An “Intensive” preconditioning methodology developed for the Cadia East panel cave project, NSW, Australia A. Catalan, G. Dunstan, M. Morgan, S. Green, M. Jorquera & T. Thornhill Newcrest Mining Ltd , Australia
Abstract The prevailing geological and geotechnical conditions of the Newcrest Mining limited’s Cadia East mining complex, which is comprised of a hard and relatively massive rock mass in high stress environment, and the requirement to achieve high production rates early, have necessitated development and implementation of a rock mass preconditioning methodology which combines closely spaced hydraulic fracturing and confined blasting. This is to help alter the rock mass to be caved thereby assisting cave performance. The first panel cave in this mining complex is called Cadia East PC1-S1. The extraction level for the PC1or first lift is located 1,200m below surface and for the second lift (PC2-S1) 1,400m also below surface. These two panel caves will be brought into operation simultaneously. In terms of geometry, the PC1-S1 is approximately 250m wide with a strike length in excess of 1,200m and a vertical extent or lift (block height) of 800m. This paper describes the development and implementation of an intensive pre-conditioning methodology at Cadia East (CE) project. At the time of writing this report, the preconditioning by closely hydraulic fracturing at PC1-S1 had been completed. Twenty one (21) drill holes had been drilled and 1,182 hydro-fractures created. Of these, 761 hydro-fractures were created at 1.5m spacing between 350m and 200m depth below the borehole collar and 421 hydro-fractures were created at 2.5m spacing between 200m and 50m underneath the collar. On the other hand, and with respect to preconditioning by blasting, 23 up holes with a total of 75,555Kgs of Emulsion had been successfully blasted.
1 1.1
Introduction The Cadia East mining complex
The Cadia East ore body is located in Cadia Valley approximately 25km south east of Orange in New South Wales, Australia (Figure 1) which it is 100% owned by Newcrest Mining Ltd. The Cadia East ore body was discovered in 1985 (Malone, 2011) and studies into the viability of the Cadia East resource commenced in the early 1990’s. The Cadia East resource, located adjacent to the Cadia Hill open pit, is a massive low grade gold-copper porphyry deposit covered by up to 200m of overburden. The system is up to 600m wide and extends to 1.9km below the surface.
Figure 1 Cadia East project location
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The Cadia East Underground Project is concerned with the development of the massive Cadia East deposit into Australia’s first panel cave. The mine will be the deepest panel cave in the world and Australia’s largest underground mine. Newcrest’s mining studies identified panel caving as the method likely to deliver the optimum technical and economic outcomes from the deposit. 1.2
Proposed mining method
The proposal is to mine the Cadia East mining complex using a series of panel caves and these will be designated as PC1 and PC2. The first level (PC1) is located approximately 1,200 m below surface and the PC2, 1,450 meters also below surface (Figure 2). Both panels will be mined simultaneously. The total mining inventory is of the order of 1,073 Mt @ 0.60g/t Au and 0.32% Cu. •
• • • • • •
Based on current mine plans, the following are some the key mining parameters: Production to start: 2012 Production rate: maximum 26Mpta or ~72,000 tpd. Mine life: +35 years. Lift 1 extraction level at ~1,225m depth. Lift 2 extraction level at ~1,475m depth. El Teniente extraction level layout. High and Post undercutting strategy. Jaw - gyratory crushers located within close proximity to footprint. Materials handling system: conveyor belt 1.5m ~200km horizontal development
Figure 2 East-West looking North section through the Cadia East project
Figure 3 shows the proposed geometry of the different panels and their position relative to each other.
Figure 3 Lifts 1 & 2 extraction level mining design configurations
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Given the geological and geotechnical attributes of the CE mining complex (considered relatively complex), the proposed mining strategy and geometry, which includes block heights greater than 400m, and the requirements to achieve relatively high draw rates and therefore productivity early e.g. up to 72,000 tons per day, a decision was taken to implement preconditioning as a means to manage cave initiation, caving rate and propagation as well as primary fragmentation. Cadia East project has incorporated an intensive preconditioning concept, which is a combination hydraulic fracturing in down-holes and confined blasting in up-holes. This will be applied before the initiation of the caves and the intent is to precondition the full block volumes. Figure 4 is an illustration of the intensive preconditioning concept and how it is to be applied at the Cadia East project first panel (PC1-S1). HF level
1 2
Block Height
Preconditioning by Blasting
Preconditioning by Hydrofracturing
@2.5m/fracture
Intensive Pre-conditioning Zone @1.5m/fracture
Cave Front Advance
Undercutting UCL level
Post-Undercut Strategy
EXT level Production Zone
Construction Zone
Preparation & Development Zone
Figure 4 Sequence to Implement the Intensive Preconditioning at Cadia East project
The perceived benefits of this overall preconditioning strategy at the CE included: • • • • • •
2
Cave propagation: ~30% faster Fragmentation: ~20% finer (<2m3) Seismicity control: more events but lower magnitude Lower abutment stresses around the cave front Draw rate up to 30% higher than the benchmarking Improved Ramp-up: 3-4 years to achieve 22Mtpa
Preconditioning
The term preconditioning has since been adopted within the caving mining industry to describe the process of "altering" a rock mass to enable better control or management of the cave mining process. The methods used for the preconditioning process are hydraulic fracturing and confined blasting but separately. Preconditioning by hydraulic fracturing (van As & Jeffrey, 2000; van As, Jeffrey, Chacon, & Barrera, 2004; Chacon, Quiñones, Gonzalez, & Barrera , 2002; Chacon, Barrera, & Jeffrey, 2004; Araneda & Sougarret, 2007) is meant to affect the overall rock mass behaviour 3|Page
through the introduction of additional and artificial large scale fractures. On the other hand, preconditioning by drilling and blasting or confined blasting (Chacon et al., 2002; Molina, Cerrutti, Henriquez, Morales, & Apablaza, 2008) is expected to damage the rock matrix and therefore altering the in-situ mechanical rock mass properties such as strength and stiffness.
3
The Cadia East Intensive Preconditioning
3.1
Preconditioning by hydraulic fracturing trials
At the Cadia East panel caving project undertook a series of full-scale preconditioning by hydraulic fracturing trials between September 2009 and February 2010 (Morgan & Catalan, 2010). The aim of these tests was to provide operational parameters in terms of the pressure requirement to generate and propagate hydraulic fractures in different geotechnical/geological domains; to define hydraulic fracture geometry; to confirm the minimum hole spacing requirement between fractures; to generate hydro-fractures down to 550m deep from the collar of the hole; and to optimise the design for the full implementation program. Three purpose- drilled HQ diamond drill holes were available for the preconditioning trial. These trials comprised three stages. For stages 1 and 2, two of the three holes had suitable intervals of good quality hole conditions between 0 - 360m. The third drill hole had suitable hole conditions between 490m - 570m for the stage 3 trial. Figure 5 shows the location of the three HF trial drill holes location in the 5250 level.
5250 Level West
UE056
UE055
PC1 S1
UE054
Figure 5 Location of drill holes used for HF trial 3.1.1
Operational parameters
During these initial trials, 53 fractures were generated. The maximum fracture depth below collar was 570m. Summary data on these hydro-fractures in terms of breakdown pressure and (Pb), instantaneous shut-in pressure (ISIP) as well as injection volumes is presented in Table 1 and Figure 6. Additionally, Figure 7 presents a graph of breakdown pressure versus depth down hole where it is observed that there is a slight increasing pressure with the location or depth of the hydrofractures. Table 1 Statistical information for CE HF trial Total fracture number
Maximum Minimum Mean Mode Standard Deviation
Stage 1 & 2 (33 HF) Breakdown 49 27 36 34 7
ISIP 22 15 19 20 2
Stage 3 (20 HF) Breakdown 52 35 41 39 5
ISIP 25 21 23 22 1
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Figure 6 Distribution of breakdown pressures and injection volumes at CE HF trial
Figure 7 Breakdown pressure versus depth down hole 3.1.2
Fracture initiation damage zone
Fracture initiation damage zone is the section of drillhole wall around the injection interval that is affected by the initiation of a hydraulic fracture. It is important to understand this characteristic as it has a direct impact on the sequencing of closely spaced hydraulic fractures. Acoustic teleview scanner images were collected pre, interim and post hydraulic fracturing. A review of the teleview images revealed that the damage at the injection interval of the HF was of three types; sub axial splitting, cross cutting or the opening of existing discontinuity. Table 2 and Figure 8 summarise these results. Table 2 Fracture Spacing and Initiation Damage Initiation type
Axial Splitting Cross cutting Opening existing discontinuities
Damage Zone size
Up to 2.4m 0.2m 0.2m
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Figure 8
3.1.3
Sub axial splitting at HF initiation point (blue line represents the initiation point and red shapes highlight the initiation damage zone)
Reducing the hydro-fractures spacing
In parallel, CSIRO Petroleum division (Jeffery, R. 2010) carried out the Stage 2 of these trails. The main objective was to place fractures along the borehole with a predetermined spacing. Slots or notches were used in order to introduce a weak initiation point for the fracture at a known location and orientation at the borehole. Breakdown at the notch then results in the preferred fracture geometry and produces a consistent spacing between hydraulic fractures placed along the borehole. Figure 9 shows the notching application and its results. UE055 Pre
UE055 Post
Pre
Post
The image cannot be displayed. Your computer may not have enou…
Figure 9 Notching at the borehole UE055 3.1.4
Fracture Propagation
Using the intersection observations of generated hydraulic fractures and the interpreted fracture path, it was possible to estimate the propagation distance of these fractures. In parallel, CSIRO Petroleum division (Jeffery, 2010) produced fracture growth curves using a numerical hydraulic fracture model. A radial fracture geometry was used, for water injected at a constant rate of 400 l/min. and the rock properties used were E = 55 GPa,
!
= 0.2, and K Ic = 2.0 MPa. The permeability
of the rock mass was then varied, to represent more or less fluid loss into the surrounding rock as a method to change the fracture growth rate. Utilising the propagation distance and the propagation time, a chart can be produced for fracture growth. The results of these analyses are shown in Figure 10.
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Figure 10
3.1.5
Propagation chart for data points plotted with simulated fracture rates with varying permeability values
Mine through hydro-fractures
In order to validate these simulations as well as orientation and propagation of these hydrofractures, additional information was collected from the mine through of proppants emplaced during the CE HF trial by CSIRO during stage 2 trial (Jeffrey, 2010). These intersections showed a good agreement in terms of the geometry, orientation and length of the hydro-fractures estimated, i.e. hydraulic fracture lengths of about 40 m and spacing of 1.25 or 2.5 m. This data demonstrates parallel growth of these closely spaced hydraulic fractures over a mapped distance of over 15 m, which is consistent with model predictions for relevant values of the governing parameters (Bunger, Jeffrey, Kear, Zhang, & Morgan, 2011). Figure 11 shows the mine through site at Cadia East. The colored lines are the four mapped fractures where they were visible along the tunnel.
Figure 11 Mine through of hydro-fracture proppants. 3.1.6
Design guidelines preconditioning by hydraulic fracturing
The full-scale trial at Cadia East project has established a basis for designing and applying preconditioning by hydraulic fracturing in the mineralisation zone. Table 3 provides the design guidelines which were defined for Cadia East project.
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Table 3 Design guidelines from HF trial (Morgan & Catalan, 2010)
Breakdown pressure (Pb) Instantaneous shut-in pressure (ISIP) Fracture Run Time Fracture size major axis Fracture size minor axis Fracture spacing Drill hole pattern Drill hole spacing EW (max axis) Drill hole spacing NS (max axis) Drill hole length Drill hole orientation
3.1.7
38-45 MPa 18-22 MPa 30 minutes 45-65m (radius) 25-30m (radius) 2.5m (single pass ) Less than 2.5m (double pass) Staggered 80m 60m 350m and 550m (downholes) Dip 78!/ Dip Direction 203 !
Excavation and drill hole configuration
Based on these design guidelines, the full hydraulic fracturing implementation program has been designed to cover the full height of each cave block. As such, hydraulic preconditioning excavations were designed from this level. These crosscuts drill chambers are designed at 60m intervals and in a staggered pattern between rows. Crosscuts have been located with 80m centre between them. Figure 12 shows the HF level layout as well as the drillhole distribution for the first block.
Figure 12 HF configuration level and staggered HF drillhole pattern
The full hydraulic fracturing implementation program has two aims. The first is preconditioning of the cave footprint, the second is treatment of key infrastructure areas to promote stress redistribution specifically around the main crusher chambers.
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3.2
Preconditioning by blasting trials
The broad objective of the full-scale preconditioning by blasting trials was to help establish optimal blast preconditioning design parameters as well as quantify the impact of the different designs in terms of the degree of rock mass modification observed (e.g. fractured and disturbed zones). The specific objectives of the CE trials were: •
• •
•
• •
To confirm the design adopted from Codelco-IM2 and Andina mine trials use as base case, in terms of blasting effects, blast geometry (stemming, drill layout, charge distribution, etc.) To establish optimal the operational and safe procedures To collect data required to calibrate and validate the HSBM numerical modelling, e.g. accelerations, damage, etc The help estimate the degree of damage (direct and indirect methods) from single and multiple hole firings To determine the optimal timing and placement of boosters To determine optimal stemming lengths
Figures 13 and 14 are illustrations of the field tests used to get data in order to achieve these results. 3rd Trial 2nd Trial
1st
Trial
1st Trial
95m@1,890Kg
BDP-012
2nd Trial
150m@3,225Kg
BDP-016
3rd Trial
2x150m@6,570 Kg
BDP-018 & 019
A #1, 2, & 3
Tri-axial Accelerometers (5000g) 80m, 55m and 25m
A #4 & 5
Tri-axial Accelerometers (500g) 6m
A #1
A #2
A #3
A #4 A #5
Figure 13 Preconditioning by blasting full-field trials configuration
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Figure 14 Field trial design blast preconditioning holes 3.2.1 Vibration/acceleration measurements
The trial consisted of measuring the vibrations in the near field from two up holes (95 and 150 meters long). The explosive used was DX5039S solid sensitised emulsion (Dyno Nobel Asia Pacific, Australia) with a VOD of 5,600 m/s. A network of 3 tri-axial accelerometers with a measurement range of ±5,000g and higher broadband resolutions (PCB Piezotronics, Inc, USA) were set out between both holes (i.e. near-field monitoring sensors) to monitor the peak particle acceleration of confined blasting. Additionally, two far-filed tri-axial accelerometers (i.e. 500g) were installed 80m and 130m from these preconditioning holes Two DataTrapII Data/VOD Recorder equipments (MREL Group of Companies Limited, Canada) were used for recording the triaxial accelerometers (Figure 15).
Figure 15 Blast monitoring system
The sampling rate for high-frequency near-field acceleration waveform was set up to 2.5 MHz for a recording time of 20ms. The electronic blasting systems were activated through a wire-break trigger method which was connected to an extra initiator programmed to fire at 10ms delay prior to the first firing detonator on the blast preconditioning holes and data acquisition systems were synchronised through the use of common wire-break circuit.
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The maximum particle motion of the reported waveform were determined using the peak vector sum values of particle motion and multi-axial vibration records were used to determine the absolute maximum particle acceleration, velocity and displacement. As part of the blast preconditioning vibration monitoring program, these data have been collected and they have been used as an analytical blast vibration prediction tool for near-field geometries. 3.2.2 Monitoring results and analyses
An analytical analysis, i.e. Holmberg-Persson (H-P) near-field prediction model, has been used in order to estimate the vibrations levels of blast pre-conditioning holes. The measured of peak particle velocity (PPV) has been plotted against the Holmberg term to obtain the regression constants K and ! of this model. Figure 16 shows these results.
Figure 16 Holmberg & Persson model - Preconditioning by Blasting at Cadia East Project
Traditional Scaled Distance vibration model has been estimated using a linear regression of distance and charge weight separately against the measured peak particle velocity generated by blast preconditioning charge holes. This regression is plotted in Figure 17.
Figure 17 Scaled Distance PPV prediction equation for blast preconditioning holes
Additionally, the H-P model has allowed carrying out a preliminary estimation of the expected rock mass damage around these holes. Predicted PPV at different distances from the charge axis for large linear charge concentration a result of H-P near-field vibration modelling are shown in Figure 18.
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Figure 18 Contour of peak particle velocity from blast preconditioning holes
The operational results of the first full-trials where 1,890Kgs of Emulsion were fired simultaneously (i.e. all detonators were initiated at the same time) are shown in Figure 19. Pre (left) and post blasting (right) results are shown on these pictures.
Figure 19 Operation result first blast preconditioning trial
Numerical analyses of these trials are presented in more detail in an accompanying paper (Catalan, Onederra, & Chitombo, 2012). 3.2.3
Design guidelines preconditioning by blasting
Based on these trials, design guidelines have been determined for this preconditioning technique. Drilling and blasting parameters of these up blast preconditioning holes at Cadia East project are shown in Table 4.
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Table 4 Drilling and blasting parameters blast pre-conditioning holes
Hole diameter Hole length Charge length Charge weight Density Emulsion VOD Emulsion Stemming plug Cure time of a special stemming plug Compression strength of the stemming plug Location initiation points Initiation time
165mm 150m 130m ~3,285 Kg 1.18 g/cm3 >5,500 m/s 20m 72hr (minimum) 50MPa (minimum) Every 8 m along the column of explosives Every point in the column is started simultaneously
Drilling patterns recommended are shown in Figure 20.
Figure 20 Drilling pattern for preconditioning by blasting at the CE project
The final pattern used and based on the Holmberg-Persson (H-P) approach was consistent with the pattern used by Codelco Andina mine and more recently by El Teniente mine.
4
Current Implementation of the Intensive Preconditioning Program
By December 2011 preconditioning by hydraulic fracturing at PC1-S1 had been completed. Twenty-one drillholes were drilled and 1,182 hydro-fractured were created. With regard of the hydro-fractures spacing program, 761 hydro-fractures were created to 1.5m spacing between 350m and 200m depth below the borehole collar and 421 hydro-fractures were created to 2.5m spacing between 200m and 50m underneath the collar. Hydro-fracturing program was achieved in a 90% with regard the planned program.
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Preconditioning by blasting program was started on October 2011 and as of March 2012, 23 up preconditioning holes equivalent to 75,555Kgs of Emulsion have been blasted. This program has been focused on the area where the cave will be initiated. The current program is ongoing. Figure 21 shows the current intensive preconditioning program undertaken at Cadia East project.
Figure 21 Intensive preconditioning applications at the Cadia East panel cave project
5
Conclusions
This paper discussed the development and implementation of a methodology referred to within Newcrest Mining as Intensive Preconditioning . This combines hydraulics fracturing (HF) and confined blasting and the intent is to alter the geomechanical characteristics of the rock through introduction of additional fractures and reduction of rock mass strength through blasting. The intensive preconditioning methodology includes HF with closely spaced fractures and blasting of fully confined blast holes charged in this case with an emulsion product and initiated using electronic detonators. Specific equipments had to be developed and/or modified for the two processes. At the time of writing this paper, both HF and blasting tests had been completed the full benefits of intensive preconditioning were still to be fully realised. Twenty one (21) drill holes had been drilled and 1,182 hydro-fractures created. Of these, 761 hydro-fractures were created at 1.5m spacing between 350m and 200m depth below the borehole collar and 421 hydro-fractures were created at 2.5m spacing between 200m and 50m underneath the collar. On the other hand, and with respect to preconditioning by blasting, 23 up holes with a total of 75,555Kgs of Emulsion had been blasted.
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Acknowledgements The authors would like to thank Newcrest Mining Limited for allowing the publication of this paper. We would also like to acknowledge to all those involved in the preconditioning trails and applications for their invaluable effort in order to incorporate these techniques at Cadia East project. It is necessary mentioned that the introduction and implementation of preconditioning by blasting at Cadia East (trials), has been part of a technology exchange agreement between Newcrest Mining Ltd and Codelco-Chile. The authors would also wish to thank the Codelco-IM2’s engineers for their contribution and support these trials.
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