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Mining TechnologyTransactions of the Institutions of Mining and Metallurgy: Section A
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Evaluation of intensive preconditioning in blockand panel caving – Part I, quantifying the effect onintact rock
Alex Catalan, Italo Onederra & Gideon Chitombo
To cite this article: Alex Catalan, Italo Onederra & Gideon Chitombo (2017): Evaluation ofintensive preconditioning in block and panel caving – Part I, quantifying the effect on intact rock,Mining Technology, DOI: 10.1080/14749009.2017.1300724
To link to this article: http://dx.doi.org/10.1080/14749009.2017.1300724
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Mining Technology, 2017http://dx.doi.org/10.1080/14749009.2017.1300724
KEYWORDSintensive preconditioning; hydraulic fracturing; preconditioning by blasting; block and panel caving; acoustic emission; rock damage thresholds; blast induced damage
ARTICLE HISTORYReceived 20 July 2016 Revised 21 February 2017 Accepted 21 February 2017
© 2017 institute of Materials, Minerals and Mining and The AusiMM Published by Taylor & Francis on behalf of the institute and The AusiMM
CONTACT italo onederra [email protected]
Evaluation of intensive preconditioning in block and panel caving – Part I, quantifying the effect on intact rock
Alex Catalana, Italo Onederrab and Gideon Chitomboc
achuquicamata Underground Project, coDelco-chile, calama, chile; bSchool of Mechanical and Mining engineering, The University of Queensland, Brisbane, Australia; cWh Bryan Mining & geology Research centre, Sustainable Minerals institute, The University of Queensland, Brisbane, Australia
ABSTRACTA combination of preconditioning by hydraulic fracturing and confined blasting referred to as intensive preconditioning has been implemented for the first time at a major panel caving operation. The reasoning behind this application was the potential to alter rock mass behaviour in a manner that could improve caving mechanics and overall extraction performance. A comprehensive sampling and testing campaign was conducted before and after the application of large scale intensive preconditioning. Intact rock data was analysed with the combined use of uniaxial compressive strength tests and acoustic emission monitoring. The aim was to define damage thresholds of intact rock and study specific boundaries associated with different degrees of preconditioning. To evaluate the intact rock failure process, four stages or damage were studied, they included crack initiation stress, crack damage stress, uniaxial peak strength and cumulative count of Acoustic Emission (AE) events. Results from this analysis indicated that between in situ and post hydraulic fracturing preconditioning, the initiation of damage occurs almost at the same stress levels, which as expected, demonstrated that hydraulic fracturing does not affect intact rock strength. However, after preconditioning by confined blasting, results indicated that the on-set of excessive damage prior to failure was occurring at lower axial stresses, this confirmed the presence of micro-fractures in the samples which may be attributed to confined blasting induced stresses.
1. Introduction
The term preconditioning has since been adopted within the caving mining industry to describe the process of ‘modifying’ or ‘engineering’ a rock mass to enable bet-ter control or management of the cave mining process. These terms are used in this context to mean the process of artificially inducing changes to the rock mass through either hydraulic fracturing or large scale confined blast-ing; which involve treating/modifying the condition of the rock mass by fluid injection or the simultaneous detonation of long and fully confined explosive charges (e.g.+100 m long charges).
Preconditioning by hydraulic fracturing (i.e. hydro-fracturing) is generally expected to affect the overall rock mass behaviour through the introduction of additional large scale fractures (treated as an additional joint set or sets). On the other hand, preconditioning by drilling and blasting (i.e. confined blasting) is likely to damage intact rock; as well as extend existing dis-continuities and thus have a marked impact on the in situ mechanical rock material and mass properties (e.g. strength and stiffness).
The intensive preconditioning concept is a combi-nation of hydraulic fracturing (in down-holes) and confined blasting (in up-holes). Both preconditioning methods are applied before the initiation of the cave and the intent is to ‘modify’ a considerable volume of the mining block. Figure 1 is an illustration of the inten-sive preconditioning concept first reported by Catalan et al. 2012. After approximately three years of data col-lection and analysis, this paper presents the results of a comprehensive evaluation of intensive preconditioning, which in this first part, focusses on its impact on intact rock. It is envisaged that the knowledge gained can be used in the planning of future block and panel caving operations.
2. The Cadia East mining complex
The Cadia East ore body is located in Cadia Valley approximately 25 km south east of Orange in New South Wales, Australia. It is 100% owned by Newcrest Mining Ltd. The Cadia East ore body was discovered in 1985 and studies into the viability of the Cadia East resource
2 A. CATALAN ET AL.
commenced in the early 1990’s (Malone 2011). 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 200 m of overburden. The system is up to 600 m wide and extends to 1.9 km below the surface.
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.
The proposal is to mine the Cadia East mining com-plex using a series of panel caves and these have been 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).
2.1. Intensive preconditioning implementation at cadia east
Preconditioning by hydraulic fracturing at PC1-S1 was completed in December 2011. Twenty-one drillholes were drilled and 1,182 hydraulic fractures were created; of these, 761 hydraulic fractures were created at 1·5 m spacing between 350 and 200 m depth below the bore-hole collar; and 421 hydraulic fractures were created at 2·5 m spacing between 200 and 50 m underneath the collar.
The preconditioning by blasting program started in October 2011 and continued until the later part of 2013. The program primarily focused on the area where the cave was initiated. After that the program was extended to the whole footprint area. Figure 3 shows a general illustration of the intensive preconditioning program undertaken at Cadia East Lift 1.
3. Intact rock strength evaluation
The evaluation of the implemented intensive precon-ditioning program has included analysis that seeks to understand the effect of different stages of precondi-tioning on itact rock strength and subsequently on rock mass strength and overall caving performance. This part deals with an evaluation of intact rock strength, the pro-cess was carried out by collecting intact rock samples before hydraulic fracturing (i.e. in situ condition); after hydraulic fracturing or before confined blasting; and after confined blasting (i.e. after intensive precondition-ing). Figure 4 shows the location of samples taken from the testing area. The testing program consisted of nine (HQ – 96 mm) diamond drilling down holes to lengths of between 325 and 350 m. Diamond core samples of about 63·6 mm in diameter and lengths of 30–75 cm were selected. A total of 125 samples were obtained and analysed. In Figure 4, the block and treatment zones are shown as shaded areas, the rings illustrate the relative location of samples with respect to each treatment area.
Figure 1. Sequence to implement intensive Preconditioning (catalan et al. 2012).
MINING TECHNOLOGY 3
3.1. Uniaxial compressive strength evaluation
The uniaxial compressive strength of intact rock is one of the most common geotechnical tests performed to determine peak rock strength. The test corresponds to the axial stress that produces the failure of a cylindrical intact rock specimen under axisymmetric loading and a confining stress equal to atmospheric pressure (i.e. with-out confinement). It is used to determine the uniaxial or unconfined compressive strength (UCS or σc) and the elastic constants, Young’s modulus, E, and Poisson’s ratio, ν, of the rock material (Ulusay & Hudson 2007).
Analysis of UCS data for rock samples collected at different preconditioning stages are given in Table 1 and Figure 5. The tests were performed by a commercial laboratory facility and the raw data made available and analysed by the authors.
Results from this analysis show little to no impact on intact rock strength measured by UCS through each preconditioning phase. As shown by the green curve, the normally distributed data shows a slight difference in strength (e.g. weakening effect) after the combined effect of hydraulic fracturing and confined blasting (i.e. after intensive preconditioning) with a mean of 131 MPa vs. 138 MPa. However, from a practical perspective the difference is insignificant. It is fair to say from this set of UCS data that the description of the rock failure process using only conventional stress and strain measurements may not be sufficient in the study of preconditioning. This is because the crack initiation and crack damage stages during uniaxial compression can be complex and difficult to detect. These complex mechanisms are important in the process of fracture formation, cave initiation and cave propagation. It was therefore con-cluded that peak strength in the form of UCS was not an
appropriate indicator to study and quantify the poten-tial effects of intensive preconditioning. For this reason Acoustic Emission was favoured and further incorpo-rated in the analysis process.
3.2. Intact rock strength analysis using acoustic emission (AE)
As outlined by Brady and Brown (1993), a result from a uniaxial compression test on rock generally shows three phases before the peak strength is reached. The first phase is a crack closure process, followed by a sec-ond phase of elastic deformation until an axial stress is reached (i.e. crack initiation, σci). This is when stable crack propagation is ongoing (σcd). Subsequently, the axial stress reaches a point of unstable crack growth and irrecoverable deformations commence. This pro-cess continues until the peak or uniaxial compressive strength is reached (σc).
Acoustic emission (AE) monitoring provides a pow-erful tool for investigating brittle rock failure, because the failure process can be associated with acoustic emissions (AE). This technique has been widely used in rock mechanic studies and engineering applications because the initiation and damage process of a piece of rock can be identified (Ohnaka 1983; Mansurov 1994; Cai et al. 2004, 2007; Manthei & Eisenblätter 2008; Cheon et al. 2011; Xie et al. 2011). By correlating AE signals such as a graph of the count of events and energy rates; or a graph of cumulative count events and energy with the stress-strain diagram, one is able to estimate the starting point of micro fractures dur-ing a uniaxial compression test. Studies have showed that AE has enough accuracy to monitor the crack
Figure 2. Section through the cadia east project (catalan et. al. 2012).
4 A. CATALAN ET AL.
strength (σc); and the initiation of failure by sliding along a macro crack typically occurs at stress levels above 0·7 and 0·8 σc.
Cai et al. 2004 proposed stress thresholds for crack initiation and crack damage of brittle rock which were obtained from uniaxial and triaxial compression tests using AE. In uniaxial compression, the crack initiation stress level for most rocks have been located in the range of 0·3–0·5 times the peak strength (σc). Subsequently, a fracture combination commences at stress levels of approximately 0·7–0·8 σc (i.e. the long-term uniaxial strength of intact rock). Finally, macro-cracks result after the peak strength is reached.
Additional work was carried out by Zhao et al. 2013 in order to characterise stress damage through acoustic
behaviour and it can be used confidently (Boukharov et al. 1995; Rudajev et al. 2000; Moradian et al. 2010; Alker et al. 2014).
Martin (1997) determined three stress levels which represent important stages for the behaviour of intact rock strength in the development of the macroscopic failure process:
• crack initiation (σci);• initiation of sliding (σcd) or long-term peak
strength; and• maximum stress which for the unconfined case is
the peak strength (σc).
Laboratory results showed that the initiation of crack growth occurs between 0·3 and 0·4 times the peak
Figure 3. intensive preconditioning application at the cadia east lift 1 panel cave (catalan et al. 2012).note that green circles show the hydraulic fracturing treatment zones.
Figure 4. location of intact rock samples used for preconditioning assessment.
MINING TECHNOLOGY 5
3.3. Evaluation of intact rock damage from intensive preconditioning
As mentioned earlier, intact rock strength and stress thresholds for crack initiation and crack damage of brittle rock were assessed using core samples collected during the intensive preconditioning drilling program, before and after all stages of preconditioning. Information such as crack initiation stress, crack damage stress, uniaxial peak strength and cumulative count of AE events were obtained from laboratory tests. It involved the definition of micro damage stress thresholds of intact rock from uniaxial com-pression strength and acoustic emission monitoring.
A detailed analysis of the intact rock failure pro-cess was conducted, identifying four stages or damage thresholds, including:
(1) σid: crack initiation damage stress, that is, the stage of elastic deformation until an axial stress is reached and AE events have been initiated,
(2) σpd: crack propagation damage stress, when stable crack propagation is initiated and AE events increase slightly,
(3) σmd: crack massive damage stress, when unsta-ble crack growth and AE events are intensified (onset of failure).
(4) σc: peak or uniaxial compressive strength is reached and characterised by rapidly increas-ing AE events (failure of sample),
emission. The crack initiation stress (σci) and crack dam-age stress (i.e. long-term strength σcd) during uniaxial compression occurred at an average stress level of 0·5 σc and 0·78 σc respectively.
Based on the analyses of laboratory tests on intact rocks using AE monitoring data, crack initiation and damage initiation stress thresholds have been proposed as a way to study and subsequently predict the start of intact rock damage. Table 2 summarises these findings in terms of these thresholds for crack initiation and damage stress respectively.
Figure 6 shows the relationship between crack initi-ation, σci, and crack damage, σcd, stress limits, normal-ised by uniaxial compressive strength, σc. This type of relationship can be used to define an envelope which identifies the onset of failure in a brittle intact rock. This approach is explored and discussed in subsequent sec-tions to evaluate the potential effect of preconditioning and the practical definition of damage thresholds.
Table 1. Uniaxial compressive strength test results.
UCSBefore hydrau-
lic fracturingAfter hydraulic
fracturingAfter confined
blastingMin (MPa) 96 90 82Max (MPa) 187 208 209Average (MPa) 138 144 131Median (MPa) 130 143 128Stand. Dev. 26 32 33coef. Var. (%) 19% 22% 25%
Figure 5. Distribution of Uniaxial compressive Strength results during to intensive preconditioning process.
6 A. CATALAN ET AL.
macro fracturing, but also with the potential for micro fractures that initiate through the positive interaction of blast induced stresses (e.g. preconditioning by con-fined blasting). The point is that a comparison between disturbed and undisturbed intact rock cannot be purely based on statistics associated with peak strength. This is the reason why the normalised damage thresholds are used, given that in cave initiation and propagation mechanisms, we are interested in identifying differences in the initiation of fracture and the onset of failure. In this particular investigation, this refers to comparing the condition of a rock material without preconditioning and a rock material that has been preconditioned.
3.4. In situ condition (before hydraulic fracturing and confined blasting)
Before preconditioning by hydraulic-fracturing, five drillholes for a total of 73 samples were prepared and tested, 38 UCS tests with AE events were obtained and confirmed as reliable. A number of tests were discarded because their failure initiated through pre-existing discontinuities. Details on the UCS with AE tests and estimation of the crack initiation damage stress (σid); crack propagation damage stress (σpd); crack massive damage stress (σmd) and peak strength (σc) were ana-lysed and documented. Figure 8 shows results of the analysis describing the relationship between the different damage thresholds defined earlier. The value of σpd/σc at 0·47 is only slightly greater than σid/σc at 0·42. However the value of σmd/σc is quite close to 1 at 0·92.
Figure 7 gives an example of the stress–strain curve associated with AE event characteristics, showing dif-ferent deformation stages of the intact core sample in uniaxial compression, it also shows a relationship between AE counts and axial stress for identifying crack initiation, crack propagation and crack massive damage stresses. Spatial 3D locations of accumulated AE events were used to visualize the gradual formation of cracks.
Once all different thresholds were obtained for each sample, and for each preconditioning stage, box and whisker plots were collated and trends associated with 25th and 75th percentile were defined to establish dam-age thresholds.
The idea behind these normalised damage thresholds was to determine a criterion that defines the degree of disturbance of a rock material and rock mass, similar to those proposed in previous studies of confined blasting (e.g. disturbed zones defined by Onederra et al. 2013). It is important to note that this is different from the adjustment of rock mass or intact rock strength (i.e. in situ condition) to model failure mechanisms. The degree of disturbance in this case is not only associated with
Table 2. Damage stress thresholds from uniaxial compressive strength tests.
ReferenceCrack initiation
stress (σci)Crack damage
stress (σcd) σci / σcd
Martin (1997) (0·30–0·40) σc (0·70–0·80) σc 0·42–0·50cai et al. (2004) (0·38–0·50) σc (0·75–0·80) σc 0·52–0·61Zhao et al. (2013) (0·45–0·50) σc (0·70–0·80) σc 0·60–0·65
Figure 6. empirical relationship between crack initiation stress (σci), crack propagation damage (σcd) and uniaxial compressive strength (σc).
MINING TECHNOLOGY 7
These relationships are used later as the base case for comparing the intact rock strength behaviour and damage boundaries, after preconditioning by hydraulic fracturing and confined blasting, in other words, after intensive preconditioning.
Figure 9 shows the results of individual tests and the relationships between the different damage limits which have been normalised by uniaxial compressive strength σc. As shown, the results are consistent with the numbers published by Cai et al. 2004 and Zhao et al. 2013 in terms of crack initiation and propagation stress thresholds.
Figure 7. example stress–strain curve associated with Ae hit for a rock sample tested in uniaxial compression and 3D locations of accumulated Ae events.
8 A. CATALAN ET AL.
damage stress (σpd); crack massive damage stress (σmd) and uniaxial compressive strength (σc) were also ana-lysed and documented.
Figure 10 shows results of analysis describing the rela-tionship between the different damage thresholds for the condition after preconditioning by hydraulic fractur-ing. The relationship between initiation and propagation damage shows a relative increase. This increase on the
3.5. Results after preconditioning by hydraulic fracturing
After the completion of the hydraulic fracturing tests, a total of 33 samples were taken from the inspection dia-mond drill hole, were prepared and tested. In this case 24 UCS tests with AE events were confirmed as reliable. Details on the UCS with AE tests and estimation of the crack initiation damage stress (σid); crack propagation
Figure 8. Analysis showing the range of normalised damage thresholds for samples before preconditioning by hydraulic fracturing (in situ case).
Figure 9. Relationship between normalised damage thresholds for samples before preconditioning by hydraulic fracturing.
MINING TECHNOLOGY 9
defined earlier (base case) are also shown for compar-ative purposes. As can be seen on this figure, there is a clear shift in the behaviour of samples under uniaxial stress. The normalised damage stress threshold bound-aries between σpd/σc and σid/σc are clearly above the in situ condition (i.e. blue line). On the other hand, the normalised damage stress threshold between σmd/σc and σid/σc is below the in situ condition (i.e. red line). This gives an indication that the crack initiation and crack propagation stages during uniaxial compression has commenced earlier.
σpd/σc ratio may be attributed to an early release of strain energy during the preconditioning process which may delay the AE events. Regarding crack massive damage stress, there is not clear evidence that the intact rock strength has been reduced and crack massive dam-age stress is initiated close to the uniaxial compressive strength, i.e. 0·93 times σc.
Figure 11 shows the distribution of these results and their relationship between the different normal-ised damage limits after preconditioning by hydraulic fracturing. Note that the in situ condition envelopes
Figure 10. Analysis showing the range of normalised damage thresholds for samples after preconditioning by hydraulic fracturing.
Figure 11. Relationship between normalised damage thresholds for samples after preconditioning by hydraulic fracturing.
10 A. CATALAN ET AL.
3.6. Results after preconditioning by confined blasting (i.e. after intensive preconditioning)
After the completion of preconditioning by confined blasting, three diamond drillholes were sampled with a total of 61 samples prepared and tested. Through this process, 33 UCS tests with AE events were confirmed
Even though a difference is identified between pre and post hydraulic fracturing, it is difficult to quantify with certainty the degree of damage after hydraulic frac-turing. This is not necessarily an unexpected result, as the hydraulic fracturing process would induce an effect at the rock mass scale (i.e. minor and major discontinu-ities) rather than at the intact rock material scale.
Figure 12. Analysis of normalised damage thresholds for samples post preconditioning by confined blasting.
Figure 13. Relationship between normalised damage thresholds for samples after preconditioning by confined blasting.
MINING TECHNOLOGY 11
and thus implying that preconditioning by hydraulic fracturing does not affect intact rock strength. This is not necessarily an unexpected result, as the hydraulic fracturing process is anticipated to have a more signifi-cant impact on existing discontinuities or create macro scale discontinuities, hence its influence is at the rock mass scale rather than the intact rock material scale.
With regards to preconditioning by confined blasting (i.e. after intensive preconditioning), results indicated that the on-set of excessive damage prior to failure was occur-ring at lower axial stresses, supporting the hypothesis that this could be caused by the presence of micro-fractures in the samples, attributed to induced blasting stresses.
From the data available, a summary of all of the calcu-lated thresholds for the different preconditioning stages is given in Table 3. These can be used to define envelopes for the quantification of damage or disturbed zones. It may also be particularly useful as another way of quan-tifying blast induced damage in other mining related applications.
Disclosure statementNo potential conflict of interest was reported by the authors.
Funding This study formed part of the principal author’s PhD thesis supported by the WH Bryan Mining and Geology Research Centre of the Sustainable Minerals Institute at The University of Queensland, Brisbane, Australia; and Newcrest Mining Limited.
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to be reliable. As with the previous analysis, Figure 12 shows the results of analysis describing the relationship between the different damage thresholds for the condi-tion after preconditioning by confined blasting.
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Figure 13 shows the distribution of these results and their relationships between the different normalised damage limits after preconditioning by confined blasting.
The relationship between σpd/σc and σid/σc does not show a significant shift from the in situ condition with regards to crack initiation and propagation damage boundaries, however, initiation damage appears to be showing a convergence to an earlier start. Additionally, there are marked differences between the damage stress boundaries for massive and initiation damage (σmd/σc and σid/σc). The analysis shows that there is a reduction in the peak strength and it can be hypothesised that induced micro fractures from blast induced stresses may have contributed to a change in behaviour which translated in a reduction of intact rock strength.
4. Conclusions
A comprehensive sampling and testing campaign was conducted before and after the application of large scale intensive preconditioning. Intact rock data was analysed with the combined use of uniaxial compressive strength tests and acoustic emission monitoring. The aim was to define damage thresholds of intact rock; and study spe-cific boundaries associated with different degrees of pre-conditioning. The assessment process was undertaken before hydraulic fracturing (i.e. in situ condition); after hydraulic fracturing or before confined blasting and after confined blasting (intensive preconditioning. Analysis showed that between in situ and post hydraulic frac-turing preconditioning the initiation of damage occurs almost at the same stress levels. During damage propaga-tion, the behaviour of samples also remains comparable
Table 3. Summary of damage thresholds for the different preconditioning stages.
Condition stress In situPost hydraulic
fracturingPost confined
blastinginitiation
damage σid
(0·32–0·53) σc (0·36–0·52) σc (0·27–0·41) σc
Propagation damage σpd
(0·38–0·57) σc (0·44–0·67) σc (0·38–0·50) σc
Massive damage σmd
(0·87–0·96) σc (0·78–0·96) σc (0·73–0·89) σc
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