Addressing the Challenges and Future of Cave Mining

news Caving

Addressing the challenges and future of cave mining

No. 56 SRK Consulting’s International Newsletter

Infrastructure development at Oyu Tolgoi

There are over 50 global cave mining projects in various stages of studies and development.

Despite the fact that the cave mining method orebody below their economic depth, and further is more than 100 years old, it is only within the exploitation of often large low-grade resources at past 20 years that this method has spread from depth would not support a more expensive mining initial cave mining centres to six continents. method. In recent years, besides the economics There are currently approximately 17 cave mining of high strip ratio, the environmental concern also operations in 11 countries. plays an important role when comparing open pit mass mining and caving. Cave mines can have a The interest in cave mining is being fuelled significantly smaller footprint than a comparable by the depletion of near surface orebodies open pit, since waste mined is only limited to suitable for open pit operations, relatively high underground infrastructure development. production rates and low operating cost. Also, a number of open pits have a continuation of the …continued LEFT EDGE OF SIDEBAR BLOCK

Addressing the challenges and future of cave mining (continued)

JAREK JAKUBEC Shaft headgear at the Resolution Copper project, Arizona, USA

Jarek has over 35 years of worldwide Cave mining principles and economics needs to be at least 100 m thick for cave mining to be economical. In the past, operating and Traditionally, cave mining was a method typical caving heights were 150-250 m. consulting experience based on the principle of undercutting rock Most of the designs which are on the in the mining industry, and then naturally letting it cave. However, drawing board today have caving lifts specialising in this mining method has been extended in excess of 350 m. Although higher underground mass to very strong rocks which would not lifts generally result in better NPV, mining methods, easily cave or would not be suitable for they also have higher business risks of diamond mining, rock caving due to very coarse fragmentation. resource sterilisation, dilution, stability mechanics and mining geology. He has To mitigate this problem, pre-conditioning of the drawpoints, and extraction level worked around the globe on more than techniques have been developed to in general. 80 mining projects in 30 countries on 5 generate more fractures and reduce the continents. Jarek worked as geotechnical fragments to a manageable size. This is By contrast, orebodies with relatively engineer at Cassiar cave mine and later, achieved by hydraulic fracturing and, in small horizontal footprints can also for De Beers Consolidated Mines in South some cases, in combination with confined be mined economically if they have Africa and Botswana. With SRK, he built blasting. Although some discussion is sufficient height and metal content to and now manages a team of mining and needed about the potential impact on in justify the capital expenditure. Good geology experts who provide consulting situ stress that is required for cave mining, examples are Northparkes Mines in services for the cave mining and diamond several cave mines are in operation with Australia, and the diamond mines in industries. Jarek has published over preconditioned rock masses and several South Africa and Canada. 25 papers on geology, rock mechanics, others are being developed. The most and mining, co-authored two mining extensive work undertaken is at Cadia East Mechanised cave mining includes guidelines books, and participated on mine and Northparkes Mines in Australia, several variations of the method, several international research projects. and Andina and El Teniente in Chile. including block, panel, incline and front Jarek is founder of the Cave Mining caving. Most of the current mines and Forum and he is a qualified person in Caving methods can be used with projects utilise either block or panel terms of National Instrument 43-101. any type of commodity since it is the caving but after several years, Ekati geological and geotechnical context that Diamond Mine, Canada successfully Jarek Jakubec: [email protected] is important. There are many parameters introduced an incline cave at their to consider but typically the orebody Koala Mine.

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Cave mining differs significantly from On a positive note, in hindsight, most Block and panel caves are very suitable other typically more selective underground of the challenges could have been for highly automated equipment like mining methods in a number of areas. prevented with better upfront knowledge, remote control loaders, trucks and Because cave mining is a bottom up correct design or draw disciplines. The crushers. A future supercave could method that relies on first establishing a other disadvantage of cave mining is potentially have less than 50-60 people large fixed infrastructure underground that the long lead time, it typically takes underground. will provide a very long term production 7 to 10 years, or longer, from initial platform, the initial capital costs are studies to production and the site may Cave mining toolbox typically very high. underestimate the logistics and skillset The ever-increasing speed of computing required for cave mining development To offset the impact of the large and the sophistication of numerical and operation. capital expenditure on project value, a modelling codes enable many mining consequent high rate of production and Because the cave mine has to be fully companies to model complex mining an increased tonnage per drawpoint is developed before all design parameters problems. Better and more reliable required. In this day and age, several are known to a high degree of confidence, instrumentation such as MPX cables, cave operations are running at upwards the design should be robust and technical Smart Marker System (Elexon), and Cave of 50,000 tpd and newer operations success should have priority over Tracker (Mining3) also provide excellent are being constructed for nameplate economics, especially when greenfield data for calibration of such models. capacities of 100,000 tpd and more. projects are considered. Although the cave However, do not count on the high mine may not require the same level of reliability of numerical models without Chuquicamata and New Mining Levels resource definition in terms of drillhole calibration. Reliable and accurate input at the El Teniente project in Chile; Oyu density as selective underground mining information for numerical models are Tolgoi projects, Mongolia; Grasberg caving method, the geotechnical and structural typically available only after the cave is complex, Indonesia; and the Resolution geology knowledge has to be typically designed, developed and operating. Copper project in Arizona all fall into the higher than for other methods. Some supercaves category. It has to be stressed The track record of predictive models of the information needed for the final that there are no examples where without comprehensive calibration, feasibility design may not be possible to tonnage over 100,000 tpd was achieved especially for greenfield projects, is not obtain from the drill core, and underground on a sustained basis from single cave very good and does not necessarily characterisation exposures may be footprint, although El Teniente produced increase confidence in the design in necessary. higher tonnage from concurrently mining comparison to other empirical tools and several caves. benchmarking. Additionally, complex Future of cave mining processes such as cave propagation, In terms of logistics, once a cave mine is Cave mining is moving to new frontiers subsidence geometry, and material in production, the execution is relatively with high production rates, strong rock flow in a cave mine cannot be yet straightforward. The production footprint masses, very high caving lifts and greater reliably modelled. remains fixed and mining consumables depth. In forefront of such projects is Jarek Jakubec: [email protected] typically revolve around secondary Resolution in Arizona where their shaft breaking with campaign maintenance was sunk to 2,100 m to develop deep Published in the Australian Centre for Geomechanics, within the production drives. It is important copper porphyry. Volume No.47, August 2018 (After article published) that strict draw control is maintained and the extraction level is not experiencing excess damage requiring repairs.

Technical challenges of cave mining As the number of cave mining projects increases, there are also heightened expectations for high production rates and caving lifts, and greater depths to be achieved. The analysis of the cave mine performance is far from satisfactory. In the past two decades, at least 12 cave footprints were put into production and all experienced some level of unforeseen difficulty related to ground conditions, fragmentation, mining induced seismicity, mudrushes, and underestimating ground support, or simply breaking basic cave mining rules, specifically in the area of undercutting and draw management.

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Accounting for micro-defects in rock mass rating

14 12 12 10 10 8 8 6 6 4

4 2 Is(50)(MPa) 2 0 Number of Fragments 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 Depth (m)

Plot showing micro-defect intensity, drop test results, and point load test strength for a drillhole in a copper porphyry deposit ANDY THOMAS

In 2001, Laubscher’s Mining Rock Mass in rock where defects are numerous and Andy, MEng, PEng, Rating (MRMR) classification system small, e.g. in porphyry-type rocks, it can be is a Geological (Laubscher and Jakubec, 2001) introduced difficult to isolate sections of defect-free Engineer with the rock block strength concept to account rock in which to obtain true intact strength. over 13 years of for scale effects and the influence of In such cases, the scale of micro-defects experience working closed defects on intact rock strength (IRS). relative to the size of core means that as a professional in Almost two decades later, rock mechanics core-scale strength tests reliably account the field of ground practitioners are still challenged by how for their influence on the rock strength. engineering. He to appropriately incorporate defects, The concept of the drop test was has extensive other than open breaks, into rock mass introduced and accompanied by the experience in designing, undertaking classification to arrive at in-situ rock mass comment that when performed correctly, and managing geomorphological, rating (IRMR). A category of such defects the drop test yields more consistent geotechnical and hydrogeological is micro-defects, which are typically non- results than the hammer blow method. investigations for projects in the mining systematic with variable geometry and The drop test consists of dropping uniform and civil sectors. Andy has worked on continuity. These fractures can sometimes length sections of the same-sized core a wide variety of projects in Australia, be difficult to identify, let alone characterise, horizontally-aligned onto a concrete floor the Americas and Asia. He completed but they can have a significant impact on and tabulating the breaks on fractures and his specialist studies at the University important mining considerations, such as through intact rock. of British Columbia where he focused the caving process and rock fragment block on engineering rock mechanics. size (Jakubec et al., 2007). The major appeal of the drop test as a Andy’s recent projects include core testing tool is its efficiency, simplicity, designing, implementing and managing Early stages of mining projects rely and repeatability; gravity is a constant so geotechnical and hydrogeological almost exclusively on rock core to define only the drop height datum for the project, investigations and stress analyses for rock strength. Assessing the unconfined e.g. core rack level, need be defined. For underground mining projects. ‘intact’ strength of the rock core is mostly rock with high micro-defect intensity, done by empirical methods, by observing significant utility can be obtained from the Andy Thomas: [email protected] how rock core breaks by hammer blow, drop test by simply counting the number and point load and UCS testing. However,

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Microdefects: GSI for fragmentation assessment?

Recently, SRK Chile was involved in It may be concluded that microdefects a caving project in Chile and in charge should not be considered for GSI of numerical modelling to analyse estimation, since microdefects are pillars and overall mine stability and to already explicitly included in BCF. define ground support. The review of Andrea Russo: [email protected] the basic geotechnical information was the first task performed, pointing out of fragments that the core breaks an underestimation of the Geological into. In a recent study, the number of Strength Index (GSI). The client justified ANDREA RUSSO fragments correlated with the micro- the lower values of GSI by including defect intensity and point load strength microdefects, as they would be acting (Figure left). during the caving process, and argued Andrea has 25 years that they should be included in the of experience in The plot shows that in the unaltered GSI estimation for fragmentation the mining industry. Post Mineral Volcanic unit, the point assessment by the software block cave He has worked in load test Is(50) values and number fragmentation (BCF). SRK’s position porphyry copper of fragments were equally low. In in this particular project was that GSI deposits, acquiring the Tonalite unit, the highly altered need only consider open joints since valuable experience in Sericite-Chlorite-Clay domain is BCF already considered microdefects geological, structural, characterised by zones where there to calculate the rock block strength, and geotechnical were high numbers of drop test and including microdefects in GSI mapping for underground mining, defining fragments, whereas testing the would have a double lowering effect. and characterising structural domains, less altered but similar micro-defect Because of this debate, SRK verified geotechnical domains, and fragmentation intensity in the Sericite domain the impact on fragmentation curves assessments. Andrea specialises in rock resulted in generally fewer fragments due to including microdefects in the GSI mass geotechnical characterisation, and far higher Is(50) values. estimation. analysing geomechanical laboratory tests, defining the caving sequence and front Although wetting the core helped, it GSI is not a direct input in BCF software, orientation, and defining ground support was not always possible to see the but it is used to scale the mi Hoek & through empirical methods. He worked on fractures in the core. It was impractical Brown parameter to mb, which is an various open-pit projects analysing slope to inspect each broken fragment (up input parameter in BCF. To evaluate the stability in Canada and South Africa. On to 15 fragments per test) to determine impact of microdefects included in GSI, an environmental project, Andrea gained whether breakage was caused by SRK estimates three fragmentation experience in rock mass grouting using the defect or intact rock or a combination curves for two geotechnical units using GIN method. thereof. Counting the number of mb input values scaled from a GSI value fragments proved to be a quick and of 70, 50, and 30, resulting in three Andrea Russo: [email protected] practical method of addressing these identical fragmentation curves. limitations and assessing how the rock at that scale behaved mechanically. 100 120 It is important not to ‘double dip’ when selecting intact rock strength reduction 100 80 to arrive at an IRMR. Although there is no current guidance on relating 80 the drop test to quantitative strength 60 values for rock mass classification 60 schemes, it can be a useful tool to 40 measure the prevalence and influence Percent passing Percent passing 40 of micro-defects on the core-scale rock 20 mass. This information can help decide 20 whether, and how much, the intact 0 0 rock strength needs be reduced or if 0,001 0,010 0,100 1,000 10,000 100,000 1000,000 10000,000 0,001 0,010 0,100 1,000 10,000 100,000 1000,000 10000,000 the reduction is sufficiently captured in Block size (m3) Block size (m3) the point load and UCS tests. Figure 1: Fragmentation curves estimated for Geotechnical Units a) and b) considering a Andy Thomas: [email protected] GSI = 70 (red dotted line), GSI = 50 (blue line) and GSI = 30 (green line)

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Geotechnical data collection and approach to modelling for Cukaru Peki

Cumulative drilling totals (R) Resource (GT) Geotechnical Drilling phases 48,000m (R) – 2014

Early Exploration Drilling Key geological and geotechnical activities

1) Geotechnical review of database 93,000m (R) – Mar 2017 45,000m (GT) X) Geological Modelling – Maiden Mineral Resource Estimate (January 2014)

2) Geotechnical Logging Training & Geotechnical 163,000m (R) – Sept 2017 QAQC visits 115,000m (GT) 3) Geological Model Exploration Drilling with iterative MRE a) Structure Model (Fault network, basal clay/ Geotechnical Data Collection updates fracture zones b) Lithological Model c) Alteration Model d) Geological Model

NEIL MARSHALL 5) Preliminary Geotechnical Model a) Rock Mass Model 4) Updated Mineral 6) Verification of Preliminary Geotechnical Resource (April 2017) Model 7) Final PEA Geotechnical Model Neil Marshall (MSc, PEA Study DIC, BSc [Hons], CEng, MIMMM) is a Corporate Consultant A geotechnical model is the fundamental Nevsun Resources Ltd. owns 100% of the with more than 30 basis for the design of an open-pit and Cukaru Peki Upper Zone; the Lower Zone years’ experience underground mines. A fully understood is a joint venture with Freeport-McMoRan in geotechnical and representative geotechnical model Exploration Corporation. The Timok project engineering for mining. provides information on the engineering is located centrally within the Timok While consulting, characteristics of the rock mass, defining Magmatic Complex (TMC), which has one half of his time was spent working in how it will behave during excavation. of the highest concentrations of copper underground and open pit mines in The model’s individual domains, each enrichment in the Tethyan Belt. Zambia and Ghana where he held various comprised of materials exhibiting internally The upper zone of the Cukaru Peki technical positions. Neil specialises in similar geotechnical properties, reveal mineralisation occurs at depths between the geotechnical characterisation of rock the critical characteristics and risks that 400 and 800 m below ground level. The masses, open pit slope design, underground the mine planners need to understand to deposit does not outcrop at surface; mining method design and evaluation, design the optimal mine.The Cukaru Peki it is buried beneath Miocene Clastic underground support and excavation Upper and Lower Zones are part of the Sedimentary rocks that unconformably design and numerical modelling. Timok copper-gold project. overlie the Upper Cretaceous Bor’ Neil Marshall: [email protected] Conglomerate and Bor’ Marl. The Unaltered Andesite sits below the unconformity and the Lower Andesite below that.

The high sulphidation epithermal mineralisation found within the Late Andesite comprises massive and semi- massive sulphide. Pyrite is the dominant sulphide mineral, and the principal copper mineral is covellite with lesser enargite, bornite and chalcocite. Gold is associated primarily with the copper sulphides.

The top of the mineralisation is constrained by the unconformity and the lateral extents are constrained by faulting. Several alteration assemblages have been grouped into four types with mostly Best practice: Geotechnical logging being carried out at the rig Phyllic and Propylitic alteration at its base.

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Downstream effects of cave fragmentation

Since the 1990s, ‘mine-to-mill’ studies to be designed for fluctuations in hardness have evaluated the impact of blast and feed size. In addition, unless mixing fragmentation on downstream plant within the cave is well predicted and The mineralisation is found within the throughput. This was because crushing understood, geometallurgical knowledge Advanced Argillic with higher grades of and grinding circuits (in particular, SAG of the orebody can be destroyed. copper and gold near the cap and then mills) are sensitive to feed size and the While mass mining offers the potential decreasing with depth. amount of fines generated from blasting. to develop lower-grade underground Significant gains in grinding circuit deposits, the downstream effects on SRK’s early involvement in the efficiency could be achieved by controlling plant performance need to be carefully exploration drilling was key to setting and optimising blast fragmentation. up the geotechnical data collection. considered. Due to the potential for A flow chart sets out the approach The advent of mass mining methods waste infiltration, pre-concentration or to geotechnical data collection like block caving has introduced the waste rejection opportunities ahead and modelling for the preliminary term ‘cave-to-mill’, which considers the of grinding should also be included in economic assessment study. variability and uncertainty in fragmentation any project study that considers mass coming from each drawpoint. In the mining methods. A logical approach was used to figure below, fragmentation estimates Adrian Dance: [email protected] develop the geotechnical model taking were made for each block in a potential into consideration the geological, caving zone underneath an existing alteration, and structural conditions for open pit. To simulate the effect on SAG the deposit. There is good correlation mill performance, these fragmentation ADRIAN DANCE between the geological framework and curves were passed through a primary geotechnical parameters. crusher model. As shown in the figure, Adrian is a Principal The visual assessment and data the variation in mill feed (80% passing Metallurgist with over modelling indicated that the spatial size, in mm) is evident as the cave 25 years of industrial variability in the geotechnical parameters propagates upward and the secondary and consulting correlated well with the geology, fragmentation generates more fines and a experience. He has alteration, and structure. A statistical smaller topsize. established himself analysis of the geotechnical parameters Between the cave and the mill, there as an authority in assessed each domain’s representation may be limited potential for controlling the optimisation of of the geotechnical conditions. fragmentation, ore blending, and/or crushing/grinding A thorough assessment of the rock stockpiling, and the plant front-end needs circuits and in adding value to operations mass was based on the geology, through process improvements. Adrian is structure, and alteration models. The an advocate of grade engineering through pre-concentration methods including geotechnical data was spatially and OPEN PIT statistically analysed relative to these coarse beneficiation to address poor mill models to understand how the geology feed quality. and structure affected the rock mass Adrian Dance: [email protected] engineering properties. The structure and alteration were controlling the variability in rock mass characteristics and the models were used to generate the geotechnical model.

The data in each geotechnical CAVE domain was used for generating parameter inputs to the cave mining study. The real value added to mine planning came from knowing how the combined individual-models created a geotechnical model that was representative of the actual conditions. SAG Feed F80 in mm 120 270 430 Author: Karl Llewelyn 200 350 CAVE FOOTPRINT Contact: Neil Marshall: [email protected]

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Geotechnical model development for caving design

SRK performed a comprehensive In addition, detailed statistical summaries redevelopment of the geotechnical of the characteristic rock-mass properties model for an existing large panel caving and their variability were provided for each operation with the aim of assisting domain for use in any geomechanical or cave planning and design in five future numerical analyses. Supplementary in situ cave blocks. Initially, separate models stress interpretations using core disking were created for the individual blocks to and drillhole breakout data were generally allow for smaller, more focused models; in agreement with past measurements however, a large all-encompassing model and interpretations. was subsequently created. The Chuquicamata underground Key insights and uses for the modelling mine in the Atacama Desert The models incorporated more than identified together with the client were in northern Chile is one of the 50 geotechnical domains based on as follows: largest planned mining projects major rock types and faults. They were • The interaction of faults is important in the world to use block caving constructed through review of the existing – each cave footprint requires the with macro-blocks option to mine geotechnical drilling (485 drillholes totalling presence of one or many major faults copper ore. CODELCO is currently ~250,000 m of core) and underground to assist caving. Stress concentration finishing the detailed engineering mapping database as well as incorporating in weaker rock mass within the fault stage and constructing the main additional data from recent targeted drilling intersections in the extraction and infrastructure for the project. The investigations. Careful characterisation and undercut levels pose an elevated underground mine is expected to recalculation of key rock mass parameters possibility of instability. Sequencing begin operations in 2020, with a ensured the final model inputs were the undercut through this region would seven-year ramp-up period and a appropriate, consistent, and thorough. be required to manage the possible nominal production of 140,000 The products comprised detailed outcomes associated with undercut tonnes per day. Datamine block models of maximum angle and initiation point. Closed The rock mechanics 25 m and minimum 5 m individual blocks Drawbells • The parameters controlling brittle team of SRK sizes, providing data for lithology type, behaviour of the rock mass (strain Chile has provided intact rock strength, rock block strength bursting of faces or local seismic technical support to the (as developed for the in situ rock mass responses during undercutting and Chuquicamata underground rating calculation, weathering, fracture cave establishment) include high project for the last eight years, frequency per m, and fracture spacing. strength rock and low joint number or including the prefeasibility, value, RQD; joint number rating; and calculated fracture frequency. feasibility, liaison, and detailed RMR, IRMR and Q’ classification values. • The presence of locally, very weak engineering stages. infill (decreasing rockmass strength) One important aspect was to has controlled drive scale deformation. independently manage each The deformation is managed with macro-block in the geomechanical mine sequencing and ground support assessment to estimate magnitude systems, and should abate once the and extension of the abutment undercut and cave have provided a stresses and tensile zones on suitable stress shadow. the macro-block pillars during the • The seismic response to mining different construction phases in has recorded larger magnitude and macro-block preparation and during more events in the south of the mine the ore column extraction involved in compared to the north, appearing the macro-block operation. This phase to be a consequence of rock mass depended on complex tridimensional conditions and structural control. This has geometry and the interaction of constrained the location of the planned different cavities; so, a tridimensional infrastructure to reduce the potential analysis was required to consider the for future damage. Preconditioning to mining sequence (Figure right). reduce the occurrence of large damaging As part of this study, empirical seismic events will become a necessity. methods, confinement-convergence Predicted cave propagation relative to RMR Ian de Bruyn: [email protected] analytical models, and 2D and 3D model (warm colours indicate poor rock)

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IAN DE BRUYN Geomechanical design for the world’s Ian has 21 years largest underground mine of experience in geotechnical engineering over a wide range of mining and civil engineering projects. He specialises in geotechnical studies for open-pit mining operations, geotechnical characterisation, and modelling for underground excavations and open pit/underground mining interaction. He has worked on projects involving complex and challenging rockmass conditions where structure and groundwater play an important role in stability. His projects have Drawbells involved site investigation, rockmass Opening characterisation, stability analysis, design and risk assessment at all levels, from conceptual through feasibility studies and working design. Ian is team leader of the geotechnical group in Perth, Australia.

Ian de Bruyn: [email protected]

“Half” Drawbells ESTEBAN HORMAZABAL

Esteban is a Civil Mining Engineer Tridimensional numerical model for the Macro-block option; red-excavations, yellow- with a Master in broken material, white-constructions (before excavations), green-West Fault zone (In situ Geophysics, and rock mass has been purposely hidden) (i.e. Hormazabal et al., 2010). specialises in applied hydrogeology. He has 23 years of experience continuum and discontinuum models pillar stability, and the stability and in geotechnical were developed and applied to evaluate support design of crusher chambers, engineering, the influence of the stresses and transfer caverns and several large rock mechanics, and geotechnical existing geological features (e.g. the excavations. Tasks developed range instrumentation, leading important open- presence of two major shear zones, the from data collection and interpretation pit and underground mining projects in West Fault, and different lithological of geotechnical data to the design of Argentina, Chile, Colombia, Mexico, Peru, units) on the mechanical response of underground excavations including Russia, and Uruguay. In addition, Esteban the excavation. more than 50 complex 3D continuum is an expert in analysis and geomechanical and discontinuum numerical models. To design of underground mining and surface SRK Chile has worked on a wide range date, the work SRK carried out for the excavations using 2D and 3D numerical of topics in the project, including Chuquicamata underground project is modelling, stability analysis, and slope the geomechanical design at the summarised in more than 45 technical design in open pits and waste dumps. undercutting level, extraction level and reports and 110 construction drawings. haulage level, the macro-sequence Esteban Hormazabal: definition, rib pillar and macro-block Esteban Hormazabal: [email protected] [email protected]

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Mass mining diamonds

DAN LAGACÉ

Dan has ten years experience working in underground Mass mining including block caving, mining took place in Canada. Today, and open pit mines, incline caving, front caving, sub-level out of some 40 diamond mines mining across multiple caving and sub-level retreat are the kimberlite, approximately half are commodities principal mining methods for primary underground and another 20 have (diamonds, base diamond deposits worldwide. Diamonds underground plans or they are exploring metals, coking have only been mined on an industrial its potential. coal and potash). scale within the past 150 years, mainly SRK has been involved in most of the He focuses on project evaluation as open pit mines. Underground mining underground diamond mining projects and mining studies from scoping to of those deposits was first implemented around the world. In Canada, the Ekati feasibility, mine design, scheduling, within the second half of the 20th Diamond Mine was the first diamond and cost estimating. Prior to joining century in South Africa. A relatively large mine to be developed near Lac de SRK, Dan worked at several mine sites number of underground mining methods Gras in Canada’s Northwest Territories. across Canada gaining experience were tested, implemented, and evolved Koala North Pipe has been developed in mine engineering and project over the past 50 years, mainly in South and mined as an open-benching, management roles. Most recently, African mines. In the mid-1990s, Alrosa, mechanised, and trackless operation to he led underground and open pit a Russian group of diamond mining test the underground mining method mining projects from early stage to companies, started developing the first and to provide access to the lower parts execution with Dominion Diamond underground diamond mine in Russia, of the Panda and Koala pipes which were Ekati Corporation. With Dominion, Dan Internationalnaya. Since then, Alrosa developed and mined underground once managed the Sable Pit project from continued to implement underground the open pit operations were completed. preliminary economic assessment to mining on several of their mines, Koala North, North America’s first project execution, and the Fox Deep including Aikhal, Mir and Udachny. China underground diamond mine, formally project from preliminary economic also experimented with underground opened in 2002. Since then, Panda assessment to prefeasibility. mining at Nhangma 701 Diamond Mine and Koala Pipes were mined by three at the end of the 1990s, but the largest principal underground mining methods: Dan Lagacé: [email protected] development of diamond underground

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Guidelines on Caving Mining Methods: The Underlying Concepts

In 2000, International Caving Study (ICS) published a practical manual on block caving by Dr. Dennis Laubscher. This first comprehensive block caving publication was co-authored by several industry experts, including Dr. Alan Guest and Jarek Jakubec of SRK Consulting (Canada) Inc. Unfortunately, the distribution of this publication was not widely available. Meanwhile the demand for practical guidelines on cave mining was increasing as new projects developed. Mass Mining Technology (MMT) recommended that Dr. Laubscher, Dr. Guest and Jarek Jakubec undertake this task. In 2017, the design book ‘Guidelines on Caving Mining Methods’ was published as a practical tool for cave mining by The University of Queensland. Dr. Dennis H. Laubsher receiving the SAIMM, Brigadier Stokes Memorial platinum medal Award in 2007

sub-level retreat, sub-level caving and Incline Caving.

Diavik Diamond Mine started open pit production in 2003. By 2005, underground development had commenced with plans to mine the A154 and A418 pipes using backfill methods. As geotechnical knowledge was gained, the mining methods were re-evaluated. The sub-level retreat method was chosen for the A154S and A418 pipes, and blasthole open stoping with cemented rockfill was chosen for the A154N pipe. In 2012, the open pits reached their ultimate depths and Diavik Diamond Mine became a fully underground operation. In 2018, open pit mining of A21 kimberlite was successfully commissioned, complementing underground mining at Diavik.

Jarek Jakubec: [email protected]

Dan Lagacé: [email protected] Jarek Jakubec and Dr. Alan Guest

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Open pit to underground transition

SRK carried out a scoping study on an SRK conducted a series of mining existing open pit operation (12 Mtpa) method evaluations to identify what the that is soon to be reaching a depth most suitable mining method options where the mine will need to transition would be to maximise recovery, while to a large-scale underground mining also maintaining production continuity method. The goal was to provide as the mine transitions from surface to production continuity at a rate that will underground. This resulted in a sub level still be suitable to maximise use of the open stoping (with backfill) mine design existing surface processing facilities. for the peripheral halo ore zone and two The ore body dips below the bottom of side-by-side block caves for the more the planned open pit and is still open massive ore area. Given the geometry at depth. Around the periphery of the of the ore body, an inclined cave could main ore body is a halo of disseminated become more of a real option, but lens-shaped types of mineralisation. additional resource drilling is required and a firmer definition of the final open- Due to this mineralisation, the ore pit bottom. halo requires smaller scale or selective mining methods, while the main A critical aspect that is often orebody lends itself to mass mining underestimated is the transition time where block caving or inclined caving frame, particularly when additional ore are viable options. body knowledge (including resource, geotechnical, hydrogeology, metallurgy and rock temperature) is still required to support detailed designs and approvals SIMON HANRAHAN for a major underground mining complex. It is not uncommon for these transitions to take more than 10 years to complete Simon is a Mining the required sequences of work from Engineer with over orebody knowledge sourcing to feasibility 30 years of global studies, construction, development, and experience as a team Many ore deposits extend vertically, a production ramp up. If the transition member and project fact which is not always known at the timeframe is underestimated, shortcuts leader across the start of mining. If initial mining is by could be required to achieve production whole resources opencast methods and ore reserves continuity. However, there is significant value chain – option, are proven to greater depths, the pits risk that if these plans are not realistic, pre-feasibility and are often planned to go deeper than the successful operation of a cave mine feasibility studies, project development, originally envisaged. In such cases, for the longer term can be comprised. and mine operations. He has significant surface plants and critical underground experience in major projects involving The results of the scoping study provided facilities -- conveyor tunnels, access the development of block-cave mines the client with a solid understanding ramps, ore passes, hoisting, and and operating open-pit mines above of a conceptual transition plan that is ventilation shafts -- are often located caving zones. Simon has carried now being used to optimise production much closer to the pit rim and the ore out project management, project continuity while making plans to continue body than desirable. This raises the evaluations, and technical reviews for life beyond an existing open pit operation. question of stability, which may be gold, base metal, platinum, industrial A key outcome of the study was that critical for operating the mine longer mineral, diamond, coal, and iron ore the client now has a vastly improved term. Key considerations to evaluate projects. He has demonstrated ability to understanding of what a transition will are: open pit stability, shaft stability, work within and lead multi-disciplinary, require, and plans are now progressing dilution, mud rushes and air blasts, and multi-cultural teams across all for a further open pit pushback to mining method selection. resource types. create adequate time to transition to an Planning Considerations: The underground mine. Simon Hanrahan: [email protected] efficient transition from open pit Simon Hanrahan: [email protected] mining to an underground operation requires extensive planning. For a large mining operation, planning could last as long as 20 years. The main factors and activities that need to be taken into account in the planning

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Planning considerations for transition from open pit to underground

PETER TERBRUGGE

Peter is a Principal Engineering Geologist and has over 39 years of experience in engineering geology, rock slope stability, site investigations, and tunnelling. His expertise includes open-pit slope design for large copper mines, diamond mines, gold mines and iron ore mines. Also feasibility studies, designing remedial measures to maintain access in critical-area, civil engineering projects, rock mass classification studies, and field mapping, mine surveying, and tunnel support.

Peter Terbrugge: [email protected] cycle are as follows: • Planning and implementing transition from surface to underground mining • Definition of the ore body can take 20 years. Planning must • The presence of an abandoned pit • Rock mass characterisation therefore commence at an early stage. above can lead to greater risks of • Definition of the boundary conditions • Surface and underground infrastructure dilution and mud rushes. • Investigation of suitable mining is often at risk as pits deepen beyond • The choice of underground mining methods planned depths. method affects the surface; and stability requirements may dictate • Underground infrastructure • Transition from open pit to underground mining often risks mud the choice of mining method. • Surface infrastructure rushes (if mud forming minerals are Peter Terbrugge: [email protected] • Ongoing open pit factors of influence present), and from sumps and surface dams. Air blasts can result from • Underground mining and layout underground collapses or mud rushes. considerations • Underground and surface effects of A schedule for an open pit to underground study

the underground mining ACTIVITY Y1 Y2 Y3 Y4 Y5 Y6 Y7 Y8 Y9 Y10 Y11 Y12 Y13 Y14 Y15 Y16 Y17 Y18 • Surface and groundwater effects Exploration Conceptual design • Risks Stage 1: Prefeasibility • Project timing Board review Conclusions: Aspects of particular Stage 2: Feasibility significance include the following: Board review Stage 2A • An economically designed pit will Stage 2B Feasibility Study have slopes close to their stability Stage 2C limits, with little scope for extending Stage 3: Final design the open pit to greater depths. Stage 4: Implementation

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Rapid cave design and production scheduling

For early stage caving projects, there commodity prices, metallurgical is usually a high degree of uncertainty recoveries, discount rate and to a lesser in the input parameters that impact the extent operating costs. It is found that mine design and strategy. When dealing mixing and footprint development costs with such uncertainty, it is better to have little impact on the placement of quickly generate multiple designs and the footprint or the overall mine strategy. schedules that cover the likely range of key Design details such as drawpoint layout, parameters rather than evaluating only a undercutting and infrastructure are not Cave Mining – Optionality in Operation few or even a single case based on precise critical for this process and costs can be but inaccurate parameter assumptions. estimated based on area, perimeter, and Selective Block Cave Option Open Pit Mine depth of each footprint. Underground Mine Underground Mine SRK has worked on many early stage projects and has developed processes Each design or schedule can then be and tools for rapidly generating an array of fed into our rapid economic evaluation Ultimate Size of Mine Plan Continuous Option Step-wise Option Limited Options conceptual cave designs and schedules model to see how it responds over a range of conditions. Designs can be for use in our rapid economic evaluation Mining and Milling Rates Few Constraints Significant Constraints Significant Constraints and strategic planning process. evaluated using different assumptions on costs, prices, material handling systems, For block caving projects, the rapid productions rates, and more. Instead Cut-off policy Dynamic (truck-by-truck) Dynamic (stope-by-stope) Shut-off Only generation of designs and schedules is of generating a single mine plan for an done using GEOVIA’s Footprint Finder> assumed set of conditions, this process It is part of the PCBC caving simulation enables the identification of robust Stockpiling Strategic Surge Surge package, and complements a suite of strategies that perform well across a proprietary software tools and templates range of conditions. used to automate and manage the Ore Type Selection Easy Possible Impossible process and collate the data. For sublevel SRK’s strategic planning process has cave projects, we use a similar mix of been used multiple times to assist Stop-start Mining Easy Possible Hard proprietary software and processes along our clients not only in assessing the with GEOVIA’s PCSLC application. Using economic viability of a caving project, but these tools, we can generate a suite of to select strategies for further study that designs and schedules, each built to a balance risks with potential reward. The Cave mining, and in particular specific scenario defined by a unique ability to rapidly generate designs and block-cave mining, stands at combination of parameter values. production schedules is a key component one end of the spectrum of of this process. mining method-related risks. It Key parameters that most influence is not so much that the risks are the cave design in terms of footprint Scott Loewen: [email protected] greater, but rather that the ability boundary and elevation are the assumed of management to respond to variations in expected conditions is much less than with other mining methods. Compounding this in the case of extraction-level stability risk is that small issues can quickly become large issues as the cave operations are slowed and the geotechnical stresses build.

A conventional open-pit mine makes few irreversible decisions in the planning phase. The size of the mining fleet, the mining rate, the cut-off policy, and the stockpiling Cu (%) Extraction Rate (%) strategy can (and should) all be >0.0 >=2.5 50 dynamically adjusted in response to >=0.5 >=3.0 70 economic and technical parameters >=1.0 >=5.0 85 to maximise cashflows and profits. >=1.5 >=7.5 100 >=2.0 >=10.0 200

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Cave mining risks - not necessarily greater, but definitely different SCOTT LOEWEN Scott is an underground Mining Engineer with 15 years’ industry Selective Block Cave experience. He has Option Open Pit Mine Underground Mine Underground Mine performed a variety of roles including mine planning, ventilation, Ultimate Size of Mine Plan Continuous Option Step-wise Option Limited Options and surveying, while working at several underground Mining and Milling Rates Few Constraints Significant Constraints Significant Constraints operations. Scott’s most recent experience before joining SRK was working with mining software companies Cut-off policy Dynamic (truck-by-truck) Dynamic (stope-by-stope) Shut-off Only in the support and development of their products. He specialises in the design, Stockpiling Strategic Surge Surge planning, and scheduling of block cave and sublevel caving projects ranging from scoping to feasibility-level studies. Ore Type Selection Easy Possible Impossible Scott is an advanced user of multiple mine design packages, developer of custom software, and an expert user of Stop-start Mining Easy Possible Hard PCBC and PCSLC.

Scott Loewen: [email protected]

The pit-slope angles, dewatering effectively set by infrastructure and strategy and support strategy can all be layout. Cut-off grades have been 80% NEIL WINKELMANN adjusted as experience is gained and defined by the footprint configuration the geotechnical context and behavior is and a dynamic policy. Responding to Neil has over 30 better understood throughout the mine economic drivers isn’t possible. years of experience life. The size and type of the processing Although sublevel caves (and other in the minerals facility (and its location) are really level-based variants) have a bit more industry. He has held the only ‘fixed’ decisions at the time flexibility, the implication of this is that senior management construction commences. there is great value in investing in a positions in Contrast this to a block cave. Many comprehensive strategic evaluation operations, technical major and effectively permanent process for cave mines generally, services, and decisions must be made before any quantitatively considering risk through business analysis. operating experience at all is obtained. sensitivity and scenario analysis, At SRK, Neil focusses on economic The orebody must effectively be and assessing the degree to which evaluation of mineral industry operations delineated on five of the six sides design decisions stand up to inevitable and projects. He has expertise in before production starts. Only the shut- variation. Getting it right up-front is economic modelling, specifically in the off policy remains available to optimise essential. This is where the value is creation of flexible models for scenario- economics. The geotechnical context really created in cave mines. In contrast, based risk characterisation and strategic must be perfectly characterised, and the value in an open pit mine is created project evaluation and optimisation. Neil designs developed to ensure stability primarily through dynamic optimisation specialises in semi-stochastic analysis of the extraction-level under the as- over the mine operations period, rather such as expected-value analysis, and yet untested loading environment. than at the initial design phase. full Monte Carlo simulations. No significant redesign on the fly Neil Winkelmann: [email protected] Neil Winkelmann: [email protected] is possible. Production rates are

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Mudrush dynamics in underground mining

Examples of very stiff red clays from Northparkes Mine and a typical low-viscosity mudpush (Photos courtesy of Rio Tinto and De Beers)

Mudrush, mudflow, or mudpush are process, and through production an inrush of 450,000 m3 of muck into the most common terms describing activities, provide disturbances as the workings. The muck originated from uncontrolled ingress of assorted well as a discharge point. Block caving tailings dams, which were located on mixture of water and solids. Mudrush operations are also susceptible to subsiding ground above the workings. is the underground equivalent of external mudrush flows because the The water, impounded in the depressed surface debris flow. It can have broken muckpile connects the surface crater of the tailings that had subsided, different origins but produce the same with the underground excavations. was seen as a major contributor to results: injury, loss of life, damage the inrush. Although mudrushes are more to property, excess dilution, and common in caves than in other mines, Over the past two decades, the mining production delays, or -- in extreme any mining activity that enables the industry developed a comprehensive cases – mine closure. Mudrush accumulation of fine particles and water risk assessment including risk rating dynamics in underground mining are is susceptible to mudrushes. Cases exist and safe operating procedures. especially complex due to confinement of injuries and fatalities from sudden Although mudrushes are difficult to and stress within the muckpile. ore pass discharges, the collapse and predict and impossible to prevent, if Block caving and sublevel caving subsequent flow of unconsolidated or taken seriously, the impact on operation operations are inherently susceptible poorly consolidated backfill, and the can be minimised with proper cave to internal mudrushes because they failure of tailings and slimes dams. In management and draw control. have the potential to accumulate water, September 1970, 89 miners were killed A mudrush seldom occurs as the result generate fines through comminution at the Mufulira mine in Zambia due to of a single cause or fault; therefore,

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A number of caving mines operate • Inspection of old workings for safely with high rainfall and mud- presence of water and/or mud forming potential using tele-remote • Sealing off all possible access points equipment and strict mudrush risk to the cave other than operating mitigation procedures, such as: drawpoints • Interception of water flow into the • Strict draw control procedures cave (surface and underground) • Comprehensive monitoring program • Pre-strip of mud-forming overburden and reliable water balance waste from the subsidence zone Jarek Jakubec: [email protected] • Tailing ponds and other sources of water and/or mud located away from the expected subsidence zone

Figure illustrates the mudrush risk assessment process

Sources of fines Source of water (material properties of ore, (surface and underground; waste, and overburden) quantities and distribution)

Examples of very stiff red clays from Northparkes Mine and a typical low-viscosity mudpush (Photos courtesy of Rio Tinto and De Beers)

Mud-forming potential any risk analysis has to take into account all contributing factors and (mud occurence in muckpile) combinations thereof. A system Cave geometry failure usually results when a combination of failures occurs Draw control and dynamics in such a way that the disturbing Mudflow potential forces exceed the capacity of the (trigger, discharge, and deposition) system to resist those forces.

Before assessing any risks to a mine, however, the following questions need to be answered: Mudrush risk analysis • Is there potential to generate (likelihood and consequences) fines? Controls • Is there potential to accumulate (monitoring and preventative measures) water? Mudrush consequences • Is there potential to form mud? (safety and economic) • What disturbance can mobilise and discharge the wet muck?

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The importance of post-drilling assessment CARL KOTTMEIER

Carl, BASc, MBA, is a professional engineer with 30 years of experience in open-pit and underground mining environments in Canada and the United States. His operational experience includes mine design and construction, mine contracting, mine operations, blast design and optimisation, technical reviews and audits, mine permitting, project management, due diligence studies, cost estimation, and financial modelling. Carl’s commodity experience includes projects involving coal, nickel, copper, molybdenum, lead, zinc, gold, and silver. In sublevel caving operations, it is make any required adjustments before Carl Kottmeier: [email protected] imperative that the production blastholes the explosives charging process begins. be drilled accurately so that there is no Blastholes that have deviated excessively risk of unblasted ore remaining between will have a considerably different IOURI IAKOVLEV adjacent drawpoints. Post-drilling burden than was planned and therefore assessment is the process of mitigating the explosives loading -- and in some Iouri’s expertise this risk by verifying that blastholes have instances the delay timing -- will have to is in engineering been drilled accurately with the correct be adjusted. activities related to length and orientation, following the Without corrective action, blasthole mine development blast design. deviation can result in unblasted ore and/ and production, Although the process can be time or poorly blasted ore remaining between compliance with consuming, post-drilling assessment is adjacent drawpoints. Additionally, quality standards, an important process that is worth the blastholes that have been inaccurately safety objectives, time and the costs involved and blasting. drilled may have excessive burdens that schedules and costs. Engineers should frequently assess how can lead to poor fragmentation, create In addition to extensive underground the blastholes have been drilled (length, excessive vibration, and in extreme cases coal production experience, Iouri’s orientation, and the spatial relationship lead to blasthole rifling. Blastholes that engineering consulting experience between adjacent blastholes) and have been drilled with insufficient burden includes due diligence reviews, compare this information to the blast can generate excessive overbreak in the operational assistance, technical studies plan. In particular, special attention should brow and pillars, cause blasthole cut-offs, (scoping, prefeasibility and feasibility) be given to any blastholes that extend and desensitise the explosives in adjacent and detailed engineering for mining towards, and overlap with, blastholes from blastholes. properties in Canada, USA, Mexico, adjacent drawpoints. South America, Africa, Europe and Asia. Further, post-drilling assessments can help Post-drilling assessment provides drillers determine what may need to be Iouri Iakovlev: [email protected] important feedback on blasthole drilling done to reduce drilling error. deviation; it allows a blasting engineer to Areas of focus may include the following:

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Automation in a cave mining environment

In general, automation is most Continued operation during shift and effectively applied to repetitive tasks, blasting breaks In an automated mine, such as production loading and the operator is located on the surface, trucking. The significant presence of and can keep the machines working these highly repetitive tasks in a cave during firing time. Hot seat changes mining environment make automation happen steps from the mine parking lot particularly attractive. and keep the machines working during shift change. The increasing automation of mining equipment is the natural evolution of Data collection The automation basic tele-remote operation. The primary infrastructure can also be used to capture benefit of tele-remote mining is in the operating data such as the ore source safety and comfort for the equipment and destination. This is particularly useful operator, having been removed from as draw point management is a critical the seat. Historically, a tele-remote driver of an efficient production plan and operator would be located at the mine effective control of the caving. site, near the machine being operated. Implementing an automated system is However, recent improvements in most effective when it is considered communications bandwidth allow before the mine is built, allowing a operators to be located wherever there suitable design and the right equipment is a good internet connection. to be specified. Automation can also be A remaining challenge for automation implemented in an existing operating technology in the production process is mine but some compromise is typically automated bucket loading. Muck piles required. Pushback from personnel may are not homogenous; subtle variations in be encountered making automation both rock size, compaction, and brow position more difficult and perhaps ultimately less mean that the optimum technique for effective. • Improving survey controls filling the bucket will differ with every It is valuable to consider automation at cycle. Although bucket filling algorithms • Improving the orientation accuracy the PFS/FS level so mine infrastructure are improving, in most applications, the of each drill setup can be designed to accommodate highest productivity (and lowest cost automation. The layout of the extraction • Reducing the number of drill setups per tonne) is currently thought to be level and design of ore-handling system per ring achieved when the operator intervenes is critical with the overall strategy • Properly fixing the drill in place with to load the bucket. for effective and efficient isolation of drill stingers and support jacks There is little actual saving in labour automated zones an important factor. • Using drill-string centralisers and costs as a result of automation allowing The technology for autonomous stabilisers one person to operate several machines. underground production is available The reality is that the automated mine For these reasons, post-drilling and appropriate for consideration. The will require the same labour, more-or- assessment was used with great opportunities and risk for autonomous less, albeit with different skills, and success at the Cassiar Asbestos Mine, haulage system are largely associated deployed in areas of technical support a sublevel block caving operation in with the efficacy of its implementation rather than in operations. northern British Columbia that operated rather than with the technology itself. until 1992. Approximately 50% of the The true advantages provided by Successful implementation of blastholes in each ring were surveyed automation are more subtle. as part of the post-drilling assessment. automation requires effective planning Corrective action was taken prior to Consistent, careful operation An and organisation. The distraction of the charging the ring blastholes to ensure automated machine will be operated far day-to-day can impact this process at the success of the blasting process. more carefully than a human operator. operating mines. Enthusiastic leadership Corrective actions sometimes included It will operate within the designed and effective change management redrilling the blastholes, using stronger parameters -- it won’t hit the wall or ride are essential. It has been said that the explosives as a toe load, and adjusting the brakes or change into reverse while journey to automation can be just as blasthole delay timing. moving forwards. rewarding as the automation itself.

Carl Kottmeier: [email protected] Iouri Iakovlev: [email protected]

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