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Innovative Cave Establishment Practices at Ridgeway Deeps

Geoff Dunstan Newcrest, Australia

L. Popa Newcrest, Australia

Abstract

Underground bulk mining methods such as Block Caving (BC) and Panel Caving (PC) are increasingly regarded as low cost solutions to extracting large, low grade underground metalliferous orebodies. Significant technological advancements in the last few decades have allowed the application of Caving at greater depths, where harder rock masses and higher magnitude stresses prevail. Successful BC and PC mines are the result of a well understood relationship between the various phases and requirements of cave establishment, Cave ramp up and steady state production each of which will be covered in this paper. Ridgeway Deeps (RWD) in Orange NSW Australia will be used as the example where innovative practices have assisted in establishing Newcrests first Block Caving Operation.

The Cave establishment phase including lateral development, Undercutting and Drawbell Construction impacts on cave construction time and the ability of the mine to achieve early production and/or reduce the ramp-up time. The success of undercutting and drawbell establishment hinges heavily on drill and blast activities. At RWD novel drill and blast design principles and methodologies have allowed the mine to challenge the industry status quo for undercutting rates as well as for the height and volume of drawbells fired in one shot.

During the Cave ramp up phase a number of management and technical systems were utilised to understand and track not only the cave back progression but also the initial formation of the draw columns.

The infrastructure required to support an operating BC was also challenged with new designs implemented in drawpoint infrastructure, roadways and supporting mine services. Many of these items improved the cave construction timing as well as providing a solid footing for the future production needs of the mine.

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Introduction Ridgeway Gold Mine is located the NSW Central Tablelands approximately 215 km west north west of Sydney in. It is part of the Cadia Valley Operations (CVO), a wholly owned subsidiary of Newcrest Mining Limited (NML) which also includes the Cadia Hill Open Pit and Cadia East Panel Cave Project. The Ridgeway orebody was discovered in 1996 and was commissioned as a Sub Level Caving (SLC) mining operation with a capacity of 4 Mtpa in 2002. Production was incrementally increased over the following years to rates above 6Mtpa until the SLC section of the mine was completed in early 2010. Additional down dip resources were identified following a comprehensive drilling program in 2004. These resources were of lower grade and identified a number of weak structures in the footwall. A number of studies into the new resource ultimately resulted in the approval of the Ridgeway Deeps Block Cave project in 2007. The Ridgeway Deeps project required the concurrent establishment of a Block Cave (BC) undercut and extraction level approximately 210m below the active SLC areas of the mine. This concurrent activity would be required for 3 years until the SLC was completed to minimize the production losses during the transition from SLC to BC mining.

State of the art Ridgeway Deeps is the first of many planned Block or Panel caves for Newcrest. An international assessment of the state of the art for caving was undertaken to determine the expected outcomes both physically and financially from the cave. The international benchmarks identified are listed in Table 1. To achieve an economically, and business plan, suitable option for RWD a number of these activities needed to be surpassed or enhanced.

Table 1: International Benchmarks

Mine Type Interest

Northparkes Block Cave Crushers, drawbell firing (#/mth),drawbell construction, undercutting rate (m2/mth), roadway type, LHD operation

El Teniente Panel Cave roadway type, drawbell construction, LHD operation, pre conditioning

Palabora Block Cave

Establishment sequence, undercut firing (m2/mth), drawbell firing (#/mth), drawbell construction, secondary break, LHD operation, roadway type

Andina Panel Cave LHD operation, roadway type, pre conditioning, undercut firing (m2/mth)

Finsch Block Cave Truck automation, undercut firing (m2/mth), drawbell construction

Gaps Following the completion of the Pre Feasibility Study a review of the expectations between the challenging assumptions of the study and the current benchmarked industry data from other Block and Panel Caves was reviewed for gaps and an assessment of the research programs required to fill the gaps was completed. For formal project approval industry or internal benchmarked rates were used though an Industrial Engineering budget was also approved to work on closing the gaps to the potential value identified in the Pre Feasibility study.

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The key gaps identified could be loosely grouped into the following categories:

Rapid Cave Establishment. This includes rapid long round development, rapid undercutting (6,000m2/month), single shot drawbells and Extraction Level infrastructure,

Cave Pre Conditioning. Within this area was hydrofracturing prior to commencing the undercut,

Cave flow assessment which included steel and electronic draw markers to reveal how flow occurs within a Block Cave

Continuous production utilising semi autonomous LHDs.

Systematic investigation During all phases of the Ridgeway Deeps Block Cave there was an assessment of the block caves deliverables and expectations. There were also a number of documented experiments on actual performance in regard to Rapid Cave Establishment including rapid multi heading development, Cave Pre Conditioning, Cave flow and semi autonomous LHD performance. The results of the research were used to enhance the benchmarked information received from other mines and industry consultants to push the limits of the individual areas within the Block Cave.

New processes and outcomes

Cave Establishment The guiding principles in reviewing the requirements on the undercut and extraction levels were to find a balance between the upfront installation requirements, time and cost, and the long term functionality of the extraction level footprint. As a key scheduling requirement of the Ridgeway Deeps Project was to enable a cave opening rate of 6,000m2/month this equated to roughly 11 drawbells per month or a new drawbell in less than 3 days. Ridgeway Deeps employed an advanced undercutting strategy above an offset Herringbone extraction level layout with a drawbell spacing of 30 x 18m and a vertical separation of 18m, floor to floor, between the undercut and extraction level, as shown in Figure 1.

Figure 1: Advanced Undercut Establishment section

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To achieve these schedule and operational criteria every aspect of the drawbell and undercutting sequence was critically reviewed to remove waste and enable the interchanging of activities in the sequence to build in scheduling flexibility whilst still achieving the technical functionality required.

Lateral Development To aid in the advance rates of the development crews a trade off between drift profile size and ground support was established. The traditional small drift profiles for undercuts and extraction levels limit the equipment available. By reviewing the triggers for the ground support requirements and matching them to the mine design and cave establishment sequence a balance which enabled rapid multi heading development was achieved. On the undercut level the number of break offs from the perimeter drive was minimized and the final pillar dimensions maximized by utilizing a trident design, as shown in Figure 2. This design whilst creating a much stronger final pillar which ensured the integrity of the undercut perimeter drive for the life of the cave, came at an operational cost of linking drives together and impacting on the efficiency of supporting services such as ventilation and power reticulation.

Figure 2: Trident Undercut design

Undercutting The Ridgeway Deeps Feasibility Study recommended the crinkle cut style of undercutting. This methodology was preferred after the review of a number of undercutting options assessed in regard to business, schedule and cost criteria:

Business return benefits of increased NPV through reduced working stocks of development and undercut production activities, reduced time period between cave initiation and production.

Reliability enhancement through the establishment of the drawbells in a stress shadow and the undercutting establishment front is within the reduced stress window; and

The upside flexibility options of increasing the undercutting advance rate upon confirmation of rapid drawbell establishment techniques resulting in a reduced cave establishment period and increased production rates.

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The Ridgeway Deeps geometry was defined by an alternating series of inclined and flat rings. Each ring was contained of 3 x 89mm diameter blastholes which were charged with a gassed emulsion and fired using MS nonelectric detonators. The inclined rings shaped the 29m high, 15.5m wide major apices, and the troughs above the drawbell Figure 3.

Figure 3: Undercut design

The undercutting process utilized confined drill and blast expertise and mining skills honed over years of SLC operation. Key points of difference to other crinkle cut undercuts employed elsewhere were the Ridgeway SLC approach of bringing undercut drives into production and the slotting technique developed to create the free face for the crinkle undercut.

With the exception of a few needed intersections, the rest of the undercut drives were not developed to break through until required, thus providing significant benefits in terms safety, productivity, technical and cost. The methodology was implemented as follows:

1. A pillar of 3m was left (drilled and ready to be charged) between the undercut drive and the slot drive.

2. The undercut stand-up rings, both inclines and flats, were precharged and preloaded respectively, previous to charging and firing the pillar.

3. The slot was then bogged wide open and high charging bund placed at the intersection for the protection of the charge-up crew

4. To prime, connect and fire the stand-up rings into the slot.

Instead of a conventional continuous slot established from an initial slot rise, or triangular slot method, it was decided that a series seventeen mini-slots would be established every 29.5m. This methodology allowed retreat mining of the slot; ensured adequate free face was created for undercut blasting and reduced the removal of blasted slot material to a minimum. The mini-slots were created by establishing a 15m high rise in which six slashing rings were subsequently fired in two stages. A minimum of blasted material was removed as it was accepted that the undercut rings could be confidently fired in conditions similar but less confined than SLC blasting. As can be seen in Figure 4 the inclined rings were fired into the slots and the flat rings were fired into the slot drive.

Figure 4: Mini slots for Crinkle Cut undercut

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Drawbell Construction

The circular design methodology Using practical mining experience and an improved understanding of confined blasting from SLC experience, one of the authors, Popa, designed and successfully implemented alternative drawbell drill and blast designs for Newcrests first block cave, Ridgeway Deeps. The associated drill and blast patterns which culminate, at the top of the drawbell, in perfect concentric circles (circular design) are considered unique. The circular designs result in an optimal distribution of explosive energy, and hole cooperation within the overall blast volume while at the same time compensating for any drilling deviation or hole loss (partial or total) that generally occurs during drawbell drilling and charging. The new methodology, allows for the rapid excavation of large drawbells in a single blast event. The circular designs have defined the Ridgeway Deeps drawbell design and influenced the undercut and extraction level geometries of Newcrests second Panel Cave [the Cadia East Project]. Figure 5 shows the pattern of one of the Cadia East drawbells which best exemplifies the circular design. The 16.5m high, 17.5m diameter, 2100m3 drawbell was successfully established in a single blast event using 136 x 76mm blastholes and 7 x 200mm reamers. The total drilling was 2057m and the charge weight 6000kg resulting in a powder factor of 1.0 kg/m3. Figure 5: Circular design (Cadia East drawbell)

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Ridgeway Deeps Drawbells The feasibility study planned that double ended drawbells would be mined to the skull shape similar to that at the Northparkes E26 Lift 1 and Lift 2 operations, and half skull drawbells at the eastern and western edges of the extraction level. The recommendation was that each double ended drawbell would be excavated using a single 14m x 0.76 m diameter blind raise in the centre of the draw bell, and 136 holes (1940m) of 64 mm blastholes drilled in fans on either side of the raise. Accordingly, the single drawpoint half drawbell required 77 holes or 1,100m of drilling. However, due to the substantial increase in height between Northparkes and Ridgeway extraction and undercut levels (from 14 to 18m), and the necessity for rapid undercut and drawpoint construction, a decision has been made to establish the drawbells using 76mm blastholes charged with a gassed emulsion and electronic detonators. There were 133 drawbells in total comprised of fourteen different designs or variations all of which were successfully established in a single blast event. The vast majority of these drawbells where 14m high skull drawbells even if, for the purpose of Research and Development and in preparation for the benefit of future Newcrest Block Caving Projects, other designs were successfully implemented. Sixteen drawbells were established around an initial 1 x 760mm diameter rise, thirty-eight drawbells around 2 x 760mm diameter rises, and eighty-two drawbells around 1 x 1060mm diameter rise (Figure 6). A summary of Ridgeway Deeps drawbell designs is given in

Table 2. Figure 6: Two examples of Ridgeway Deeps skull drawbells designs

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Table 2: Ridgeway Deeps drawbell design variations

Ridgeway Deeps experience in developing the eastern drawpoints, half drawbells Figure 7, has shown that due to limited drilling space and orientation of blast holes the design is very sensitive and difficult to implement, and hang-ups in the half drawbells are more difficult to release than the larger skull drawbell.

Figure 7: m3 Single drawpoint half drawbell

item Designnumber

of drawbells

drawbell height

Rise type number of holes

number presplit holes

number of V-holes

toes distance from BT

distance beam to

brow

distance to brow

CHANGE

1 m 1 1 14 1 x 760 68 2 x 9 4 1.5 0.7 9.52 m2 1 14 1 x 760 68 2 x 9 4 1.5 0.7 9.53 m3 7 14 1 x 760 70 2 x 9 6 1.0 0.7 9.54 2 x m3 1 14 1 x 760 70 + 70 2 x 9 6 + 6 1.0 0.7 9.51 M 1 3 14 2 x 760 115 2 x 9 9 1.0 0.7 9.52 M 2 4 14 2 x 760 119 2 x 9 13 1.0 0.7 9.5 13 V-Holes3 M 3 18 14 2 x 760 113 2 x 9 13 1.0 0.7 10.5 Brow at 10.5m along centreline4 M 4 2 14 1 x 760 112 2 x 9 12 1.0 0.7 10.5 one rise - 12 V-holes5 M 5 8 14 2 x 760 105 2 x 5 13 1.0 0.7 10.5 Five pre-split holes6 M 5.1 5 14 2 x 760 105 2 x 5 13 0.5 0.7 10.5 drilled 0.5m from B/T7 M 6 11 14 1 x 1060 104 2 x 5 12 0.5 0.7 10.5 1 x 1060mm rise8 M 6.1 4 14 1 x 760 104 2 x 5 12 0.5 0.7 10.5 1 x 760mm rise9 M 7 56 14 1 x 1060 98 2 x 5 12 0.5 1.2 10.5 distance to brow increased to 1.2m10 M 7.1 2 14 1 x 760 98 2 x 5 12 0.5 1.2 10.5 rise diameter decreased to 760mm11 M 7.2 1 18 1 x 1060 98 2 x 5 12 0.5 1.2 10.5 18m high drawbell - under no flats undercut12 M 9 3 25 1 x 1060 98 2 x 5 12 0.5 1.2 10.5 25m high drawbell - SLC undercut13 M 10 1 25 N/A 118 2 x 5 core 0.5 1.2 10.5 25m high drawbell - SLC undercut14 M 8 5 14 1 x 1060 106 2 x 5 12 0.5 1.2 10.5 full drawbell - single drawpoint

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The design used to establish the western edge single drawpoint drawbells (Figure 8) used all of the learnings from the preceding drawbell designs and in terms of volume it is 10% larger than the standard skull drawbell and it offered the following advantages:

Simplified and rapid development and production operations Rapid drawbell establishment Because of its circular shape maximizes and promotes flow towards the drawpoint

Figure 8: M8 - Single drawpoint drawbell

A trial was undertaken at Ridgeway Deeps which investigated the establishment in a single blast event of 25m high drawbells under an SLC type Undercut. To this end the first 55m of the undercut (between adjacent crosscuts XC35 and XC33) was converted to an SLC Undercut. The mining of the SLC undercut was successfully completed and the remaining section reverted to crinkle undercut. The geometry of the SLC undercut has shaped three 25m high skull drawbells as seen in Figure 9. The design of the 25m drawbell (M9) is not dissimilar to the standard design (M7) - the same numbers of holes are drilled from identical collar positions. What defines and separates these two designs are the overall size and degree of difficulty in terms of Drill and Blast (Figure 9).

Figure 9: Section of SLC undercut and design pattern of associated M.9 drawbell

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Table 3: Comparison between M7 & M9 designs

Parameters Crinkle Cut High UndercutDrawbell Design M7 M9Drawbell Height (m) 14 25Method of Extraction Single Shot Firing Single Shot FiringRise Diameter (mm) 760 or 1060 1060Number of holes 98 98Drilling Metres (m) 1225 1672Charged Metres (m) 800 1200Charge Weight (Kg) 4000 6000Electronic Detonators 110 152Insitu Tonnes (t) 4000 6000Powder Factor 1.0 1.0

Extraction Level Infrastructure Infrastructure requirements on the Extraction Level were identified as the key area where scheduling flexibility was paramount. Historically in Block and Panel caves the extraction level infrastructure follows a rigid installation sequence, which often then results in lower than scheduled establishment rates. The philosophy employed at Ridgeway Deeps was to design each component and the equipment required to complete the task such that the activities could be carried out in any order, subject to equipment and access availability. Infrastructure activities were defined primarily as drawpoint support and sprays, roadways and permanent services including those for automated LHD operation and geotechnical monitoring equipment.

Prior to development arriving at the Extraction Level at Ridgeway Deeps there were a number of comprehensive reviews of the infrastructure required and how it interacted with the other activities on the level during construction, i.e. development, drawbell drill and blast and loading. These reviews involved both Project and Operational personnel on site and Australian and International consultants

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working on the Project. It was identified that it is not only the actual infrastructure being installed that impacts the establishment of the extraction level but the supporting tasks, storage and additional resources required that impact the amount of real estate required and the delivery schedule.

The two biggest time and interference activities were identified as the steel sets at the drawpoint and the 80MPa concrete roadway. Common to these activities was the preparation and curing times of the concrete pavement, either in the extraction drive or the drawpoint apron. The preparation time impacted on the ability to undertake other activities in the area, whilst the curing time also impacted access but more importantly added to the heat on the level and therefore the operating environment.

Brow Infrastructure A critical review of the need for steel sets identified that they were only being installed in

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withstand the dynamic load of a single shot drawbell blast,

withstand the impact of a 5 tonne rock rolling down a 50 degree, 15m high rill slope, and

Be easily replaceable.

Of the >200 flow beams installed only a handful were damaged during the firing of the drawbell and

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Concrete delivery was limited due to other construction activities in the mine, reducing the efficiency of the roadway crew, and

Doing the drawpoint aprons after initial production required the drawpoint rills to be stood up and supported to enable the full apron length to be poured. This support material then had to be fed through the materials handling system when the drawpoint was brought back on line.

Previously installed roadways or aprons in incomplete Extraction Drives had to be removed as the ground movements due to the moving cave front had impacted the relative levels along the drives and the drainage of the level was compromised.

Cave Monitoring and flow A number of cave monitoring systems were used in combination to track the progression of the cave back. This was incredibly important at Ridgeway Deeps as the Ridgeway SLC was still operational some 200m vertically above the Ridgeway Deeps Undercut level. The transition from SLC to BC production was a major focus of site and corporate management as Ridgeway made a significant gold contribution. The longer the 2 caves could be operated in parallel the greater the metal production and the lower the cost base. An overview of the system will be outlined in this chapter, for a detailed description of the cave monitoring systems the authors suggest reviewing the paper by Lett and Capes et al (2012)1 of these conference proceedings.

The basis of the cave monitoring system design was that of redundancy to ensure continuous monitoring of the back independently of access. The systems used were open holes, extensometers, TDRs and a seismic system. As the Block Cave is located in a down dip extension of the Ridgeway orebody being mined by the SLC a few of the SLC monitoring systems, seismic system, extensometers and TDRs, were able to expanded to cover the Block Cave area which reduced the installation time of the system, Figure 11.

Figure 11: Monitoring System

To get enhanced coverage of the Eastern side of the cave back a dedicated monitoring drive was put in at approximately the top of the block cave and just outside the worst case prediction for subsidence. From this drive an array of monitoring systems were installed and access to the area was maintained for the entire period of cave back growth. The diamond drill holes installed to conduct the trial

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Hydrofracturing program within the Eastern Volcanics section of the orebody were also used as open holes or had other sensors installed within them. The correlation between the various systems was to such a high confidence level that the Geotechnical department was able to see the various zones of caving within their data. All of the systems were assessed on a regular basis and used to update the swell and breakthrough predictions. On the back of the monitoring systems the mining strategy in the SLC was adjusted and an additional 2 months of mining was achieved without incident.

Within the production areas of the Block Cave there were a few issues during the ramp up:

Oversize fragmentation within the first 30m above the undercut,

Cave back propagation delays in the corners and near structures, managed by targeted draw strategies,

Interactions between the construction, development and production work teams,

Ventilation issues between the production and establishment sides of the footprint, this was solved by essentially forming 2 ventilation circuits on the extraction level which shielded the production area of the footprint form the development firing and construction heat issues,

Some delays in the commissioning and operating methodology of the extended materials handling system which limited the production rate and reduced the preferred product size from the cave, and

There were implementation and operational issues with the semi autonomous loader fleet. Some of the productivity issues were a function of the delayed implementation of the fleet until the cave was to be at full production rather than during the earlier stages of cave ramp up and the fragmentation experienced during ramp up.

All of these issues were managed and overcome by the combined mine teams and the ramp up of production tonnes from the block cave was within the expectations of the Feasibility Study.

During the establishment of the undercut a series of cave flow marker trials were conducted. These marker trials and the results were reviewed within the Newcrest and Mass Mining Technology 2 (MMT2) technical groups. The results showed provided some significant insights into the preferred flow paths of material in a Block Cave using the Advanced, Crinkle Cut style, undercut.

Steady State Production 12 months after the completion of the final drawbell, November 2011, the Ridgeway Deeps was to some degree in steady state production. The production targets outlined in the Feasibility Study had been reached and the mine was being set up for an increase in production toward 8Mtpa (Figure 12Figure 12).

The cave back has progressed to its predicted final limits and the underground subsidence impacts have been within expectations, actually allowing the ongoing use of the SLC crusher and conveyors.

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Figure 12: Ramp up performance

Acknowledgements The authors would like to thank all of those who have worked on the Ridgeway Deeps Block Cave for their input and insights. We would also like to thank Newcrest Mining Limited for authorizing the publication of this paper and their support in advancing Block and Panel Cave mining.

Reference 1. Lett, J.L., Capes, G.W. (2012), Characterisation of Caving and Subsidence for Operational Enhancements at the Newcrest Ridgeway Gold Mine, Proc. Massmin 2012, Sudbury.