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Article G&T_Gomes 19-02-13

Design Aspects of the Permanent Underground Infra-structure for the Chuquicamata Mine in Chile

Alexandre R.A. Gomes Gustavo Reyes Juan Carlos Ulloa The mining complex Chuquicamata, of the National Copper Corporation of Chile (CODELCO), is located in the Atacama Desert, 1.650 km north of Santiago, and 2.870 m above sea level. Since the large copper ore reserve that lies below the “Chuquicamata” open pit (Figure 1) will no longer be economically feasible to mine from the end of this decade, the mine is currently developing the required infra-structure to switch the operation to an underground mine type, where the block caving method with macro-blocks will be used to mine out copper ore. The future mine will count with four production levels, corresponding to one of the largest underground mining operations in the world, with a production rate of about 140 thousand tons per day. This article presents design aspects of the permanent underground infra-structure works, which encompass several deep and steep tunnels and vertical shafts for the future mine access and operation.

Figure 1. Chuquicamata Open Pit

1 Project Overview The operation of the future underground mine will require both temporary and permanent infra-structure works. The former correspond to those associated with the block caving mining process itself, being transitory and dismantled as the respective mining level is exploited and exhausted, and the latter, to those that shall support the mining operation throughout its time life. The permanent underground infra-structures will include a principal access tunnel, a principal ore transport tunnel and several interconnected ventilation tunnels and shafts, together with other ancillary structures, such as adits, access areas and ramps. The general layout of those infra-structures is presented in Figure 2. The integral mine feasibility study, which included an advanced basic engineering design for the construction tender, was commissioned to Hatch Chile, with Geoconsult as specialist consultant for tunnelling and geotechnical disciplines and Cementation for the deep shafts design. Construction of these infra-structure tunnels have started in early 2012 and the underground mine operation is expected to initiate in 2018. This article presents design aspects associated with the permanent underground infra-structure works, whose relevant technical data is shown in Table 1.

Table 1. Permanent Underground Infrastructure Characteristics

Underground Infra-structure Length or Depth (m)

Cross-section area (m

2)

Longitudinal inclination (%)

Bidirectional traffic access tunnel for personnel, 7.451 49-72 -8.7%

1 km depth

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equipment, supply, waste and machinery

Transport tunnel (for the mineral conveyor belt that transports the ore to the surface)

6.248 43-63 -15%

Five (5) fresh air injection tunnels and five (5) air extraction tunnels

~4.350 73-102 -15%

Deep Ventilation Shafts 970 95 vertical

Several ancillary structures, such as connection/ventilation shafts, cross passages, parking and emergency bays, niches, etc. Total of 844km of tunnels, 175km of chimneys and 2km of shafts

Figure 2. Layout of Permanent Infrastructure Works (Plan and View)

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2 Design Methodology The design was carried out on the basis of a step-by-step comprehensive methodology, as illustrated in Figure 3, which incorporated the Austrian Society for Geomechanics [1] recommendations for the geotechnical design aspects.

Figure 3. Design Methodology

3 Geological Setting The project geological setting consist of plutonic (granites, diorites, granodiorites, tonalites and monzodiorites) and meta-plutonic rocks (amphibolites) of the Chuquicamata complex, as well as their respective hydrothermal alterations (weakly leached and structurally leached rocks). Along the alignment, tunnels are expected to encounter mostly fair to good rock quality. Nevertheless, also some adverse geological conditions are expected, such as in minor faults zones and areas affected by leaching and weathering with different degrees of alteration. Ground water conditions are mostly favourable and no major water inflows are expected, even though the presence of sodium sulphate and other metals in the water, probably derived from adjacent mining chemical processes carried out at the surface, could negatively affect the tunnel durability. The zone is characterized by seismic events of low to medium magnitude; however the region as a whole may be considered as highly seismic with more than 80 events per day. Another relevant factor for the tunnel design and construction is the in-situ stress conditions, with stress anisotropy and low to very high tunnel overburden, reaching up to 1250m below surface.

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4 Geometric and Functional Design The geometric design of tunnels was the result of a compromise among relevant aspects, such as geotechnical-structural behaviour, constructive and operational issues as well as durability and safety requirements. The alignments of the access and ore transport tunnels are mainly governed by the need to maintain longitudinal gradients to acceptable limits, resulting in straight lines from the surface (entrance portals) to the first production level. For the mine traffic access tunnel, due to its hybrid mine/transportation character, specific tailored safety and operational criteria had to be defined. For the ventilation tunnels, alignments were governed mainly by the mining layout. Injection tunnels consist of parallel tubes, starting at the surface and ending at the mining level. Due to the great depth, alignments are subdivided in three sub-horizontal segments interconnected by vertical intermediate shafts, with about 180 m depth. Extraction tunnels are connected to the surface by means of very deep shafts (with about 970m depth), but their inner part shows a layout similar to the injection tunnels, connecting the system to the different mining levels. The tunnel cross sections of the access, ore transport and ventilation tunnels are shown in Figure 4.

Figure 4. Layout a) Access Tunnel; b) Ore Transport Tunnel; c) Air Injection, Air Extraction

5 Geotechnical and Structural Design 5.1 Geotechnical Model and Boundary Conditions The initial activity comprised the analysis of the geotechnical investigation data, obtained from previous investigations and experiences (e.g. open pit mine operation and exploratory galleries) and also carried out along the tunnels´ alignment specifically for the new underground works, in terms of sufficiency, coherence and confidence levels. Field investigations included mapping of both surface and existing underground galleries, geophysical profiles, drilling of boreholes with core sampling, geophysical scanning (e.g. caliper, gamma-ray, sonic probe, resistivity and neutron logs), in situ verification of rock mass parameters (e.g. Point Load Test and permeability), among others. Laboratory testing of selected core samples included Unconfined Compression Strength (UCS), Tensile Strength (TS), Triaxial Test (TX), etc. and water chemical tests, among others. The existence of massive gravel and sterile mine dumps in certain portions of the tunnels´ alignment hindered the execution of drillings, so that conditions had be inferred by extrapolation of geotechnical data at these sections For this project, the existing stress field around and below the open pit was determined by means of a tridimensional numerical model (FLAC3D, Itasca Inc.), Geotechnical Update [2], in regional scale, where the stress-field was calibrated on the basis of several measurements carried out with the “hollow inclusion” and hydraulic fracturing methods, both inside and outside of the influence field of the open pits. The average deviation error between the model and the individual measurements is about 44%. High stress-field anisotropy was identified, where the principal stresses axis is oriented at an East-West direction. The maximum estimated stress ratios (horizontal/vertical) were about 2.0 and 0.81 for the East-West and North-South direction, respectively. Following that, ground types with similar geotechnical characteristics, identified at the influence zone of the permanent underground infra-structures, were grouped in distinct “Basic Geotechnical Units (BGUs)”, as listed in the Table 2.

a) b) c)

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Table 2. Basic Geotechnical Units (BGUs)

Granodiorite (GRD) Anphibolites (ANF)

Silificate Anphibolite (ANF-SIL) Granodiorite Elena Sur (GES)

Weakly Leached Rock (LXD) Structurally Leached Rock (LXE)

BGUs were described both in qualitative (descriptive) and quantitative terms, always considering maximum, minimum and average values for each specific set of key parameters. Each tunnel sector was then characterised with the respective BGUs, considering also a description of eventual singular local characteristics, ground behaviour and key parameters variation. Based on the above, boundary conditions associated to each tunnel section could also be determined for the tunnel and ground behaviour analysis, taking into account geometric features, overburden, particular situations, geotechnical and hydro-geological conditions and stress regime.

5.1 Assessment of Ground Behaviour Generally, the expected ground behaviour is categorized in one of the relevant ground failure modes or as a combination of them. The Austrian Society for Geomechanics [1] proposes 11 general categories with possible sub-categories. For the purpose of the present article, Table 3 shows a simplified list of major sources of opening instability in relation to the main controlling affecting effects, as summarised by Gomes [3]:

Table 3. Main Sources of Opening Instability – Gomes [3]

Instability Sources

Failure mechanisms mainly controlled by

(GD) Gravity and discontinuities (e.g. block and wedge failure, etc.)

(SI) Stress-induced (with/without association with discontinuities) due to stress redistribution or dynamic loads (e.g. buckling, plastic or brittle fractures and creep).

(GW) Effect of groundwater (e.g. flowing ground, water pressure exerting, water inflow, etc.)

(MI) Presence of minerals (e.g. swelling, slaking, etc.), affected by water and environment

(CX) Presence of complex conditions, such as faults, mixed face, stratification, bim-rocks, frequently changing ground, highly disturbed zones, low overburden, etc. (e.g. unstable ground, ravelling, cave-in, etc.)

The actual assessment of the ground behaviour is carried out with the support of engineering tools (e.g. empirical, analytical or numerical methods) and descriptive understanding of the ground failures modes, in the light of savvy engineering judgment. Since geotechnical analyses are intrinsically affected by uncertainty - both in terms of input data and the inherently limitations of engineering tools – results must be treated probabilistically and properly portrayed in the tender documents to allow for flexibility and an adequate risk management during construction. In all cases, the actual tunnel behaviour must be monitored, verified and corroborated on site (observational approach). A list with some approaches considered for the analyses is provided in Table 4:

Table 4. Approaches Applied for the Geotechnical Design of Tunnels

Descriptive Methods Approaches

Assessment of failure modes Geotechnical description and monographs of the relevant ground structural features analysed in conjunction with stress regime, water conditions, presence of minerals, as required.

Empirical Methods

Potential stress-induced (SI) failure modes in the two limits of the rock competency scale

Extent of squeezing in rocks with GSI≤40 (Hoek and Marinos, [4]) and verifications of brittle failure depth (and rock burst potential) in massive to moderately jointed rock with GSI≥70 (Martin et al., [5], [6] and Hoek and Brown, [7]).

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Rock quality and fabric of Jointed Rock

Q’ [8] and RMR [9] indexes were considered in the range GSI=40 to 70, to support engineering judgment in the definition of required support measures.

Analytical Methods

Stress-strain behaviour (SI) and Limit-state analysis

Ground characteristic lines (Carranza–Torres & Fairhurst, [10]), stress distribution around the cavity (Feder, [11]), face and roof stability and others.

Stability analysis controlled by discontinuities and gravity (GD)

Stereographic projection and kinematics analysis of wedges (e.g. software "Unwedge" of Rocscience Inc., based on Goodman and Shi, [12]).

Numerical Analysis

Stress-strain behaviour (SI) under anisotropic loading

conditions (e.g. ground, water pressure and seismic loads)

2D Finite element models (e.g Phase2D V7.0, Rocscience) 3D Boundary element models (e.g. MAP3D, Mine Modelling Pty) and 3D Finite-difference methods (e.g FLAC3D, Itasca Inc.) to assess tunnel sections where 3D stress conditions were relevant, such as in fault zones, tunnel crossings and enlargements.

Based on the above-mentioned approaches, the ground behaviour was assessed at each tunnel section, describing the response of the ground to the full face excavation of openings, without the influence of supports, division of face or auxiliary measures (Goricki et al. [13]). This assessment considered the respective BGU characteristics and properties variation, as well as the specific influencing factors at each tunnel sector, such as the tunnel geometry (shape/size) and orientation, geo-structural characteristics, stress and water conditions as well as adjacent ground conditions. These assessments, together with the geotechnical interpretation of the tunnel alignments, allowed a risk assessment of potential geotechnical hazards, which could affect safety and constructability aspects, and the preparation of a Geotechnical Baseline Report (GBR) to support the tender process.

5.2 Ground-Structure Interaction – System Behaviour For the system behaviour analysis, a preliminary Excavation and Support Classes (ESCs) concept was established on the basis of engineering judgment, experience in similar conditions, ground behaviour analyses and recommendations based on the Q classification system (NGI [8]). These ESCs concepts were considered for the analyses of the supported tunnel behaviour and its interaction with the ground (system behaviour) for different combinations of ground quality (range of expected variation) and applicable ESCs. The applied approaches were fundamentally the same as described earlier in section 5.1, but incorporating the support effect on the modelling. In each analysis, results were compared to pre-specified acceptance criteria for the rock mass (e.g. stress concentration/relaxation, disturbance/damage zones around the excavation and magnitude of deformations and strains), and applied for the dimensioning of the support elements. This interactive process allowed the optimization of the initially proposed ESCs, as to best fit the range of stress-strain conditions and expected ground behaviour. Additionally, the ESCs optimization also took into account the analysis of construction cycles, incorporating further efficiency in the construction logistic and maximizing excavation rates. Extensive probabilistic analyses of construction methodologies and working cycles were carried out with the software @Risk (Palisade Corporation), based on the Monte Carlo Method, together with the assessment of methods for muck handling and use of state-of-the-art construction equipment, delivering a comprehensive and traceable estimation of construction time and cost with the consideration of relevant influencing factors. As a result of that, a complete set of Excavation and Support Classes (ESCs) and a probabilistic forecast of ESCs distribution along the tunnels´ alignment was made for each tunnel, forming the basis for compensation clauses in the tender documents, together with the respective technical and

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administrative specifications and material take-offs. These ESCs were designed to cover the complete range of expected geotechnical conditions, as described in the project construction baseline. Table 5 shows an example of the concepts applied for the Access Tunnel ESCs regarding applicable instability sources and the related excavation and support methods.

Table 5. Excavation and Support Classes (ESCs) Concept – Access Tunnels - Sections (43-49m2)

ESCs Instability Sources Proposed Method of Excavation and Support

GD GW SI CX Excav. Round

Shotcrete (e in cm)

Bolt pattern (each m)

Forepoling Heading

ESC1 ≤5-6m 5 2m

ESC2 ≤5-6m/ 10 1,7m full face

ESC3 ≤4.5m 15 1,5m

ESC4 ≤3.5m 15 1,3m eventual

ESC5/ESC6/ ESC-Portal

≤1,5m Event. Mech. Excav

25-30 1,1m regular Sub- div. / Invert

obs: 1. GW: no relevant water inflow expected, but cannot be excluded in faults or weak zones; MI:

no swelling or slaking mineral is expected. 2. Installed support measures are generally proposed as structures of permanent character,

except in case of exceptional water inflow, where an inner lining installation is foreseen.

Figure 5. Progress in the Construction of the Access Tunnel

6. Conclusions A step-by-step methodology was applied for the design of the permanent underground infra-structure works of the Chuquicamata Mine, aiming at the development of a comprehensive baseline and a traceable framework for both the tender process and the actual construction stage. The design included the development of tailored solutions for the specific functional and operational needs of this future behemoth underground mining operation, and also catered for the particular nature of an underground infra-structure geotechnical design, where geotechnical information is hardly totally complete and accurate and relevant design and construction decisions have to be made in an earlier stage of the project. Hence, a probabilistic construction baseline was developed, together with suitable and flexible excavation methods and permanent support measures, so that tunnelling documentation and monitoring could enable the actual completion of the geotechnical design process during the construction stage.

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7. Acknowledgments The authors would like to express their gratitude to the National Copper Corporation of Chile (CODELCO), and the companies Hatch and Geoconsult Latinoamerica, for the permission to publish this article.

8. References

[1] Austrian Society for Geomechanics. Guideline for the Geomechanical Design of Underground Structures with Conventional Excavation, 2010.

[2] Chuquicamata Underground Project, 2009 Geotechnical Update, 2009, Itasca Denver, Inc.

[3] Gomes, A. R.A., “Geotechnical Design of Tunnels”, Keynote Speaker “Conferencia Internacional de Túneles y Construcción de obras subterráneas”, Elite Training; Lima, Perú, 2009

[4] Hoek, E and Marinos, P. 2000, Predicting tunnel squeezing problems in weak heterogeneous rock masses.

[5] Martin, C.D., Kaiser,P.K., & McCreath,D.R. 1999. Hoek-Brown parameters for predicting the depth of brittle failure around tunnels. Can. Geotech. Jour.

[6] Martin, C.D. and Christiansson, R. 2009, Estimating the potential spalling around a deep nuclear waste repository in crys-talline rock. Accepted for publication in Int. J. Rock Mech. Min. Sci. 46(2), 219-228.

[7] Hoek, E. and Brown, E.T. 1980. Underground excavations in rock. London: Inst. Mining and Metallurgy.

[8] Norwegian Geotechnical Institute, NGI (1999). Use of the Q-system in weak rock masses – Rep. nr: 592048-1.

[9] Bieniawski, Z. T.: Engineering rock mass classifica-tions. Wiley: New York, 1989.

[10] Carranza–torres, C. & Fairhurst, C. (2000). Application of the Convergence-Confinement Method of Tunnel Design to Rock Masses that Satisfy The Hoek-Brown Failure Criterion. Tunnelling and Underground Space Technology.

[11] Feder, G. 1978. Versuchsergebnisse und analytische Ansätze zum Scherbruchmechanismus im Bereich tiefliegender Tunnel. Rock Mechanics (6). 71-102.

[12] Goodman, Richard E., Introduction to Rock Mechanics, second edition, 1989.

[13] Goricki A., Schubert W., Riedmueller G., 2004, New developments for the design and construction of tunnels in complex rock masses, International Journal of Rock Mechanics and Mining Sciences.

[14] Gomes, A. R.A., Reyes G., Ulloa, J.C, “Geotechnical Design of Underground Infra-strucutre Works for the Mine Chuquicamata in Chile”, World Tunnel Congress 2013 Geneva, Undergounf – the way to the future! G. Anagnostou & H. Ehrbar (eds), 2013.