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NUMERIC MODELLING IN UDEC OF THE TINYAG MINE

EXPLOITATION

MODELO NUMRICO DE EXPLOTACIN DE LA MINATINYAG EN UDEC

ALFONSO RODRIGUEZ DONOIngeniero de Minas. Departamento de Ingeniera de los Recursos Naturales y Medio Ambiente, Universidad de Vigo, 36310

Vigo, Espaa. Contacto: [email protected]

LEANDRO R. ALEJANODr. Ingeniero de Minas. Departamento de Ingeniera de los Recursos Naturales y Medio Ambiente, Universidad de Vigo, 36310

Vigo, Espaa. Contacto: [email protected]

DAVID CORDOVADCR Ingenieros Sociedad Limitada. (Geomecnica en Minera y Obras civiles). C/Altamira 124 urb. Camino Real. La Molina.

Lima 12. Peru.

RESUMEN: El objetivo de este artculo es realizar un modelo numrico simplificado de las labores de explotacincon minera subterrnea (mtodo de hundimiento por subniveles) de la mina Tinyag, para estimar el posible

comportamiento del macizo rocoso afectado por las labores y, as, detectar posibles mecanismos de rotura (roturacircular, vuelco, etc.). El modelo tambin resulta interesante para identificar los mecanismos de hundimiento y

subsidencia asociados. En definitiva, se pretende tener una visin aproximada de los riesgos geomecnicosexistentes. Para llevar a cabo este modelo se ha utilizado el cdigo UDEC, cdigo numrico basado en el mtodo delos elementos discretos.

PALABRAS CLAVE: modelo numrico, UDEC, hundimiento por subniveles, rotura, subsidencia.

ABSTRACT: The aim of this paper is to perform a simplified numeric model of the labours in the Tinyagunderground mining exploitation (sublevel caving method), in order to estimate the possible behaviour of the rockmass affected by the labours, and to detect possible failure mechanisms (circular failure, toppling, etc.). The modelis also interesting to identify the subsidence mechanisms associated. After all, it is expected to have an approximatevision of the existent geomechanical risks. We have performed this model using UDEC, a code based in the discreet

elements method.

KEYWORDS: Numeric modelling, UDEC, sublevel caving, failure, subsidence


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1. INTRODUCTION

The Tinyag mine is located at Iscaycruz (Figure1) Oyn province, Lima department, Peru -and belongs to the mining company LosQuenuales S.A.. The mine is 4700 m.a.s.l. in theAndes.

Figure 1. Situation map of the Tinyag deposit

The upper part of the deposit till an elevationof 4544 m.a.s.l. has been exploited by means ofopencast mining (Figure 2). This exploitation hasalready been concluded because it had beenreached the economic limit. Below thatelevation, it is planned to continue theexploitation by means of underground mining,using the sublevel caving method, to be precise.This mining method has been proposed byCrdova et al. 2007 [1]. Their work is based onthe studies of other authors (Kvapil [2]; Bull &Page, 2000 [3]; Laubscher, 1994 [4]; Rustan,

2000 [5]), on the design proposed by aconsulting Chilean company (Krstulovic yOvalle, 2004 [6]) and on the experience acquiredin the design of the Rosaura mine (Crdova,2004 [7]). Furthermore, we think that it can beproved that the sublevel caving method is asuitable exploitation method in this case,following the method suggested in Nicholas &Marek (1981) [8].

Figure 2. Photograph of the open pit exploitation

Tinyag is a semi-tabular deposit, orientedparallel to the stratification, 200 m long, and

between 15 and 25 m thick. The ore is scatteredin a skarn, forming sulphide massive bodies,mainly Zn. The average grade in Tinyag is 7.7 %Zn. In the hanging wall there are horizons ofpyrite, oxides, silica, quartzite-marls andquartzite. In the footwall there are horizons ofpyrite, shales and altered shales, dolomitic shaleswith sandstones, and sandstones with shales.

The aim of this work is to carry out a simplifiednumeric model of the labours in the Tinyagunderground mining exploitation, in order to

estimate the possible behaviour of the rock massaffected by the labours, and to detect possiblefailure mechanisms. It is foreseen that circularfailure or toppling could be the most probablefailure mechanisms. The model is alsointeresting to identify the subsidencemechanisms associated to the sublevel cavingmethod. After all, it is expected to have anapproximate vision of the geomechanical risksthat we are going to face. We think that thenumeric model will make an importantcontribution to achieve these goals.

We have used UDEC (Universal DistinctElement Code), developed by Itasca (2004) [9],to carry out this model. UDEC is a numeric codebased in the discrete elements method. UDECsimulates the behaviour of discontinuous mediasubjected to loads. The discontinuous medium isrepresented by means of discrete blocksconnections, which can behave as rigid or


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deformable blocks. Joints and discontinuities areconsidered as contour conditions betweenblocks. This allows great block displacements,including its separation and rotation. This isconsidered one of the main advantages of UDECamong other codes.

This code includes several behaviour models,which make easier the modelling of differentmaterials and geological structures. UDEC isbased in lagrangian calculations, very suitablefor the simulation of great displacements anddeformations in a medium formed by blocks.UDEC is especially suitable to detect physical

instabilities in rock mechanics engineeringproblems, related with open pit or undergroundmining.

2. MODEL DEVELOPMENT AND

RESULTS

The model was developed taking into accountthe geological sections and the geometry of themine, provided by David Crdova, consultant ofthe exploitation. There have been also used theavailable consultancy reports to propose asimplified model (Figure 3).

Figure 3. Simplified model of the geological cut of the Tinyag mine

Therefore, we distinguish 9 different rockmasses, plus a rock fill added to the sill. Thosematerials have been numbered from 12 to 21, asfollows:

12. Rock fill13. Skarn mineral14. Shale-sandstone15. Earthy pyrite

16. Earthy pyrite17. Sandstone18. Marl19. Quartzite20. Marl21. Quartzite

The skarn mineral, the earthy pyrite and the rockfill have been considered as discontinuous

material. This material constitutes blocksaccording to three joint sets, which form anglesof 0, 70 and 110 degrees to the horizontal. Therest of the layers have been modelled as acontinuous deformable media.

Iscaycruz has a geomechanical database with theformat recommended by the ISRM (Brown,1981 [10]). This database is periodically

updated. The geomechanical properties of thedifferent rock masses (table 1) are referred to thestrength and elastic parameters. These propertieswere estimated according to the Hoek &Browns failure criterion (Hoek & Brown, 1980[11]; Hoek & Brown, 1988 [12] and Hoek et al.,2002 [13]), with the help of Bieniawskis RMR(1989) [14], and also with the aid of the RocLabsoftware (Rocscience, 2002 [15]).


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Table 1. Geomechanical properties of the Tinyag rock mass

LithologyCohesion

(KPa)

Friction

angle

Deformation

modulus (MPa)

Poisson

ratio

Bulk Modulus

(K) (MPa)

Shear Modulus

(G) (MPa)

Pyrite 80 24 285 0,3 238 110Sandstone 120 28 1500 0,28 1136 586

Marl 100 24 620 0,28 470 242

Quartzite 180 31 2500 0,25 1667 1000

Ore 130 31 1010 0,28 765 395

Skarn 130 31 1600 0,28 1212 625

Shale 140 29 1800 0,26 1250 714

Rock drill 5 35 80 0,35 89 30

The shear strength characteristics of the existent

joints in the different rocks are controlled by thefriction and cohesion parameters of the Mohr-Coulomb failure criterion (Mao-Hong Yu, 2002[16]). The determination of these parameters wasbased on in-place characterization of JRC, JCSand basic friction angle. The results of theseessays are presented in table 2.

Tabla 2. Properties of the discontinuities

LithologyCohesion

(KPa)

Friction angle

Pyrite - -Sandstone 53 30

Marl - -Quartzite 66 30

Ore 63 32Skarn 74 30Shale 3 27

On the other hand, we put forward theexploitation sequence pointed out in figure 4,where the ore is being extracted in 4 phases,following a numerical sequence from 1 to 4. Thisexploitation will produce a progressive

subsidence and affections to the rock mass.

Thereby, the model would be the one shown infigure 5, where we can observe that phase 1 hasalready been exploited. As we can observe infigure 6, the collapse of the blocks takes place asthe model is running. These blocks tend to fillthe exploited area. The blue arrows represent thedisplacement vectors of the different blocks.

Figure 4. Exploitation sequence in the Tinyag mine

Figure 5. UDEC model of the Tinyag mine. Phase1


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Figure 6. Collapse of the blocks after the mining ofthe ore in phase 1

In figures 7 and 8, we can observe theexploitation phase 2 in its beginning and in itsending. At the end of phase 2, we can observethat a certain subsidence has been produced inthe surface. The subsidence phenomena shouldalways be taken into account when the sublevelcaving method is used. This phenomenon hasbeing studied from the 70s (Hoek, 1974 [17]).We can estimate the area affected by subsidenceby means of the fracture and drop angles(Cavieres and Daz, 1993 [18]), but the aim ofthis paper is just the numerical modelling of the

subsidence. The authors of this paper havealready some experience in this subject (Alejanoet al., 1999 [19]).

Figure 7. Phase 2 - Beginning

Figure 8. Phase 2 - Ending

In the exploitation phase 3 (figures 9 and 10), wecan observe that, besides the subsidence, certaintoppling begin to take place. We can identify thisphenomenon through the cracks and stepsproduced in the surface.


When the model reaches the phase 4 (figures 11,12 and 13), large deformations and large shearstresses are produced. These stresses induceplastic deformations that could give rise tocircular failure (red asterisks in figure 13).


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3. CONCLUSIONS

A numeric model of the Tinyag mineexploitation, in Peru, has been built with the aidof the UDEC code. The underground miningmethod selected is the sublevel caving method.The aim of this work is to assess the behaviourof the rock mass affected by the exploitation and

to identify the possible failure and subsidencemechanisms that could take place.

In the numeric model, the exploitation has beencarried out in 4 phases, and we have been able toobserve how the collapse of the rock mass takes

place, as described by other authors like Kvapil(1992) [2]. We have observed also that asubsidence occurs in the surface, as it is usual insublevel caving and as it actually happened inthe Tinyag mine, as we can see in figure 14.


Finally, we have observed that toppling of theblocks in the hanging wall could take place and,as a matter of fact, tensile cracks have arisen inthe mine wall, as we can see in figure 15.Moreover, we have observed that large shearstresses and large plastic deformations can showup. And we think that these stresses could giverise to a circular-type failure in the back wall(figure 13).

Therefore, we believe that the appropriatestudies and the adequate actions should be takento avoid these studied phenomena to take placeor, at least, to avoid any human or equipmentloss.


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Figure 13. Phase 4 Plastic deformations (red asterisks) produced by shear stresses

Figure 14. Subsidence in the bottom of Tinyag mine. Figura 15. Tensile crack in a slope of the Tinyagmine.


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Acknowledgements:

The authors thank the Spanish Ministry of Scienceand Technology for funding under contract referencenumber BIA2006-14244 for the research projectentitled Analysis of rock mass post-failure

behaviour.

REFERENCES

[1] CRDOVA, D., CUADROS, J. & ALEJANO,L.R., 2008. Practical considerations and models of thesublevel caving exploitation Tinyag in Peru. 5thInternational Conference and Exhibition on MassMining, Lule, Sweden. 2008. p. 751-760.

[2] KVAPIL, R., 1992. Sublevel caving. In: SMEMin. Eng. Handbook Littleton, CO: Soc Min MetalExplor; p. 1789814.

[3]BULL, G. Y PAGE, C.H., 2000. Sublevel Caving Todays Dependable Low-Costs Ore Factory.MassMin, Brisbane, Australia. 2000. p. 537-556.

[4] LAUBSCHER, D., 1994. Cave miningthestate of the art. J. South Afr. Inst. Min. Metall.1994;94:27993.

[5] RUSTAN, A. (2000). Gravity flow of brokenrockwhat is known and unknown. In: Proceedingsof the MassMin 2000, Brisbane. p. 55767.

[6] KRSTULOVIC, G. Y OVALLE, A., 2004.Estudio de perfil, explotacin subterrnea de minas

Rosita y Tinyag. Metlica Consultores S.A. InternalReport. Santiago. Chile.

[7] CRDOVA, D., 2004. Geomechanicalevaluation of the mining method in Rosaura.PERUBAR S.A. (In Spanish). Unpublished Report.

[8]NICHOLAS, D.E. Y MAREK, D., 1981. Designand operation of caving and sublevel stoping mines.Ed. American Institute of Mining, Metallurgical andPetroleum Engineers.

[9]ITASCA CONSULTING GROUP, 2004. UDEC.Universal Distinct Element Code. Version 4.0.Minneapolis. Minnesotta. USA.

[10] BROWN, E.T., 1981. Rock CharacterizationTesting and Monitoring. Ed. Pergamon Press.Oxford, RU.

[11]HOEK E Y BROWN E.T., 1980. UndergroundExcavations in Rock. Champan & Hall: London,1980.

[12]HOEK, E. Y BROWN, E.T., 1988. The Hoek-Brown failure criterion - a 1988 update. Proc. 15thCanadian Rock Mech. Symp. (ed. J.C. Curran), 31-38. Toronto, Dept. Civil Engineering, University ofToronto.

[13] HOEK, E., CARRANZA-TORRES, C. YCORKUM, B., 2002. Hoek-Brown failure criterion 2002 edition. In R. Hammah,W. Bawden, J. Curran &M.Telesnicki (Eds.), Proceedings of NARMS-TAC2002, Mining Innovation and Technology. Toronto,

2002.pp. 267-273. University of Toronto.

[14] BIENIAWSKI, Z.T., 1989. Engineering rockmass classifications. New York: Wiley.

[15] ROCSCIENCE, 2002. RocLab. Rock massstrength analysis using the H-B failure criterion..Rocscience Inc. Toronto.

[16] MAO-HONG YU; Advances in strengththeories for materials under complex stress state in

the 20th Century; Applied Mechanics Reviews;American Society of Mechanical Engineers, NewYork, U.S.A.; May 2002; 55 (3): pp. 169218.

[17] HOEK, E., 1974. Progressive caving inducedby mining an inclined orebody. IMM Section A:

A133-A139.

[18] CAVIERES, P.H. Y DAZ, J.A. ,1993.Evaluation of angles of break and fall to define thesubsidence of the El Teniente mine. TechnicalReport, (In Spanish ). El Teniente Division,CODELCO, Chile.

[19] ALEJANO, L.R., RAMREZ-OYANGUREN,P. & TABOADA, J., 1999. FDM predictivemethodology of subsidence due to flat and inclinedcoal seam mining. Int. J. Rock Mech. & Min. Sci.36, 475-491.

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