec, 2017).
However, the potential rewards led to continued research and development. Smart marker trials 2 conducted in actual sublevel caving operations in the 2000s provided a better understanding of rock flow and ultimately led to the development of cave flow modelling algorithms and interactive draw principles. Other technical and operating advances, often linked to the leaps in computing power, have contributed greatly to sublevel cave implementation. These include interactive draw principles, rock flow modelling algorithms, greater understanding of mud-rush mechanisms and mitigation measures, and numerical stress and subsidence modelling. Entire conferences are devoted to the cave mining such is the level of analysis and operational optimisation being undertaken.
Modern sublevel cave practices minimise dilution by striking the right balance with ore recovery, basically by not being greedy. While dilution and recovery factors are dependent on deposit geometry contemporary sublevel caving practice aims to extract the blasted ore evenly from all available drawpoints based on draw tonnages that are predetermined by computer modelling for each blast ring. This approach limits the mixing of the caved waste and creates what is commonly termed a waste blanket above the ore.
The sublevel caving toolbox available today enables miners to accurately model rock flow and estimate ore recovery and dilution, the key drivers of the method. The ability to model varying sublevel cave configurations accurately allows miners to reduce investment risk and maximise project returns with a high degree of confidence.
Sublevel caving and opportunities
- Improved returns and reduced risk profile
Sublevel cave mining starts at the top of a deposit and progresses downwards level by level through the deposit as the ore is extracted. The top-down mining sequence is a major advantage because it reduces both the lead-time to production and enables capital development expenditure to be spread over the life of the operation – which is usually the best way of increasing NPV and IRR for a given deposit.
- Amenable to automation
Sublevel caving in its most common configuration utilises a transverse drill drive layout meaning identical ore drives and longhole drill patterns are used throughout the entire orebody. The repeating design and operating activities making it highly efficient and highly amenable to automation. Sublevel caves, like block caves, are about as close as underground mining can get to a production line and are often referred to as rock factories.
- Increased resource recovery
Sublevel caving has a low operating cost per tonne which permits a greater proportion of the resource to be exploited at a higher production rates and lower operating costs per tonne when compared to non-caving underground methods. While increased scale often improves project economics, the optimum configuration will depend largely on the deposit geometry, grade distribution, and company goals 3.
- Reduced environmental footprint
Environmental footprint and closure costs compels miners to derive maximum benefit from a resource once developed and minimise disturbance. Sublevel caving significantly reduces surface footprint compared to open pit mining and is less energy intensive than open pits, where large quantities of waste must be moved, and non-caving underground methods which require backfill, are less productive and can be more energy intensive.
Appian and sublevel caving
Appian has seen an increase in deposits considering sublevel caving and is investing into the technology and process to provide a differential advantage, both economically and ESG wise. Appian expect that many new opportunities will arise from reconfiguring existing deposits to sublevel caving. Some deposits will have been overlooked because of a lack in confidence or understanding of the only method available to economically extract them (sublevel caving). Others will provide improved investment characteristics when optimised around sublevel caving and become more compelling propositions.
Appian’s Santa Rita mine in Brazil is an operating open pit operation mining and processing 6 million tonnes of nickel sulphide ore per year to produce around 15 kt of nickel and 5 kt of copper in concentrate. The surface mining operation has a remaining life of approximately 7 years. The deposit increases in grade and thickness with depth and has a combined Indicated and Inferred underground resource 4 of approximately 168 Mt at 0.6% nickel, 0.2% copper and payable levels of cobalt and precious metals.
Appian’s initial investment thesis was based on an eight-year mine life followed by an underground sublevel caving operation to exploit the significant and improving resources at depth. Beyond eight years of pit life the pit stripping ratio increases and underground mining is preferred. While grades improve with depth, they remain relatively modest for underground, but the large scale at sublevel caving cut-offs enables healthy operating margins 5 across a range of nickel prices.
The underground sublevel cave has the following advantages:
- Low mining operating costs (<US$20/t)
- Top-down sequence enables a fast production ramp-up and delayed capex
- High resource recovery and extended mine life
- High production rates enabling full utilisation of the current mill
- A contemporary mining method which is well understood and becoming more prevalent
In comparison, SLOS options were shown to generate lower operating margins (less than 25% at and below $7.00/lb Ni price), lower production rates (maximum of 4.5 Mtpa estimated) and generally inferior project economics (IRR and NPV).
Figure 2. Santa Rita PEA sublevel cave layout
Source: Appian Capital analysis and Atlantic Nickel
The excellent results achieved by more recent sublevel caving operations is rapidly purging many of the early misconceptions associated with the method. Miners are increasingly aware of the opportunities sublevel caving presents and are using the knowledge accumulated by industry to reconsider and reconfigure their plans. Sublevel caving is perhaps one of, if not the most elegant of the underground mining methods, being logical, highly productive, and efficient. The full potential of sublevel caving has now been unlocked by technology and its prevalence will increase in the years ahead.
- Cave and caving terms used throughout this article are not used in the colloquial sense, but instead in the context of cave mining where cave and caving terms refer to the natural caving-in or breaking without assistance, of ore and waste material as part of the mining process.
- Marker trials are used to understand material flow typically for block caving and sublevel caving. Marker trials involve placement of durable coded markers to be placed within the rockmass prior to caving (block cave) or blasting (sublevel cave). Markers are logged as they are retrieved during production and provide an indication of time based material flow around the discrete extraction point(s).
- It is not uncommon for smaller scale configurations, withreduced capex and higher grades, to generate superior IRRs (perhaps at the expense of NPV) and reduce overall execution and financial risk.
- Based on the 2020 NI43-101 technical report. An updated resource based on primarily infill drilling with some steps out holes drilled in late 2020 and early 2021 will be available mid-2021.
- The underground PEA resulted in an underground sublevel caving inventory of 134 Mt at 0.54% Ni and 0.18% Cu with a resultant NSR of US$46/t (@ $6.50/lb Ni, $3.00/lb Cu).