1 INTRODUCTION

Cerro Vanguardia (CV) is an Au-Ag mine located in the central steppe of Santa Cruz province, Argentina. The deposit consists of 102 epithermal low-sulphydation veins 19km long, 3.5 - 10m wide and dipping 60° - 90° NE (Heather et al. 2004). Country rock consists of rhyolitic ignimbrites of the Jurassic Age, moderately to intensely fractured and widely al-tered by hydrothermalism. The stratigraphic se-quence dips 7 / 146 on average and comprises strati-fied, grainy, brecciated and massive slabby ignimbrites. This sub-horizontal stratigraphy is fa-vourable for open pit mining, allowing the excava-tion of very steep walls which exhibit consistent good performance.

Mining is being carried out mainly through open pits and, more recently, underground longwall de-velopments. Pits are up to 1000m long, 150m wide and 200m deep, with global slope angles of 53º - 58°, 64° - 70° interramps, 20m benches and 6 - 10m berms. Pre-split blasting techniques are widely em-ployed to ensure design compliance under tight dilu-

tion constraints, stripping ratio being in excess of 25 (Adamson et al. 2011).

2 GLOBAL STABILITY ANALYSES

Global and bench-berm stability analyses were per-formed with the 2D Finite Element Method (FEM) utilizing Phase2 (V8) software. General guidelines for stability analyses for open pits were followed from Read & Stacey (2009).

After definition of geotechnical domains and model, critical sections for each pit were defined based on their heights and general inclination angle; only the most unfavorable cases were simulated. Typical pit geometries presented 100 - 200m depths, 15 - 20m high benches, berm widths between 4 and 6m and bench inclinations from 75° to 87°.

The procedure consisted of the simulation of the excavation process according to the exploitation program. Initial stresses were calculated with the K0 procedure, assuming Kx = Ky = 1.0. Since no in-situ measurements were available, sensitivity analyses were performed in order to assess the implications of this assumption; as expected, in-situ initial stresses

Cerro Vanguardia Open Pit Mining: Database Management for Stability Analyses

I. García Mendive, G. Rellán, U. Sterin & A. O. Sfriso SRK Consulting Argentina SA, Buenos Aires, Argentina

G. Erz Cerro Vanguardia SA, Santa Cruz, Argentina

ABSTRACT: Cerro Vanguardia is an Au-Ag mine located in the central plains of Santa Cruz, Argentina. The deposit consists of several veins 3.5-10m wide and 19km long, related to epithermal mineralization of the Chon-Aike formation by Jurassic volcanism episodes. The exploitation is carried out by means of several open pits in the range 100-240m deep. It is to be expected that more than fifty pits will be developed during the lifetime of the project. The nature of the mineralization process caused different degrees of argillic alteration in the surroundings of the veins, especially in the form of poor quality rock bands that could potentially compromise the stability of the pit walls. Two geotechnical rock-mass characterization programmes were carried out in 2012 and again in 2015 and were employed to perform numerical analyses for slope stability assessments. This work compares and contrasts the two analysis methodologies implemented for the two studies, with the purpose of showing a data management procedure aimed at reducing uncertainty by several geotechnical pro-cedures. Geostatistical distribution and density of data conditioned the definition of limits between geotech-nical units and the accuracy of the boundaries between them. Wide variations both in quality and amount of data implied that key parameters had to be estimated by wholly different methods. Ultimately, enhanced char-acterization of geotechnical units and the study of detailed analysis sections led to targeted recommendations on excavation procedures and eventually produced a more consistent and reliable pit design.

have a negligible effect on the results of 2D numeri-cal analyses of slope stability.

Rock-mass features and minor discontinuities were considered in the analysis by the standard pro-cedure of reducing the Hoek-Brown strength param-eters for each geotechnical domain. According to the blasting methodology applied and guidelines put forth in Hoek (2012), a damage factor (D) equal to 0.7 was considered in a zone adjacent to the pit face.

The global factor of safety (FoS) was determined by using the shear strength reduction factor method-ology (SRF). Stability was analysed at global scale, with a deterministic approach and an acceptance cri-teria for overall slopes: FS > 1.3. Where appropriate, modifications to the geometry were proposed to comply with adopted safety requirements and the project´s excavation to date.

3 SITE-WIDE GEOMECHANICAL DATABASE

3.1 Geology The Cerro Vanguardia epithermal Au-Ag deposit is situated within the Deseado Massif (De Giusto et al. 1980). It encompasses a 60.000km² area in which Ju-rassic volcanism is the predominant episode, associ-ated with the rifting originated in the Atlantic Ocean opening (Uliana et al. 1985).

Heather et al. (2004) have performed the latest geological study of Cerro Vanguardia Project, ac-cording to which the deposit consists of 102 epi-thermal low-sulphydation veins mineralised with Au and Ag, stored in rhyolitic ignimbrites of the Jurassic Age. Veins span a length of 193km, measure 3.5m in width on average (10m maximum) and dip 60° - 90° NE. Mineralization generally occurs as veins and stockworks, together with quartz, chalcedony, opal, baryte, calcite and adularia.

Hydrothermal alteration mostly comprises silici-fication, sericitization, adularization, argillization and propylitization. Alteration minerals are adularia, sericite-illite, smectite and kaolinite (Zubia et al, 1999).

Site stratigraphy consists of a repetitive series of felsic ignimbrites divided into stratified, grainy, brecciated (subdivided into base breccia, breccia and superior stratified) and massive slabby (composed of seven subunits).

3.2 Major structures Within the Cerro Vanguardia anticline, the strati-

graphic sequence dips 7° (on average) in direction 146 SE (Heather et al. 2004). Faults oriented 324° / 83° determine the general fracturing of the rock mass, moderate to intense, with clear fracture pat-terns. The main direction at mine scale is 328° / 87°.

Fractures with densities greater than 5 per metre have a marked principal direction, namely 323° / 85°, which correlates them directly to veins and veinlets (dominant orientations are respectively 323° / 86° and 323° / 89°).

3.3 Intact Rock Previous characterization of intact rock units (IRUs) presented in Hormazábal et al. (2004) was based on geotechnical mappings and laboratory tests on core drilling performed at four pits. In order to simplify stability analyses for a multi-pit mine, Hormazábal et al. (2004) suggested sorting the rock mass into sim-ple categories that grouped similar mechanical prop-erties. These basic geomechanical units (BGUs) were classified according to their degree of alteration into soft argillic, soft, and hard, the latter being sub-divided into three subcategories (small blocks, big blocks, massive), in relation to the degree of fractur-ing of the rock mass (see Table 1). In addition, re-cent geotechnical reports have included a subdivi-sion of the Soft unit on the basis of fracturing.

Table 1. Summary of the geotechnical units of CV. ______________________________________________ Geotechnical Alteration Fracture unit frequency ______________________________________________ Soft Argillic Argillic (intense) — Soft Argillic (light) — Hard – Small blocks Siliceous Medium Hard – Big blocks Siliceous Low Hard – Massive Siliceous Very Low ______________________________________________

More recently, this data set was augmented by a se-ries of test results made available in 2012 and 2015: 68 uniaxial compression, 16 tension, 121 triaxial, 23 Young’s modulus and 239 unit weight tests. Samples were taken from several veins, both on the footwall, hangingwall and the orebody itself. Materials en-compass grainy, brecciated and slabby ignimbrite, as well as argillic and siliceous alterations.

Those new tests contained a fair description of the lithology, albeit not of the alteration degree or type. Therefore, in the 2012 and 2015 analyses described herein, intact rock and geotechnical units as defined in (Hormazábal et al. 2004) could not be retained and had to be redefined according to the data availa-ble.

4 THE 2012 DATABASE AND PROCEDURE

4.1 Rock matrix Scanlines were the only source for rock mass charac-terization and definition of geotechnical domains, and they only included lithology, not alteration. Thus, IRUs were redefined for each pit, individually, on the basis of lithology/stratigraphy and then subdi-

vided according to matrix strength into “Soft” and “Hard” (see Table 2). Strength parameters were ob-tained by fitting Hoek-Brown failure envelope to test results; given the small size of the database available for each unit, the parameter mi was adjusted in ac-cordance with reference values for each lithology.

Table 2. 2012 Intact Rock Units for ODcb7. _____________________________________________ Lithology Strength Vci mi Ei [—] [—] [MPa] [—] [GPa] _____________________________________________ IBR* Soft 46.4 15.4 18850 IBR* Hard 65.3 26.1 32800 IGR** — 115.0 26.0 43400 _____________________________________________ * IBR: brecciated ignimbrite ** IGR: grainy ignimbrite

4.2 Discontinuities Structural domains were defined on the basis of dis-continuity data captured by line mappings and vali-dated through geological interpretation of pit wall photographs.

4.3 Rock mass The rock was classified with Bieniawski’s Rock Mass Rating (Bieniawski 1989), which was calculat-ed for each structural set: the lowest RMR89 was re-garded as controlling rock mass instability and the rest were ignored. Additionally, the rating adjust-ment for discontinuity orientation was applied for each pit, on a case-by-case basis.

Intact rock strength for RMR calculation was tak-en as that informed in scanlines, which indicated li-thology but not alteration. Where available, it was complemented with lab test data, albeit it is worth noting that important differences were observed be-tween the two. Roughness rating was based on the joint roughness coefficient (Barton 1973).

The Hoek-Brown strength criterion was employed for the rock mass. GSI was calculated as “dry” RMR89 – 5 (Hoek 1994).

5 THE 2015 DATABASE AND PROCEDURE

The quantity, quality and spatial distribution of the data available in 2012 proved insufficient to fully address the uncertainties in material rock properties, implying that large ranges of values had to be con-sidered for the sensitivity analyses of each parame-ter, yielding a poorly defined risk scenario. This fact motivated the execution of additional exploration studies aimed at enriching the geomechanical char-acterization. A fairly large set of non-oriented drill holes and additional mappings were performed. As a result of this field effort, a much enlarged and consistent database was accessible in 2015.

5.1 Rock matrix Hormazábal et al. (2004) defined IRUs on the basis of fracturing. In the light of later (2003 and 2011) tests, the authors have not observed differences in in-tact rock characteristics that could be ascribed to block size, except for the Hard Massive unit. In other words, fracturing could not be directly attributed to intact rock behavior that could be inferred from it. On the other hand, definition of geotechnical do-mains was based on corelogs complemented with mappings, all of which contained UCS data but scarcely any lithology or alteration. Therefore, 2015 IRUs shown in Table 3 were classed based solely on strength: each unit was enriched with new lab tests and thus the confidence interval 95% for the mean was diminished. It is acknowledged that this classifi-cation deserves further attention, observation of the pit walls while mining being the most direct method of improving the consistency of the approach.

Table 3. 2015 Intact Rock Units. ____________________________________________ IRU J Vci mi Ei [—] [kN/m3] [MPa] [—] [GPa] ____________________________________________ Soft Argillic 22.1 11.4 12.0 5.0 Soft 23.0 21.6 14.8 15.8 Hard 24.2 41.1 15.6 21.9 Hard Massive 25.0 73.2 23.3 22.5 ____________________________________________

5.2 Rock mass Rock mass characterization was primarily based on non-oriented geotechnical core logging, which con-stituted the most abundant geomechanical infor-mation source. Scanlines and mappings were also available and mainly employed to validate GSI and UCS obtained from corelogs.

The system described in (Dempers et al. 2010) was used for logging the cores. This methodology enables the direct computation of RMR90 (Laubscher 1990), Q (Barton 1974) and GSI (Hoek 1995). These parameters were determined for 3m core runs. The direct computation of GSI is not optimal when it comes to large databases, as it requires the use of charts. Therefore, GSI was estimated as “dry” RMR89 – 5 (Hoek 1994).

The estimation of UCS, RQD, spacing and groundwater ratings for RMR89 was straightforward. Conversely, to calculate Jc, an equivalence had to be established with the ratings defined in (Dempers et al. 2010). In particular, persistence and weathering were lacking and had to be adopted from scanlines.

In (Dempers et al. 2010), the joint ratings are cat-egorized by the following angles of incidence: 0 - 30°, 30 - 60° and 60 - 90°. Accordingly, one RMR89 was calculated for each group and the minimum was adopted. As usual, the synthesis of large amounts of experimental data into a small set of indexes simpli-fies the picture at the cost of loss of detail and so-phistication. Observation of the pit walls will also al-

low for the refinement of the proposed classification procedure for Cerro Vanguardia pits.

Figure 1 (left) presents the RMR89 obtained under these assumptions, alongside RMR89 for Jc = 10 and for Jc = 20. As shown, the foregoing considerations are equivalent to considering Jc < 10. Figure 1 (right) shows the variation of RMR90 and RMR89 (for joint groups). In all cases, RMR90 and RMR89 follow the same general trend, except for outliers.

!

"!

#!

$!

%!

&!

'!

(!

)!

*!

"!!

""!

"#!

"$!

"%!

"&!

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

40 50 60 70 80 90

+,-./-01/234

Level [m]

RMR!"##$%&'()RMR Jc=20

RMR Jc=10

RMR

Figure 1. Left: RMR89 calculated for Jc = 10 and for Jc = 20. Right: RMR90 and RMR89 for three joint groups.

6 APPLICATION TO PIT OSVALDO DIEZ CUTBACK 7

Pit Osvaldo Diez is one of the main producers of the Cerro Vanguardia operation. Mining in this location started in 2002 and continued uninterrupted since then. Several cutbacks were performed. The authors were involved in the design and analysis of Cutback 7, which is employed here as an example of the ap-plication of the two material databases to a single problem.

Analyses completed in 2012 and 2015 for pit Osvaldo Diez Cutback 7 (ODcb7, Fig. 2) are com-pared and contrasted. The section analyzed in 2012 almost coincides with the Domain 2 section defined in 2015.

Domain 2 comprises highly altered rock in the uppermost 20 - 30m, underlain by competent rock. According to core log data, fracturing is medium to low (RQD > 50) without any major gouge bands. Fracturing does not correlate with the degree of al-teration: RQDs of 80 - 100 have been recorded for

cores having UCS < 5MPa (unconfined compressive strength estimated via Schmidt hammer).

In both 2012 and 2015 analyses, staged excava-tion of the cutback was simulated by 2D finite ele-ment models in Phase2, where each stage was al-lowed to converge to full equilibrium before the new stage was simulated. A shear strength reduction cal-culation was carried out at the final stage to assess the FoS of the final intended wall.

Figure 2. Plan view of ODcb7 pit and sections analyzed.

6.1 2012 analysis Available information in 2012 consisted of four 20m-long scanlines distributed between levels 185 and 205. A unique domain was defined and there-fore, the critical section was that with the steepest geometry (Fig. 3). Geomechanical parameters and mesh considered in the FEM simulations are in-formed in Table 4 and Figures 4-5 respectively.

The Young’s modulus for the rock mass was es-timated via (Hoek & Diederichs, 2006). :

� �� � 1

2/10.02]MPa[E=[MPa]E 11/1560irm ¸¹·

¨©§

��

�� �� GSIDeD

A Poisson ratio equal to 0.26 was considered for

all geotechnical units. The FoS calculated for the fi-nal excavation was 1.63. Shear strains corresponding to the FoS stage are displayed in Figure 6.

Table 4. Geomechanical parameters, 2012. ______________________________________________ Lithology J GSI Vci mi Ei [—] [kN/m3] [—] [MPa] [—] [GPa] ______________________________________________ IBR Soft 26.0 44 46.4 15.4 18.9 IBR Hard 26.0 44 65.3 26.1 32.8 IGR 26.0 44 115.0 26.0 43.4 ______________________________________________

2015

2012

Figure 3. Geometry and materials considered, 2012.

Figure 4. Mesh, 2012.

Figure 5. Detail of the mesh, 2012.

1 1

Critical SRF: 1.63MaximumShear Strain

0.00e+0006.00e-0041.20e-0031.80e-0032.40e-0033.00e-0033.60e-0034.20e-0034.80e-0035.40e-0036.00e-0036.60e-0037.20e-0037.80e-0038.40e-0039.00e-0039.60e-0031.02e-0021.08e-0021.14e-0021.20e-002

1320

1300

1280

1260

1240

1220

1200

1180

1160

3140 3160 3180 3200 3220 3240 3260 3280 3300 3320 Figure 6. 2012 Shear strains in SRF Stage.

6.2 2015 FEM simulation On top of the database available in 2012, additional information provided by eight loggings was pro-duced for the 2015 analyses, which enabled the defi-nition of five geotechnical domains. Fortuitously, the

section analyzed in Domain 2 (Fig. 7) almost coin-cided with the section studied in 2012. Geomechanical parameters and mesh considered in the FEM simulations are informed in Table 5 and Figure 8-9 respectively. Rock mass Poisson’s ratio and Young’s modulus are estimated considering Hoek et al (2000) and Hoek & Diederichs (2006), respectively.

The FoS calculated for the final excavation was 1.24. An alternative geometry was proposed by wid-ening the benches up to 10m at levels +95 and +115masl; the corresponding FoS was 1.36. Shear strains corresponding to the safety calculation stage are displayed in Figure 10.

Figure 7. Geometry and materials considered, 2015.

Table 5. Geomechanical parameters, 2015. ___________________________________________ Lithology J GSI Vci mi Ei [—] [kN/m3] [—] [MPa] [—] [GPa] ___________________________________________ Soft Argillic 22.1 40 11.4 12.0 5.0 Soft Argillic 22.1 45 11.4 12.0 5.0 Soft 23.0 50 21.6 14.8 15.8 Hard 24.2 45 41.1 15.6 21.9 Hard 24.2 50 41.1 15.6 21.9 ___________________________________________

Figure 8. Mesh, 2015.

Figure 9. Detail of the mesh, 2015.

1 1

Critical SRF: 1.63MaximumShear Strain

0.00e+0006.00e-0041.20e-0031.80e-0032.40e-0033.00e-0033.60e-0034.20e-0034.80e-0035.40e-0036.00e-0036.60e-0037.20e-0037.80e-0038.40e-0039.00e-0039.60e-0031.02e-0021.08e-0021.14e-0021.20e-002

1320

1300

1280

1260

1240

1220

1200

1180

1160

3140 3160 3180 3200 3220 3240 3260 3280 3300 3320

IBR Soft

IBR Hard

IGR

Soft Arg - 40 Soft Arg - 45 Soft - 50 Hard - 45

Hard - 50

Hard - 50

Critical SRF: 1.24Maximum

Shear Strain0.00e+0005.50e-0041.10e-0031.65e-0032.20e-0032.75e-0033.30e-0033.85e-0034.40e-0034.95e-0035.50e-0036.05e-0036.60e-0037.15e-0037.70e-0038.25e-0038.80e-0039.35e-0039.90e-0031.05e-0021.10e-002

220

200

180

160

140

120

100

80

-260 -240 -220 -200 -180 -160 -140 -120 -100 Figure 10. 2012 Shear strains in FoS stage.

7 CONCLUSIONS

Two analysis methodologies were presented for the stability assessments of pit Osvaldo Diez cutback 7 at Cerro Vanguardia mine, Santa Cruz Province, Ar-gentina. The geological, structural and rock mass model at deposit scale were outlined, which form the basis for the analyses performed in 2012 and 2015.

The respective databases were compared and con-trasted along with the processes and criteria that were implemented to obtain the relevant mechanical parameters.

The stability analysis performed in 2012 showed a very robust Factor of Safety 1.63 but a wide uncer-tainty coming from many sources, mainly the quanti-ty, quality and spatial distribution of the data availa-ble. This fact motivated the execution of additional exploration studies to enrich the geomechanical characterization; thus a much enlarged database was accessible in 2015.

The stability analysis performed in 2015 for the same pit wall showed a Factor of Safety 1.26. While 1.26 is much lower than 1.63, the uncertainties relat-ed to the 2015 analysis were much less than those of the 2012 study; scatter was lower, and ultimately the confidence in the design was higher. Furthermore, the finer detail of the geomechanical characterization of the rock mass allowed for a slight change in the wall design which raised the Factor of Safety to 1.36, a value deemed reasonable for this wall and remaining level of uncertainty.

The improvement of the geotechnical database —both at deposit and at pit scale— and the study of numerous and detailed analysis sections led to tar-geted recommendations on excavation procedures and, at times, slight variations in slope geometry which eventually produced a more consistent and re-liable pit design.

8 ACKNOWLEDGMENTS

The authors gratefully acknowledge the participation of Cerro Vanguardia SA, as well as the permission to publish and present this work. Marcos Calvente G. and Ariel Abad produced many of the models of the 2012 and 2015 studies, respectively. Their con-tributions are acknowledged here.

Herein are also credited Esteban Hormazábal, Pe-ter Terbrugge and Oskar Steffen, SRK consultants who acted as reviewers for 2012 and 2015 studies.

9 REFERENCES

Adamson, W.R., Muñoz, V., & Sarapura, G. 2011. Application of Technology for Final Wall Damage Control at Cerro Vanguardia, Argentina. Explo Conference Melbourne.

Barton, N. 1973. Review of a new shear strength criterion for rock joints. Engineering Geology (Elsevier), 7, 287-322.

Bieniawski, Z.T. 1989. Engineering rock mass classifications. New York: Wiley

Dempers, G. D, Seymour, C. R. W. & Harris, M. B. 2010. Op-timising geotechnical logging to accurately represent the geotechnical environtment, Second Australasian ground control in mining conference, Sydney.

De Giusto, J.M., Di Persia, C.A. & Pezzi, E. 1980. Nesocratón del Deseado. Turner, J.C.M. (ed.) Geología Regional Ar-gentina, Academia Nacional de Ciencias 2: 1389-1430, Córdoba.

Heather, Perez & Caram (2004). Final report, geological map-ping program, Cerro Vanguardia district.

Hoek, E. 1994. Strength of rock and masses. ISRM News Jour-nal 2 (2), 4–16.

Hoek, E., Kaiser P. K. & Bawden, W. F. 1995. Support of un-derground excavations in hard rock. AA Balkema, Rotter-dam.

Hoek, E. & Karzulovic, A. 2000. Rock mass properties for sur-face mines. Slope stability in surface mining, 59-67. Colo-rado. SME.

Hoek, E., & Diederichs, M.S. 2006. Empirical estimation of rock mass modulus. International Journal of Rock Mechan-ics and Mining Sciences 43, 203-215.

Hoek, E. 2012. Blast Damage Factor D. RockNews. Hormazábal et al. 2004. Optimización geotécnica rajos Cerro

Vanguardia. Laubscher, D. H. 1990. A geomechanics classification system

for the rating of rock mass in mine design. Journal of South African Institute of Mining and Metallurgy 90 (10), 279-283.

Read, J. & Stacey, P. 2009. Guidelines for Open Pit Design. CSIRO Publishing.

Uliana, M., Biddle, K., Phelps, D. & Gust, D. 1985. Significa-do del vulcanismo y extensión mesojurásicos en el extremo meridional de Sudamérica. Revista de la Asociación Geoló-gica Argentina 40: 231-253.

Schalamuk, I.B., de Barrio, R.E., Zubia, M., Genini, A. & Echeveste, H. 1999. Provincia auroargentífera del Deseado, Santa Cruz. Zappettini, E. (ed.) Recursos Minerales de la República Argentina, Instituto de Geología y Recursos Mi-nerales, SEGEMAR, Anales 35 (2): 1177-1188, Buenos Ai-res.

Zubia, M. A., Genini, A. D. & Schalamuk, I. B. 1999. Yaci-miento Cerro Vanguardia, Santa Cruz. Zappettini, E. (ed.) Recursos Minerales de la República Argentina, Instituto de Geología y Recursos Minerales, SEGEMAR, Anales 35 (2): 1189-1202, Buenos Aires.

Critical SRF: 1.24MaximumShear Strain

0.00e+0005.50e-0041.10e-0031.65e-0032.20e-0032.75e-0033.30e-0033.85e-0034.40e-0034.95e-0035.50e-0036.05e-0036.60e-0037.15e-0037.70e-0038.25e-0038.80e-0039.35e-0039.90e-0031.05e-0021.10e-002

220

200

180

160

140

120

100

80

-260 -240 -220 -200 -180 -160 -140 -120 -100