5th International Conference and Exhibition on Mass Mining, Luleå Sweden 9-11 June 2008

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Preconditioning implementation on rock bulks in Codelco Chile and its results

R. E. Molina Civil Mining Engineer, CODELCO CHILE GCPMS

C. B. Cerrutti Civil Mining Engineer, CODELCO CHILE Subsidiary IM2

J. O. Henriquez Civil Mining Engineer, CODELCO CHILE GCPMS

R. F. Morales Civil Mining Engineer, CODELCO CHILE El Teniente Division

R. A. Apablaza Civil Mining Engineer, CODELCO CHILE Andina Division

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Preconditioning implementation on rock bulks in Codelco Chile and its results

R. E. Molina Civil Mining Engineer, CODELCO CHILE GCPMS

C. B. Cerrutti Civil Mining Engineer, CODELCO CHILE Subsidiary IM2

J. O. Henriquez Civil Mining Engineer, CODELCO CHILE GCPMS

R. F. Morales Civil Mining Engineer, CODELCO CHILE El Teniente Division

R. A. Apablaza Civil Mining Engineer, CODELCO CHILE Andina Division

Summary The exploitation of deeper and mass ore bodies, results in having to operate complex rocks, exposed to higher stress, increasing its difficulties regarding the caving, fragmentation and in some particular cases of relevant seismicity. Codelco has been researching for alternatives that allow to improve the underground exploitation of own resources. Codelco has developed several Preconditioning tests (PC) on bulk rock, through hydraulic fracturing technology and confined blasting and through a combination of both techniques

The current paper summarizes several experiences of rock mass preconditioning carried out on the three underground mines of Codelco Chile, describing the characteristics of the sectors of implementation, their designs and the results obtained. By way of conclusion, the learning method of the technology for the capitalization of the benefits of the applications with the Block/Panel Caving exploitation method it is showed as key.

1 Introduction In hard rock underground mining, the most low-cost method is “Block / Panel caving”, see figure 1, that use gravitational caving and to extract the ore from lower level.

Caving method has been used in mainly Codelco underground mines that because the ore bodies were placed in fractured rock, a favorable condition for this method. However, nowadays the ore rocks characteristics are few fractured, sealed fractures and hard filling so the caving method don’t have the same performance.

Figure 1 Block caving method isometric

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Figure 2 Codelco underground production plan profile

The Codelco plans considers a increasing of underground mining production, since 200 Ktpd in actual productive sectors, until productions that could be triple during the next 20 years (see Figure 2), who are placed on deeper ore bodies, rock mass very competent, high stress and lower grades of cupper. Conditions more complicate to use the conventional caving methods.

To improve the exploitation outcomes with more strict conditions, several innovative processes are being carried out to have new technologies to be considered in future projects so to maintain or improve the current productivity levels and costs of the current method. One of these development lines is the rock bulk preconditioning technology (PC).

The main objective for the development of the PC technology has been to build a technically efficient process that allows trying in situ and before the exploitation, the hard rock mass, to transform them into a material with a better caving, fragmentation, seismic response with high standards of safety and productivity within environments of high stress, hardness, as well as with competitive operational costs.

2 Description of the Techniques The development technological alternatives to carry out the PC are: Preconditioning with Hydraulic Fracturing (PCHF) and Preconditioning with Explosives (PCE), and a combination of both; this is, to produce a first fragmentation using HF and right after, to carry out a blasting process.

2.1 PC with Hydraulic Fracturing The development of this innovative line has included adapting the hydraulic fracturing technique, used from a long time within petroleum industry, to the conditions and requirements of the copper underground mining.

HF is to pressurize a segment of a drilling, to inject fluid through pressure, usually water, to start a tension fracture on intact walls, or to extend, or open a pre-existing fracture, and that way, to propagate it to inside the rock bulk, see figure 3.

PCHF is a feasible technology to be implemented on hard rock, like first stages to the exploitation. Facing the caving start conditions, as well as, the active exploitation front conditions, and, the conditions of higher stress in-situ, as it was proven in El Teniente. The technique development is available to be used in up and down welling, which will depend on the availability of drift, dimensions and accesses to the area, from which the drilling and operations of hydraulic fracturing will be carried out. Other aspects to be considered are the availability to connect the required supplies (electric, pneumatic, hydraulic) and their interference to the mining operations.

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Figure 3 Hydraulic Fracturing

In theory, the fractures produced are developed heading to the σ1 σ2 plane, see figure 4. The rate and time of pressure parameters is the key to determine the fracture reach, and according to that information, the design of the implementation is established.

Figure 4 Orientation regarding tensional aspect

The up welling operating technological implementation has been carried out up to the height of 90 m 3¾” Hole, and in down welling operation up to 140 m depth. Both cases developed with fracture spacing of 1.5 m and fracture reach of 40 m in average

2.2 PC with Explosives Differently from the PCHF, referent to technology, this concept is supported with wide use of explosive in mining industry, even though in this case, in confined application, without free face. This development line is been studying by Codelco regarding its theorist frame and development of technique for industrial application.

The PCE is based on the confined and precision blasting,, see figure 5. This development has been possible through the use of high precision electronic detonator, a technology that has allowed a substantial decrease of dispersion of the range delays to a few milliseconds and a limited amount of delayed intervals.

The methodology consists in using an appropriate characterization of the rock mass and the explosives to settle a confined blasting design, assuring the rupture effect of the desired portion of rock mass volume with controlled damage on surrounding.

The development of the technology is available to be implemented on hard rock, during stages previous to the exploitation, as it was proven during the tests carried out in Andina and El Salvador Divisions. The operation has been materialized with the Corporation own technical resources with the assistance of

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consultants in drilling and blasting. The implementation of PCE has been developed in holes 5¾” diameter, with up welling and down welling form, with a scope of 100 m height. This value is currently delimited by the characteristic and density of the column of the explosive.

Figure 5 Confined blasting principle

2.3 Mixed The Combined or Mixed Preconditioning concept is to use both technologies, PCHF and PCE. The hypothesis considered to blasting in an environment previously fractured with PCHF that would have two fundamental effects:

• The lack of continuity generated from the PCHF would work as reflective surface for the waves’ field generated from the detonation of explosives loads; as result, these fronts of waves would be “trapped” among the fractures of the PCHF to increase its possible new fractures.

• The waves coupling possibility, which have been already conditioned with PCHF out of the area is minimized, decreasing the volume of the bulk that could experiment a relevant seismic event, post blasting.

Nowadays, this hypothesis is in verification stage.

3 Implementation Sectors The PC technological development started in the year 1999 in Division Andina and it was a challenging idea to incorporate news technologies to change hard rock underground mining. The concept was intervened the rock mass before its exploitation to improved it more easily for gravitational caving method.

In its initial stage, PCHF technology was considered. Nevertheless, the in-situ conditions in test sector weren’t appropriate for generate sub-horizontal fractures with PCHF. That originated a research and development program in confined blasting (without free faces), at large scale.

Figure 6 shows the experimental sector location, which was in 7000 m2 area, with around 2.18 Mt of mineral, 100 m of block caving height. Because its being a panel border area, it was a fitted sector on two of its faces that was an unfavorable caving condition. For more details see appendix.

The objectives of this first stage were:

• Development the technology and procedures necessary to materialize the implementation.

• To ensure the primary fitted rock caving.

• To reduce the size fragmentation expected.

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Figure 6 Experimental area in Andina Division

Table 1 Main background information

Nº Well 19 Explosives amount 29500 Kg. Well diameter 5 ½” Load length 85 meters Explosive RS-95 emulsion Well length 100 - 112 m. Density 1.15 gr/cm3 Area 7000 m2 Ignition system Electronic detonators Blasting time 26 ms

Figure 7 Fragmentation measurements with PC and prediction without PC

Main results:

• Fragmentation size reduction 50% of P80, figure 7 shows the measurement carried out on four drifts with PCE and the expected prediction without PCE,

• Reduction of hydraulic radio for Caving 35%, referent to empirically estimated.

During the year 2001, another experimental stage was started in El Salvador Division, the objective was to launch the PCHF technique. And parallel, looking an additional benefit, the combined PC with both

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technologies, PCHF and PCE, called mixed PC, was tested, the test purpose was reduced the mass blasting to modular blasting.

The working was divided into two areas; one for base reference without PC with natural caving and other with PC, in 10.000 m2 area and located on sector ICO central, 2600 level, see figure 8. The primary column height was about 100 m and 2.5 Mton of mineral. The objectives of this stage were:

• Developed the PCHF implementation on primary rock.

• To had a comparison base reference without PC.

• Evaluated operational parameters of method.

• Evaluated PCHF effect, either mixed or not (PCHF+PCE).

For more details see appendix.

Figure 8 Experimental area in El Salvador Division

Table 2 Main background information

Nº Well 9 Load length 80 metros Well diameter HQ 96mm Well length 100 m Fractures Radio 40 m Blasting strategy In Module Separation between fractures 1,5 m Nº of modules 4 Ignition system Electronic detonators

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Main results

• Fragmentation Size, informed at 30% of experimental block extraction was 45% less regard to P80, see figure 9.

• Decreased secondary reduction, 51% less events regard area without PC.

• Hang-ups, how happen below 5 meters were similar behavior both with and without PC. Between 5 to 12 meters was reduced the hang-ups events in 84%. Over 12 meters there weren’t hang-ups in PC block.

• Damage rate, measured in the draw points. In PC area there was only first frame break, in reference area was break up to second frame and Shotcrete release. The draw points affected by break were 44% less in PC area.

Figure 9 Fragmentation measurements with PC and base line without PC

In 2004, a new experience was started in El Teniente Division. The goal was validated PCHF technology, in higher stress and higher seismic risk rock mass geomechanical conditions that are into underground mines of the Corporation. The objectives of this stage were:

• To assess the effect of PC in the intact caving sector (first caving with upper crater connection).

• To assess the seismicity behavior and size fragmentation.

The testing was in 10.200 m2 of Diablo Regimiento Mine, figure 10, located on the southern of ore body, underneath El Teniente 4 Regimiento Mine and similar level than Esmeralda Mine; with 3.87 Mton and 150 m. solid height block. For more details see appendix.

Figure 10 Experimental areas in El Teniente Division

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Table 3 Main background information

Nº Well 6 Nº fracture 446 Well inclination 54º Time per fracture 20 min Well diameter HQ 96mm Pressures: Fractures Radio 40 m Packer 32 Mpa Separation between fractures 1,5 m Break 23 Mpa Propagation 18 Mpa

Main results

• Seismicity: A base reference Gutenberg-Richter curve was established based on similar sectors of El Teniente. The result with PC was successful, no relevant magnitude seismic events occurred, either rock burst (seismic and associated damage).

• Caving: Connection to the upper crater time was evaluated. The methodology was based on seismic; subsidence and production information. Figure 12 shows an example of effect with and without PC. The results showed that the connection occurred after 10 months following the extraction beginning, a 57% less to reference case.

Figure 11 Seismic frequency and magnitude graph

Figure 12 Estimated limits of the bounding, after 10 months of operation, with PC connected and without PC in Caving

• Fragmentation: there weren’t variation referent to fragmentation prediction.

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In the year 2004, due first experiences results measured that showed the PC potential benefits, Division Andina continued to technological development up for standardization and optimization of mixed PC technique focused to put it at industrial scale use.

The test sector was the H area, Norwest to III Panel underground Río Blanco Mine, in 22.000 m2, 2.4 Mton of rock and 40 to 130 m of block height, see figure 13. For more details see appendix.

Figure 13 second experimental area in Andina Division

The objectives of this stage were:

• Developed standardization, instrumentation and operational practices.

• Evaluated the mixed PC behavior, Size fragmentation, productions rate.

Table 4 Main background information

Nº Well 9 Explosive 29.500 Kilos (49 wells) Well diameter HQ 96mm Charge length 20 to 100 m Fractures Radio 40 m Well length 40 to 130 m Separation between fractures 1,5 m Blasting strategy In Module Nº fractures 262 Nº of modules 10 Ignition system Electronic detonators

Main results

• Size fragmentation was 30% P80reduction, regarding the prediction value without PC (D80(LB)=2.10m; D80(PA)=1.30 m), see figure 15.

• Decrease secondary reduction 26% regarding the zone PC 2001. See figure 14..

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Figure 14 Reduction rate

Figure 15 Fragmentation measurement in the sector with PC and base line of prediction without PC

4 Result of implementations Even though this innovative technological project is being developed and experimented yet, its current stage is enough to use it technically with efficiency and safety in the current underground exploitation of the Corporation.

The benefits are, on one hand, direct, based on results achieved according to each validation protocols of tests carried out and, on the other hand, analysis and interpretation of results that are being carried out to define parameters and design criteria that can be transferred to projects as potential technical and economical improvements of the method.

4.1 Achieved benefits The results had been evaluated according to immediate effect the PC operation and the impact measured during extraction ore process.

Immediate effects:

• The PCHF do not cause instability on mining infrastructure and, each fracture development doesn’t cause risky seismic response.

• The PCE operation produces few local damages on the undercut level, sort spalling of minor thickness, mainly on drift roof and on collars well blasting. The production level had minor effects,

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being similar to draw bells blasting damager. Vibrations monitoring, in near and far field showed an expected level.

During ore extraction was detected that the PC modifies rock mass in situ resistance to material with higher feasibility for caving methods and without adverse effects. Mains were:

• Inducted seismicity: The PCHF behavior showed an increase on frequency of events low magnitude and significative decrease of the events of a magnitude that causes damage. The interpretation of this is that the energy is dissipated into a higher amount of events. The consequence is seismic risk decrease during exploiting hard rock.

• Caving: in PC sectors a reduction area for caving was achieved, according to results obtained in Andina and El Salvador tests, both implemented in recessed areas.

• Caving connection time: It is highly reduced the connection time to crater using PCHF in new caving sectors. That was showed on El Teniente test.

• Fragmentation and Operational rates: Decreases the over size, there was lower hang-ups and less secondary reduction. The reduction has been obtained in El Salvador and Andina tests, with mixed PC and it has not been obtained with PCHF tests.

4.2 Benefits to be evaluated A way to capitalize the technological development benefits is incorporating improvements on design, planning and operations activity of the mines. For this, new parameters have been defined and they can be used when implementing PC, to validate and transfer the improvements to the exploitation method. These are the following:

4.2.1 Extraction speed

The PC effect is for the first extraction state, during the in-situ rock column breakage, where a favorable scenario is visualized because the PC increases the extraction/breakage relation. See figure 16.

The geomechanical restriction to keep in-situ rock column breakage safe without PC, is restricted extraction speeds for the first 30% of extraction, with PC it has been demonstrated that only the first 20% is reduced. During his first 20%, as well, the extraction speed can be faster without a risky response during the exploitation.

There are yet some gaps to know the maximum limit that can be achieved with PC. What was already proven is that the maximum production capacity of the materials handling method, which during the column breakage stage, there wasn’t high level risk response of the rock mass.

4.2.2 Area and use availability

There are direct relation between the active area and the open area through the availability and use factors of production area. So, when improving, (increasing) these factors, the size of the open area is decreased and because of that, the Active Volume so it contributes to improve the geotechnical condition of it.

In quality terms, the availability and use of area must be increased depending the availability of the draw points, particularly regarding the fluidness in the draw points, less secondary reduction and draw bells deceiling activity, less repairing or damage on the levels.

4.2.3 Extraction and breakage angle

The breakage angle varies due to the effect of the rock mass with PC behavior regarding the relation height of extraction/ breakage height. Was defined the relation 1:5 for rock mass with PC. Without PC this relation is 1:3, Figure 16 shows this effect. This is regarding the characteristics of the rock in El Teniente ore bed.

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Figure 16 Break extraction relations

The extraction angle is a planning definition that must ensure the caving propagation maintaining a broken material column close to the caving front, avoiding greater gaps that could cause air blast. This parameter can be improved according to the break angle due to the use of PC, see figure 17.

Figure 17 Extraction angle and breakage angle relation with and without PC

4.2.4 Hydraulic radio

This parameter was indirectly evaluated through seismic tomography and empiric methodology 0. This with other parameters like a better caving, breakage angle, etc. allow knowing the hydraulic ratio requirements in advance for the caving must be less, however, not enough data is available to quantify this effect.

5 Technology development status With the favorable results obtained during the experimental phase of the PC carried out up to date, the Corporation started a second phase of technological development called “Improvement in the knowledge of the PC technology”. The main scope of this phase is to acquire a better knowledge to validate, industrially, consolidate and deploy the PC implementation. The development will be carried out in the current sectors of the Corporation being exploited, with Technologies of PCHF, PCE and mixed PC, to improve the current design as well as to asses its impact, so they can be incorporated to future projects, to the mining design and planning of the exploitation of the primary ore. The development lines are:

• Improvement of the technique knowledge

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• Improvement of the technique

• Evaluation of the impact of the technique

• Technological transference

6 Conclusions This technology development has been focused on PC implementation, its benefits have been proven and it has been discovered great potential benefits to face difficult geomechanical environment in which the geological resources of the Corporation are located.

This enforces the necessity of continuing improving the knowledge and analysis so the PC impact are properly quantify within the mining industry, to convert its benefits into concrete actions, during the stages of mining design (lay-out, draw point spacing, undercut and caving methodologies, supports, material handling system), mining planning (caving sequences, area incorporation and extraction policies) as well as, unitary mining operations.

Acknowledgement The authors would like to thank to the Underground Mining Project Corporate Management of Codelco, to the Mining Resources and Development Managements of the Mines in El Teniente, Andina and Salvador Divisions, and to the Institute of Mining and Metallurgic Innovation IM2 of Codelco, for the opportunity to publish this article. It is also recognize the participation of all professionals of the previously mentioned organization who have contributed working for the technology development and to issue this report.

References API M07DE12 “Profundización en el Conocimiento de la Tecnología de Preacondiciona-miento” (“A better Knowledge

of the Pre-conditioning Technology”). Fidel Báez, Rodrigo De Nicola, et. al. 2006 Final report IM2 Corporate Project Underground Mining, Phase I “Evaluación del Estado Actual del Desarrollo y

Aplicación Tecnología de Pre Acondicionamiento” (“Evaluation of the Current Status of the Development and Implementation of Pre-conditioning Technology”). Jaime Henríquez, Carlo Cerrutti, at. al. Diciembre 2006

Project IM2 IC 14/04 Salvador, final report, “Evaluación de Prueba de Preacondicio-namiento En Roca Primaria De División Salvador” (“Evaluation of the Pre-conditioning Test on Primary Rock of Salvador Division”), Álvaro Zamora, Carlo Cerrutti Abril 2006

Project IM2 IC 16/04 Andina, report of results, “Estandarización y Optimización de la Tecnología de Preacondicionamiento de Roca Primaria para Uso a Escala Industrial en Métodos de Explotación por Hundimiento de División Andina” (“Standardization and Optimization of the Primary Rock Pre-conditioning Technology for its use on Industrial Scale in Methods of Exploitation through Caving in Andina Division”), Álvaro Zamora, Carlo Cerrutti Abril 2006

Project IM2 18/99 - 071/01 Andina, Project Final Report “Estudio de Metodología de Acondicionamiento de Macizo Rocoso para Hundimiento” (“Study of the Methodology of Conditioning of Rock Bulk for Caving”), Enrique Chacón, Víctor Apablaza, Luís Quiñones, June 2003.

SIT-010-2006, report of results “Validación de la Tecnología de Preacondiciona-miento en Roca Primaria con Fracturamiento Hidráulico En División El Teniente” (“Validation of the Pre-conditioning Technology in Primary Rock with Hydraulic Fracturing in El Teniente Division”), Ricardo Morales, Rigoberto Molina, Raúl Espinoza, Carlo Cerrutti, Noviembre 2006.

Report SGM- I-021-2006 “Conexión Diablo Regimiento – Teniente 4” (“Connection Diablo Regimiento – Teniente 4”) Ricardo Parraguez , Eduardo Rojas, Hugo Constanzo, August 2006

Report IM2 “Potencial de Valor Tecnología de Preacondicionamiento” (“Potential Value of the Pre-conditioning Technology”), Jaime Henríquez, Carlo Cerrutti, January 2007

International Caving Study, “Block Caving Geomechanics”, E. T. Brown, 2003

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Appendix Mains Parameters

Andina 1999 Salvador 2001 El Teniente 2004 Andina 2005 Cantidad

Área 7.000 [m2] 10.000 [m2] 10.200 [m2] 20.000 [m2]

Tonelaje 2,0 [Mton] 2,52 [Mton] 3,87 [Mton] 2,40 [Mton]

Altura Primario 130 [m] 100 [m] 140 [m] 40 a 130 [m]

Módulo Minero

Malla 13x13 / 13x15 / 13x17 225 [m2] 17 x 20 [m] 13x13 / 13x15 /

13x17

Xc Calle 4 x 3,6 [m] 4,3x3,8 [m] 4,5x4,5 [m] 4 x 3,6

Xc Zanja 3,6 x 3,6 [m] 4,0x3,5 [m] 4,5x4,5 [m] 3,6 x 3,6

UCL 3,6x3,6 [m] 3,0x3,0 [m] 4,0x4,0 [m] 3,6x3,6

Litología

Granodiorita, Brecha

magmática, Andesita

Andesita, PórfidosAndesita Primaria, Brecha Turmalina

Hidrotermal

Granodiorita, Brecha

magmática, Andesita

Geológicos Geotec.

FF/m 3 a 5 1 a 4 3 a 5

RMR 70 a 80 70 a 80 55 a 58 70 a 80

GSI 60 a 80 70 a 85 60 a 80

UCS 167 a 187 83 a 226 100 a 120 167 a 187

M Young 50 a 70 40 45 a 60 50 a 70

Velocidad de onda

Vp / Vs [m/s] 5.600 / 3.500 5.700 / 3.450 5.980 / 3.480 5.600 / 3.500

Esfuerzos insitu

σ1 / σ/ 2 σ3 28,6 / 22,8 / 17 51 / 33 / 23