Geotechnical Design of Large Openings for Mining Operations in Chile

Proceedings of the World Tunnel Congress 2014 Tunnels for a better Life. Foz do Iguau, Brazil.

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1 INTRODUCTION

The permanent infra-structure of major mining projects in Chile often comprises of a number of large underground openings, which are used for housing operational installations, such as crusher chambers, material handling systems, electro-mechanical equipment and working offices, among others facilities. Generally, in accordance with the mine operational layout, these openings are interconnected by means of tunnels, drifts and vertical or inclined shafts, resulting frequently in a cluster of various closed-located underground structures.

This paper summarizes the general methodological approach implicitly considered for the geotechnical design of such large underground openings, with particular reference to the authors experiences on a number of projects for mining operations in Chile. Examples of two case histories are presented, where the first case consist of a large multi-cavern complex excavated with relatively shallow overburden conditions, whereas the second case corresponds to deep seated large openings excavated under high stress conditions.

2 GEOTECHNICAL DESIGN METHODOLOGY

2.1 General In general terms, the design of openings shall

aim at accomplishing the following objectives: (1) To allow the rock mass to reach a new

equilibrium state with the use of adequate excavation sequences and support measures, assisting the mobilization of the rock mass self-support capacity;

(2) To reduce to acceptable levels the effect of the construction on the environment and adjacent structures and;

(3) To provide the required functionality and durability of the openings, in accordance with its usage and expected lifetime.

To achieve it, the design process shall follow a comprehensive methodology, which shall be extended throughout the construction stage. The following flowchart attempts to summarize the main steps and relevant aspects implicitly considered in the geotechnical design of large caverns and openings:

Geotechnical Design of Large Openings for Mining Operations in Chile.

A. R.A. Gomes GEOCONSULT Latinoamrica, Santiago, Chile. J. C. Ulloa GEOCONSULT Latinoamrica, Santiago, Chile.

ABSTRACT: The paper summarizes the methodological approach considered for the geotechnical design of large underground openings, with particular reference to the authors experiences on a number of projects developed for mining operations in Chile. The stepwise design approach includes the geotechnical model, the assessments of the stress field, relevant boundary conditions, the analysis of the rock mass behavior and the ground-support interaction, additionally addressing some significant aspects associated to the construction stage. Two case histories of large underground facilities developed for the mining sector in Chile are presented to illustrate typical challenges and conditions faced by the design and construction of these types of underground structures.


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Figure 1. Geotechnical Design Methodology applied for Large Openings (Gomes A., 2013).

2.2 Geotechnical Model Large openings design and construction

hurdles are usually closely related to the complexity of the geological and hydrogeological conditions of its host rock mass. Therefore, good geotechnical knowledge of the ground is required for the selection of the optimum opening location and layout.

The site investigation campaign habitually includes mapping of surface and existing underground excavations, drillings and geophysical investigations, in situ verification of rock mass parameters and laboratory and water tests. In the case of mining projects, the foregoing mine operation typically provides substantial amount of geotechnical and observational/monitoring data from previous excavations. This accumulative experience and specific geotechnical knowledge, together with specific investigations, can contribute significantly to the optimization of forthcoming design and construction processes.

For the geotechnical model, ground types with similar geotechnical characteristics are grouped in distinct Basic Geotechnical Units (BGUs), which shall be characterized both in qualitative (descriptive) and quantitative terms (key parameters), accounting for the

information coherence and confidence levels and the probabilistic variation of each specific set of key parameters. The following aspects shall be covered in the description of each BGU:

a) Lithology, alteration and mineralization; b) Intact rock and rock mass

characterization: - Qualitative data: sketches, photos,

characterization and understanding of basic failure mechanisms) and;

- Quantitative data: key geotechnical parameters and classification indexes (e.g. RQD, IRS, GSI, Q, RMR and MRMR). c) Geo-structural characterization and

properties of micro, meso and macro structures. d) Hydro-geological model (permeability and

water flow and inflow). The geotechnical model defined at the design

stage shall be continuously updated during the construction stage by means of mapping and monitoring of the actual geological conditions, as to mitigate unexpected conditions and allow for design adjustments and optimizations.

2.3 Assessment of the Stress Field The natural stress field is a factor of major

relevance for the design of large openings. It is assessed by means of stress measurements (e.g.


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hollow inclusion and hydraulic fracturing methods) used to calibrate tri-dimensional numerical models that considers the existing local geo-morphology and ground conditions. Since the stress field and the seismic activity caused by mining is a decisive factor in the operation of deep underground mines, a mine-scale stress field model is generally available, which can be further refined to assess the prevailing stress field at a specific mine location. Otherwise, for projects where stress data is minor or inexistent, the stress field needs to be extrapolated from general stress maps or calibrations based on measurements carried far away from the actual construction site.

In any of the above cases, it is not unusual to observe large average deviation errors between the numerical modelling stress fields with respect to individual measurements. It occurs also due to rock mass jointing and inhomogeneity, which causes deviations in the local stress fields. Hence, it is recommended to carry out additional stress measurements at the cavern site, as to allow for a probabilistic calibration (e.g with the Rosemblueth method) of the effective local stress field and associated ground behavior.

2.4 Geometrical and Functional Layout and Geotechnical Boundary Conditions

A large underground opening complex shall be carefully analyzed with respect to all relevant boundary conditions in an opportune design stage, including selection of optimum location, orientation and layout of the structures, as well as definition of adequate support measures and constructive methods. The shape of the cavern(s) and ancillary structures are majorly defined by their functional requirements; nevertheless other aspects such as the ground characteristics and behavior, the proximity of existing adjacent openings (vibration and deformation limits) as well as constructability requirements (e.g. accesses, ventilation, interference with the mining operation, etc.) must also be taken into account. Commonly, the openings roof presents a curved shape, with vertical walls. In some cases, where combination of poor rock mass conditions and stress may require it, a wall with curved shape could help to reduce the damaged rock zones and support requirements; however, due to the prevailing rock conditions, this solution has not been extensively applied in Chile. As far as

possible, sites with the presence of strongly inhomogeneity between rock mass units (e.g. large difference in stiffness and strength) or with low cover (e.g. under open pits active zones) are avoided. The opening axis orientation is defined as a compromise between: crossing weak zones in the least distance; be aligned with the intersection of the main discontinuities families and; avoid the largest stress anisotropy ratio acting on a perpendicular plane to the opening.

Figure 2. Stress Anisotropy Concept.

The minimum distance between openings (pillars) is typically determined on the basis of experience and supported by results of detailed numerical analysis. Particular attention shall be given to large openings under high overburden (e.g. 700m) and tectonic stresses, due to the risk of spalling and rock burst, as well as for cases of shallow rock cover conditions (e.g. 1-1,5 times the span) and interference with other openings, where stability issues associated to rock relaxation and tensile stresses can control the design and increase geotechnical risks.

2.5 Analysis of the Rock Mass Behavior and the Ground-Support Interaction

2.5.1 General

These analysis are carried out with the support of engineering tools (e.g. empirical, analytical or numerical methods), in the light of experienced engineering judgment. Since geotechnical analyses are intrinsically affected by uncertainty - both in terms of input data and inherently limitations of engineering tools results must be treated probabilistically. This condition must be properly expressed in the design to allow for flexibility and an adequate risk management during the construction stage.

2.5.2 Rock Mass Behavior Once the preliminary geometrical layout and

associated excavation sequence and support of the openings are defined, the rock mass


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behavior can be assessed. The sources and basic relevant failure mechanics of the rock mass shall be identified, e.g., discontinuities-controlled, stress, dynamic/seismicity loads, groundwater and presence of minerals (Gomes A., 2013)

Simplified analytical and empirical methods, together with elastic tridimensional numerical analysis (B.E.M.) are typically carried out for an earlier assessment of the ground response to unsupported excavation. Depending on the results obtained, a first optimization and adjustment of the general opening(s) layout can be carried out, before more complexes numerical analyses (e.g. discontinuous modelling) are carried out to obtain a more detailed assessment of the ground behavior.

Results obtained from the analyses are compared with predefined failure and stability evaluation criteria, such as those shown in the following Table 1.

Table 1. Examples of Stability Evaluation Criteria.

Parameter Variables Stress concentration 1 / UCS

Shear failure potential (1 3) Deformation of roof and walls Relaxation or tensile stresses

Critical level of strains Mohr-Coulomb failure criteria

Hoek-Brown failure criteria

V/B; V/H (3) (3)

f(1, 3) f(1, 3)

* Acceptable levels may vary in function of the ground composition, presence of nearby openings, functional requirements, etc.

2.5.3 Ground-Support Interaction - System Behavior

The preliminary excavation sequence and support systems are considered for the analyses of the ground-support interaction (system behavior) for different combinations of ground quality (range of expected key parameters variation).

In each type of analysis carried out, results are compared to pre-specified acceptance criteria for the rock mass, as mentioned previously. Similarly, the proposed support elements are checked to verify their degree of utilization in terms of load, energy absorption and deformation for each analyzed scenario, and the compliance with the required structural safety factors. Numerical modelling can be either continuous or discontinuous, iso- or anisotropic, based on various available methods

(e.g. F.E.M., F.D.M., D.E.M.) and with application of different constitutive models and failure criteria (e.g. Hoek-Brown, Mohr-Coulomb, etc.). The most suitable methods are chosen in function of the rock mass composition and the type of failure mechanism to be analyzed (e.g. squeezing, brittle failure, structurally controlled failure, creep, etc.).

The process is interactive, allowing an optimization of the initially proposed excavation sequence and support, as to mitigate stability issues and improve the proposed construction scheme.

2.6 Construction Baseline Scheme The previous design stages allow the

establishment of a construction baseline scheme and associated compensation clauses, where specific excavation sequences and support measures are defined and specified. The analyses of the proposed construction methodology and working cycles shall ideally be treated in probabilistic terms (e.g. Monte Carlo Method) to deliver a comprehensive and traceable estimation of construction time and cost, with the consideration of relevant influencing factors. Additionally, a broad risk assessment and management plan shall be outlined, identifying all possible geotechnical and natural risks, deviations and critical aspects that can negatively affect the baseline scheme. Those risks shall be controlled during the construction, supported by a continuous mapping and monitoring of the actual ground and support behavior (observational approach), as an inherent part of the design process.

3 CONSTRUCTION ASPECTS

Even though constructability aspects are largely defined in the earlier design stages, a detailed analysis of existing interferences, ventilation requirements, effective working time at the headings, transportation of muck and personnel, HSEC issues, accreditations, among other relevant factors, shall also be carried out by the contractor in close cooperation with the client (Mine owner). The construction baseline scheme shall be revised and made its own by the Contractor, which shall plan in detail how the design will be properly developed on site to fulfill the expected contractual performance (e.g. time, costs, quality, safety, etc.). This is a


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main issue, particularly considering the fact that the mine is permanently in operation, with its own working schedules and agenda, which must be catered for by the contractor.

Caverns and large openings are typically excavated with the use of Drill and Blast methods. Road headers can be also be applied in weaker and less abrasive rock masses, also as a measure to mitigate blasting vibration to nearby structures (but there is no record of cases of use of this method for cavern excavations in Chile). Excavation works usually starts from the access tunnels, which shall reach the top of the caverns, allowing the excavation of a central pilot tunnel, which is further enlarged to materialize the cavern roof. Whereas the roof geometry and structural function requires a more careful excavation sequence, benches (walls) can generally be excavated more massively with consideration of larger excavation blocks and rounds (except in poor rock conditions).

Figure 3. Example of Excavation Sequence (ACT, 2008)

The typical ground support system is habitually constituted by shotcrete, plain or reinforced with wire mesh or fibers, grouted bolts and cables and ground anchors in particular cases. Longer cables have the advantage that they can be more easily installed in limited headroom conditions and therefore are frequently used as long term rock support. In some cases, a permanent inner in situ concrete lining is provided, depending on the usage and functional requirements of the opening (e.g. vertical ore chutes). On the other end, there can be cases of openings constructed without provision of linings, and supported only with bolts and cables and light wire meshes in walls (to protect against rock falling). Cement or chemical pre-grouting or ground treatment can be locally required to improve the ground or to control water inflow in weak zones.

Another major issue in the construction of large openings consists of an adequate control of both the blasting damage and over-breaks and the avoidance of geometric deviations, particularly as those openings are typically excavated simultaneously from different headings and a final irregular geometry can negatively affect the opening stability, increasing the need of maintenance and decreasing the opening functional durability.

4 CASE HISTORIES

4.1 Andina Mine PDA Phase I - Caverns for Operational Facilities

The Andina Mine of Codelco, which has both surface and underground mining operations, is located at 3.000 m.a.s.l. at the Andes Cordillera, in a zone with abrupt geography, high altitude and adverse climate (up to 12m of snow, white wind and temperatures below -20oC during the winter period). For these conditions, the use of the underground space for the installation of operational facilities is a natural and ideal solution.

The Project PDA Phase I, comprised the expansion of the mining production from 25k to 60k tons/day and included the construction of new major underground structures (~200.000 m3 of excavations) in addition to the already existing infra-structure, for housing of operational facilities, such as crush and milling chambers, copper concentrate flotation systems, ore chutes, electrical rooms, among other facilities.

Figure 4. View of Main Underground works PDA Phase 1 Andina Mine

These large openings were located at close vicinity to each other and to the existing


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facilities, being excavated in fair quality Granodiorite (RMR 55 to 58) with the presence of some localized major structures (faults). The openings were located within one of the valley slopes, away from the ore extraction sectors, so that rock cover varied from about 80m to a minimum of about 50m due to the natural slope. The caverns presented vertical walls and curved roofs, generally with large span and height (e.g. milling cavern with 28mx37x58m). Chutes were cylindrical-shaped with an inner diameter of up to 23m and heights of up to 30m.

For this case, the geo-structural conditions of the rock were a key issue, so that the analyses were carried out with both continuous and discontinuous 3D numerical modelling (to verify the effect of major structural systems). The detail design also included wedge stability analysis and analysis of the ground-support behavior with specific bi- and tri-dimensional models. Analyses results showed that compressive stress concentrations were of minor significance, so that the greatest challenges corresponded to the reinforcement of zones with relevant ground relaxation and tensile stresses, and zones with the presence of major faults and dikes.

Figure 5. Modelling of Caverns at PDA Phase 1 Project (Karzulovic A., 2008-2010)

Typical rock permanent support consisted of shotcrete with grouted bolts and cables with various lengths. The shotcrete at the roof was additionally reinforced with wire mesh and the walls were provided with a loose light mesh to control minor rock falls. In some areas, long pre-stressed cables have also been considered, to improve confinement conditions and bridge interferences between multiple openings.

Construction works were carried out from 2007 to 2010, with the use of the drill and blast

method. Accesses were provided through new adits, built specifically for the new facilities, as well as through the existing underground tunnels and structures. The remote site location, the limited working space and restrictions imposed by the mine operation required a strict coordination of works between contractor and the mine, logistic for access, transport of muck, ventilation, etc.

The construction was followed by an extensive monitoring plan and continuous mapping of geological conditions. Particular attention was given to the structural conditions, which in some areas showed the presence of major structures (faults) with loose infilling and water inflow, which required special excavation methodology and ground treatments. These facilities were in operational in December 2010.

Figure 6. Excavation of Crusher Chamber at PDA Phase 1 (ACT, 2006)

4.2 Mina El Teniente Crush Chambers at the Diablo Regimiento Sector

The Mine El Teniente of Codelco is currently the largest underground copper mine of the world, producing 403k tons of Cu and 5.6k metric-tons of molybdenum with the use of the panel caving method. The mine is at present developing El Tenientes New Mine Level Project (NMLP), which shall increase ore production by mining the ore deposit (reserves 2k Mt) located under the current ore body under extraction.

The present operation includes 6 productive sectors, located at different elevations, which extracts ore from the porphyry copper deposit that surrounds the Braden Breccia Pipe, a massive volcanic intrusive body roughly circular in plan view and an inverted cone in depth, with variable diameter (max. 1200m).

The largest part of the mining infra-structure is placed within the Braden Pipe, which consists of a cemented breccia with practically no joints (weak cement behaviour). Even though, some


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large openings for crush chambers and material handling system are also located at the perimeter of some productive sectors footprint (mining polygon), within the so-called mafic complex (denominated CMET) and constituted by diabases, porphyric basalts and gabbros, with strong alteration and multi-directional stockwork (previously classified as Andesite primary rock), being also crossed by some intrusions and dikes. The mentioned openings are built in depths that go in excess of 1000m in certain cases, i.e., under the effect of high stress conditions.

Figure 7. Crusher Chambers at Diablo Regimiento (Cavieres & Pardo, 2006)

The Diablo Regimiento corresponds to one of these productive sectors, which include five crusher chambers, each of them with a 9 to 13 years lifetime (Cavieres & Pardo, 2006). Each of these chambers denominated CH-1 to CH-5, encompasses about 5,000 m3 of underground excavations. The CMET rock mass at the sector presents RMR between 75 to 90 and hydraulic conductivity lower tan 10-8m/s, so that water inflow is generally not expected, except at localized weak zones. The crush chamber is constituted mainly by the discharge station, the vertical chute, crusher room, being the latter the largest opening with 31mx13m and a length of 22,5m. The caverns present curved roofs and vertical walls.

To study the geomechanical behaviour of these openings various analysis were carried out during the feasibility design stages, considering the different stages to which the cavern will be subjected to (before, during and after its construction) to identifying the most relevant parameters that impact its stability (stress field, structural geology) and defining the chambers support requirements (Cavieres & Pardo, 2006).

Besides, in some cases, these openings could also be affected by the stress redistribution caused by the mining activity (e.g. abutment stresses due to the rock caving process)

Figure 8. Effect of Abutment Stress due to Caving

The analyses included continuous and discontinuous 3D numerical modelling, where the effect of major structural systems could be evaluated, providing insight in the impact of different geo-structural ubiquitous systems orientations and gravitational stress field options analysed for the various cavern sections and layouts.

Figure 9. Crusher Chambers Geometry and Modelling

The detail design also included specific wedge stability analysis and analysis of the ground-support behaviour with both bi- and tri-dimensional models. Analysis showed that the geological structures had the largest impact on the caverns stability compared to the other analysed aspects. The most critical issue corresponded to the sub-horizontal systems that mostly affected the stability of the chambers roof, which could be controlled by grouted bolts installed for the excavation advances (temporary support). Whereas stress concentrations around the openings could reach up to 100 MPa and the rock damaged zone could reach depths of 6m,

Cavern


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rock permanent support (shotcrete reinforced with fibers and mesh, with 5-10m long grouted cables), was proven sufficient to maintain the opening behaviour within acceptable performance levels.

Construction was carried out with the use of the drill and blast method, under similar restrictive logistic conditions as previously described for the Mine Andina. Similarly, the construction was followed by an extensive monitoring plan and continuous mapping of geological conditions, allowing timely identification of singularities and anomalies. Currently, four out of five chambers are already in operation, where the first chamber construction was finished in the year 2005.

Figure 9. Excavation at Crush Chamber at at Diablo Regimiento CH-4 (Geoconsult, 2009)

5 CONCLUSIONS

A general methodological approach for the geotechnical design of large underground openings has been outlined, with particular reference to the authors experiences on a number of projects for mining operations in Chile. The methodology, presented as a stepwise process, can be followed to attain a traceable and flexible geotechnical design development, including the opportune definition of key aspects throughout all design stages up to the design hypothesis monitoring and validation during the construction period. The two case histories presented, illustrated typical challenges and conditions faced by the design and construction of these types of underground facilities, where these methodology has been implicitly applied and proven adequate.

6 ACKNOWLEDGEMENTS

The authors are in debt with the many institutions and professionals that, individually or collective, contributed in one form or other to the basis of knowledge upon which this paper is based on. They include consultants, contractors and not the least, the staff of the geomechanical departments of the National Copper Corporation of Chile (Codelco). Additionally, the authors would like also to express their gratitude to Codelco for the permission to mention two of its projects in this article.

7 REFERENCES

Cavieres, P. & Pardo, C. (2006). Geomechanical Evaluation of a Crusher Chamber Excavation at El Teniente Mine, Codelco Chile. Proceedings of the International Symposium on in-situ Rock Stress, ISRM Regional Symposium, Norway.

Geoconsult Latinoamrica-Amec (2008-2010), Gomes, A. et al. Various internal reports and documents prepared for and financed by Codelco-Chile VP for the design and construction stages of the PDA Phase 1 project.

Geoconsult Latinoamrica (2009), Gomes, A. & Ulloa, J. (Cavieres, P.) - Expert opinion on the Crusher Chamber Cavern N4 for the Diablo Regimiento project Fase III. document prepared for and financed by Codelco-Chile El Teniente Division.

Gomes, A. R.A., Reyes G., Ulloa, J.C, (2013) Geotechnical Design of Underground Infra-structure Works for the Mine Chuquicamata in Chile, World Tunnel Congress 2013 Geneva, Undergound the way to the future! G. Anagnostou & H. Ehrbar (eds),.

Jarufe, J. & Vasquez, P. (2008). Mine-Scale 3D Stress Model for the New Mine Level Project, El Teniente Mine, Codelco, Chile. Proceedings of the 1st Southern Hemisphere International Rock Mechanics Symposium, SHIRMS 2008, Perth, Western Australia, Volume 1.

Karzulovic, A., Cavieres, P. and Gonzales, G. (2006) El Teniente Mine Conceptual Field Stress Model, Study DT-CG-2006. Prepared for and funded by the New Mine Level Project, VCP.

Karzulovic Asociados (2008-2010) various internal reports and documents prepared for and financed by Codelco-Chile VP for the PDA Phase 1 project.

Lorig L., Silva R. (2005), 3D Geomechanical Modelling of Caverns, document prepared for and financed by Codelco-Chile Andina Division.

Lorig L. (2001), Considerations for the 3D modeling in the analysis of primary rock caverns, TM-0105801 Consultant Contract, Itasca Chile, document prepared for and financed by Codelco-Chile El Teniente Division.

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Geotechnical Design of Large Openings for Mining Operations in Chile

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