costa paper

Slope Stability Santiago Chile, November 2009

Geological and geotechnical characterization and failure mechanism of Friable Hematitic Ore in Vale iron ore mines. West of Iron Ore Quadrangle, MG Brazil

The main typology in Vales iron ore mines is Friable Hematitic Ore. This typology is the primary focus for geotechnical consideration due to its abundance and also because it is one type that commonly occurs in final open pits and has a low mechanical resistance.Catastrophic slope failures have occurred in iron ore mines leading to either fatalities or loss of production. The main causes of failure are the shear mechanisms in the pre-existing anisotropy and stress concentrations in the toes of slopes which are commonly and predominately composed of Friable Hematitic Ore.To address this issue, it is common practice for geotechnical engineers to leave a buttress of ore at the toe, leaving reserves.The design for slope stability is based on Limit Equilibrium Elements analysis and in some cases with the aid of numerical analysis. Parameters are normally derived from lab tests and geomechanical classification.In this paper, on the other hand, four approaches are used: petrographic and microstructural analysis of thin sections, testing and characterization of resistance in lab samples; geological and geotechnical characterization and back analyses with computational models (finite element and limit equilibrium):

Thin sections were analyzed to determine micro-characteristics of the rock and discontinuities and to verify their response upon deformation. About twenty samples of various mines representing the Friable Hematitic Ore were used.Laboratory tests were carried out in blocks of various sizes and different directions, to determine strength and deformability parameters of intact rock, as well as physical and chemical parameters of the rock type.Geological and geotechnical characterization to macroscopically characterize rock types at bench scale. A rock mass classification systems adapted to iron ore was also used during mapping. Finally, the data obtained in previous stages was used in computational back analysis using the softwares PHASE2 and SLIDE (Rocscience).

This combined approach will best determine deformation and strength parameters, improve the understanding of failure mechanisms and thus promote a more complete and comprehensive understanding of the behavior of such materials during pit excavation. From this, pit design can be optimized so that maximum ore can be mined while ensuring necessary safety.

Abstract

T. A. V. Costa

Vale

R. P. Figueiredo

UFOP Universidade Federal de

Ouro Preto

P. B. Franca

Vale

INTRODUCTION

Justification In Vales ferrous south division, more than 70% of all rich iron ore deposits are composed of Friable Hematitic Ore (FHO). FHO is

one of the lithological ore types of lower geomechanical resistances that occur in final open pits. After mining of FHO, the resistance characteristics of friability and strain hardening alter and this combined with the relative low strain levels, make typology the target for geotechnical investigation.

Geological and structural settings impose a stratigraphical position on the FHO and as such, FHO is commonly present at the base of the slopes and therefore in the highest stress portion. This configuration is very common in Vales open pit mines and some global slope failures have occurred with low strain conditions but very rapidly leading to either fatalities or loss of production.

Usually geological mapping and geomechanical classification are sufficient to determined the main failure mechanisms while, limited equilibrium analyses, using Morh Coulomb stress parameters, is efficient to determine the best safety factor.

Santiago Chile, November 2009 Slope Stability

Increased world iron ore demand leads to increases in the mining production rate and this effectively leads to increases in the average depth of mining for open pit operations. For these large open pits, a high level of finite element analyses, structural mapping, correlation with microscale samples and lab analysis is necessary to ensure good comprehension of slope failure mechanisms and accurate determination of the safety factor.

ObjectivesThe main objectives of this paper are: geological and geomechanical characterization in micro and macro scale using triaxial tests

to correlate strength parameters and main failure mechanisms for FHO slopes. Secondly, these approaches will help open pit design optimization in terms of maximizing mined ore whilst ensuring the necessary level of safety.

MethodologyTo obtain the expected results, four approaches were used:Geological and geotechnical characterization was adopted to characterize rock types at a bench scale. A rock mass classification system adapted for iron ore was also used during mapping.Laboratory triaxial tests were carried out on blocks of various sizes and in different directions, to determine strength and deformability parameters of intact rock, as well as physical and chemical parameters. The software ROCDATA (Rocscience) was used to analyze the samples in order to evaluate the results for different stress criteria and deformation levelsThin sections were analyzed to determine the intact rock and discontinuity micro-characteristics and verify their response upon deformation.Finally, the data obtained in previous stages was used in computational back analysis using the software PHASE2 and SLIDE (Rocscience).

LocalizationVales iron ore division is centred in the state of Minas Gerais, Brazil, more specifically in the western side of the Iron Ore Quadrangle.

The main iron ore mines of the southern ferrous division (DIFL), are deposited in a linear group of hills geologically called the Moeda Syncline and Curral Homocline.

The mines valued in this paper include the guas Claras and Mutuca mines in the north (close to Belo Horizonte) and Tamandu, Capito do Mato and Pico mines in the south (close to Itabirito town). A map of the mines and their locations can be seen in figure 1.

Figure 1 - Localization map showing the DIFL/ Vale worked mines and the Iron Ore Quadrangle configuration.

GeOlOGICal SeTTINGS

The studied area is localized in the western part of a geotectonic structure called the Iron Ore Quadrangle, which is located in the southern border of the So Francisco Craton. The structure is delineated by a roughly quadrangular arrangement, with Proterozoic banded iron formations


(BIFs) of the Minas Supergroup composted with hundreds of meters of metamorphic rocks. Within this formation exists an especially rich iron ore called Itabirito from the Itabira group, which will be the focus of this paper. The Minas Supergroup comprises, from bottom to top, the Caraa, Itabira, Piracicaba and Sabar groups, a sequence of psamitic pelitics rocks (Dorr, 1969).

Below this is the Archean greenstone belt terrains of the Rio das Velhas Supergroup and domes of Archean and Proterozoic crystalline rocks (Machado et al., 1992; Machado and Carneiro, 1992; Noce, 1995).

The regional structure is the result of two main superpositional deformational events (Chemale Jr. et al, 1994). The first produced the nucleation of regional synclines in the uplift of the gneissic domes during the Transamazonian Orogeneses (2,1 2 Gyr). The second is related to an east/ west vergent thrust belt of Pan African Brasiliano age (0,8 0,6 Gyr) (Marshak et al., 1992). This event deformed the earlier structures and was mainly responsible for the deformational gradient. The intensive structuration developed folds, foliation, shear zones and an eastward increased metamorphic zoning and follows the deformational gradient to greenschist to lower amphibolites facies (Hertz, 1978).

Regionally, the main iron ore deposits of Vales southern division are located in the western low strain domain of the Iron Ore Quadrangle, most of them in the eastern limb of the Moeda Syncline (Mutuca, Tamandu, Capito do Mato and Pico mines). The main trend of the syncline is N S but the structure has also been deformed around Bao dome. To the south, the structure is interconnected with the Dom Bosco Syncline and partially truncated by the Engenho Fault. To the north it is continuous with the Serra do Curral. In this junction, a northwest verging asymmetric anticline and an interference saddle was developed due to the refolding (Rosiere et al., 2003).

The Moeda Syncline has been partially affected by the younger Brasiliano tectonic plate movement and this affect occurred mainly in the eastern limb with the local development of ductile brittle to brittle shear zones. These shear zones cut all lithologies or are sub parallel to the bedding planes. Several strike slip faults cut across the structure dividing it in several segments. The Serra do Curral represents the overturned southeastern limb of a truncated NW vergent syncline anticline couple. It was highly strained and rotated by a right lateral movement of the NE SW trending oblique ramp of a thrust fault. In this segment, the northwestward inverted limb of a syncline is truncated near the contact of the minas Supergroup in the underlying Rio das Velhas Supergroup by shear zones elated to the thrust. This occurrence is the foundation for the guas Claras mine. All the structures and geological settings can be seen in figure 2.

Figure 2 - Geological map of northwest portion of Iron Ore Quadrangle, lithostatigraphical column and localization of the worked mines

eXPeRIMeNTal

Geological and Geomechanical CharacterizationThe high grade iron ores focused on in this paper, typically have iron (Fe) contents less than 64% and very low proportions of silicon

oxides (SiO2,), aluminum oxides (Al203) and others contaminants. Usually, Vales rich iron ores contain more than 64% Fe and are divided


by lithotypes. Normally, this division is based on laboratory testing of crushed material resistance. These tests result in three different classifications; Hard Hematite Ore (more than 50% above 6.35 mm), Medium Hematite Ore (50% to 25% above 6.35 mm) and Friable Hematite Ore (FHO) (less than 25% above 6.35 mm).

Only hematite mineral and specularite can be identified. In some cases, a cataclatic mass of friable material predominates. The hydroxyl minerals as goethite are superficially or fault zones restricted. Structural settings are especially hard because of the friability characteristics which make it difficult to identify the structure. and the hydroxy minerals.

At a slope scale, FHO have a dark metallic gray color and are very friable. In general, the ore bodies in the Minas Supergroup open pit mines have the following configuration; compact hematite ore is contained within the centre of the fold whilst friable hematitic ore dominates in the surface, fold limbs, shear zones and brittle failures.

The visual porosity is very high and apparent cohesion is very low but, in most cases, the original (S0) or the tectonical foliation (Sn) structuring was preserved. Particularly, the meso banding can be determined by the interface between more massive layers with metallic bright color and low cohesions layers (more friable) with opaque gray color.

Using field evaluations three main different types could be identified in the mines studied, characterized by:

BeddingBedding layers with less cohesion in-

terface with more cohesive layers. Higher lateral extension, present in the folds limbs, above the itabirite rocks and in low deformation areas. This is the main type throughout all open pits.

At a bench scale, shear failure in a planar surface is the main failure mecha-nism.

The anisotropy can be easily determined and controls all failure mechanism.

The resistance parameters are con-trolled by the thickness and percentage of more cohesive bedding layers.

BrecciaVisually, as a homogeneous mass and

fine matrix associated with collapsed structures (non tectonics structures) and brittles faults. Very low apparent cohesion and high porosity. Bench scale failure typi-cally occurs through shear mechanisms usually in circular surfaces.

It is more difficult to determine the an-isotropy and the discontinuities in field; and in the same way, the resulting defor-mation cracks are only visualized close to the failure.

Commonly, can be associated with stress zones and stress failure mechanisms.

FoliatedIn field valuations it is easy to determine

the anisotropy by the millimetric foliation, strong anisotropy, low apparent cohesion and specularity contained. Geologically there is low lateral extension associated with shear zones and ductile faults, con-centrated in high deformation areas.

In bench scale, failures are common thro-ugh planar surfaces in shear mechanisms.

The resistance parameters are determi-nate by the specularite mineral contents in the foliation.

Occurs in very restricts portions in the mines but commonly associated with the global scale failures.

Even using a geomechanical classification adapted for Vale iron ore mines to characterize the FHO, it is not ease to determine the three FHO types. The low apparent cohesion of Vales FHOs and a cohesion below 3 (H-3) limited the use of RMR classification (adapted Bieniawisk, 1989). The best classification possible using RMR is class IV (below 30) for Bedding FHO and class V for Breccia and Foliated FHO (about 20). The main problem with the use of RMR classifications is the identification and characterization of discontinuities.


In the same way, for GSI characterization, the use of specific GSI tables (Marinos and Hoek, 2004) provides a range of GSI value from 5 to 45. This range is too large and does not provide enough specific information to accurately determine parameters, in order to determine type.

As a result, Vales geomechanical studies have focused on a way to minimize this problem in order to improve the geomechanical classification system for friable rocks in iron ore mines.

Laboratory AnalysesThe reports from Mutuca, guas Claras, Tamandu and Pico mines were evaluated based on physical analyses, granulometry, density

and humid percentage. Additional tests for triaxial deformation were completed on samples from different directions and with different sizes of FHO. This evaluation showed:

The granulometric curve shows a fine sand to silt composition with the following parameters;

the specific gravity samples has an average of 3.93 (s.d.=0.38)humidity percentage of 15% (s.d = 5%) void ratio equal to 0,75 (e=0.75).

All the triaxial results (35 in total) were grouped in the same graphic using the software, ROCDATA. This resulted in a cluster of points divided into low and high stress levels using the value 1. An average of about 2 MPa was used as the cut off limit, with values below this classified as having low stress ratio (1< 2 MPa), and values above classified as having a high stress ratio value (1>2 MPa). This can be seen in Figure 3. The result parameters from this division are presented in Table I.

These differences were used to consider the anisotropy parameters, using lower values for parameters parallel to anisotropic and higher values for the parameters perpendicular to anisotropy. This approach was used because it was not possible to determine (in reports) the real relationship of the test direction and the anisotropy.

Figure 3 - Triaxial texts using Morh Coulomb adjustments, showing the limits used to divide the high and low values.


Litotype Frictional Angle Cohesion Morh Coulomb Curve

FHO parallel 34 20 Kpa 12 MPa

FHO isotropic 34 65 Kpa All 1

Table I - Stress parameters adjusted for high and low stress deformation levels and adapted for anisotropy.

Micro Scale CharacterizationsUsing a total of 20 thin sections from the mines of guas Claras (5), Tamandu (10) and Capito do Mato (5) the following characteristics

could be defined: mineralogical composition, crystal shape, micro porosity, cementation, discontinuity, granulometry and structural settings.

The petrographic analyses showed a monomineralic composition essentially of hematite crystals in variable shapes and sizes. Secondly, martite, specularite, magnetite; quartz and clay minerals were present as gangue. The main cementation mineral was goethite which was especially abundant in surface areas and in fault zones.

Of the porosity observed, 70% of the total porosity was attributed to the intergranular properties whilst the intragranular and fissural porosity combined was 30%. The anisotropy is easily the determining factor in compounding the banding or the foliation, using the orientation of the crystals and the void alignments. In some cases it was possible to see two anisotropy directions. The minerals shape, size and content of different mineralogys determined different types of anisotropy, characterized by microbanding while the micro scale determined three different types of FHO.

BeddingIn total, of the thin sections analyzed, 24%

of hematite crystals were sinuous, granular or lobular shape. The size analysis showed that the hematite crystals had the biggest relative min-eral sizes (more than 1 mm). The orientations of crystal were determined by the recrystallization portion. Anisotropy was largely determined by the interface layers, the level of porosity and the tabular shape of hematite. The void percentage was relatively medium. The bedding had typical layers of predominately high porosity granular graniloblastic crystals interfaced with a layer of tabular lepidoblatsic low porosity layers.

Foliated: Composed of tabular hematites (37%), the micro plates were oriented with the 0,15 mm size crystals and specularite showing a lepidoblastic morphology of 11%. The specularite was substantially lengthened by deformation. The porosity content was low as a result of the goe-thitic cementation. The anisotropy is easily deter-mined because specularite is commonly found in the discontinuities.

Breccia: Have typical bimodal composition: in clast layers, there was a predominately gran-ular crystal of hematite with a high porosity; in matrix layers, there was predominately macro plates of hematite oriented with the flow and with low porosity content. The anisotropy is less in-tense but can be determined. This configuration is a result of brittle deformation.

The anisotropy could be defined by the layers of fine hematite minerals.

Back AnalysisIn order to better understand the failure mechanism, the geological and geomechanical information as well as the lab tests were used

in two back analysis valuations.


The Morro do Patrimnio rupture, a large scale failure, occured in April, 1992 in the guas Claras mine. This involved a slope failure of more than 240 meters height and resulted in the mobilization of about 2 millions tonnes of material.

The Mina Velha rupture, a small scale failure, occurred in November, 2006 in the Pico mine when a 75 meter high slope failure occurred and mobilized about 100.000 tonnes.

In case studies limited equilibrium and finites elements analyses were used to understand the failure mechanisms and check the stress and strength parameters. The parameters used can be viewed in tables II and III.

Resistance Parameters

cp//

(Mpa)c

p+(Mpa)

cr

(Mpa)

p//()

p+

()

r()

(KN\m3)

FHO 0,070 0,092 0 33 34 6 27

Table II - Stress parameters used in SLIDE analyses.

Table III - Stress and strain parameters used in PHASE2 analyses.

The Patrimonio Failure, largely studied by Franca (1997) and others, occurred in the Serra do Cural Hill and the failure resulted in the movement of material to a height of 200m, a width of 100m and a reduction in the mountain crest of 30 meters. The failure surface occurred in the geological contact between FHO and Friable Siliceous Itabirite (or the bedding of FHO) and was typically planar circular with a base of displacement in the slope toe. Figure 4 A shows the land slide failure that occurred at MAC in 1992 whilst figure 4 B shows the typical SLIDE cross section used to study the failure mechanism.

Figure 4 - Patrimnio failure in 1992 (guas Claras mine), in B typical SLIDE cross section and geological setting.


Figure 5 shows the SLIDE analyses. Figure 5 A shows a representation of the slope failure in relation to the geological conditions and geometry. In figure B the same configuration using a pre existing surface according to the FHO bedding is shown.

For both analyses the geometry was unfavorable for stability for the use of all types of FHO. The factors of safety (SF) values were below 1 and a study of all the possibilities indicated that there were at least two probable types of surfaces for failure. Using SLIDE analyses, shear failure mechanism was possible in both cases for non circular, (multiple surfaces) and planar surface (using the FHO bedding and the geological contacts).

Figure 5 - SLIDE analyses, in A multiples surfaces and B pre-existing surface using the geological contact and the anisotropy parameters, parallel.

Figure 6 shows the PHASE2 analyses. In A using a pre existing surface, the rupture surface and the total displacement evaluation in field is shown. The concentration of the total displacement at the bottom of the toe cannot be seen as the concentration occurs in the middle. In B the total displacement is not concentrate in the toe of slope but the displacement portion can easily be determined in the contact between the FHO and FSI.

Figure 6 - PHASE2 analyses, in A pre existing surface, in B a free stress deformation. Both cases show the total displacement valuation and a low concentration in the toe.

At the Pico do Itabirito mine, a failure occurred in April 2002 whereby material of 70m high by 40m was released resulting in a loss of 3m in the 1240m bench. For the Mina Velha slope failure analyses, two different groups of stress parameters were used. Figure 7 shows in A the land slide just below Itabiritos Peak and in B the typical cross section used in PHASE2 and the geological composition.


Figure 7 - shows in A the land slide failure just below Pico do Itabirito and in B the typical cross section used in PHASE2 and the geological composition.

For SLIDE analyses of the Mina Velha slope failure, two different groups of stress parameters and non circular surfaces were used. Figure 8 A has non favorable stability parameters, parallel to anisotropy, typically used in Breccia and Bedding FHO types. In this analysis, it was possible to determine a single surface to represent the failure.

In Figure 8 B for favorable stability parameters, perpendicular to anisotropy and typically used for Foliated FHO type, it was possible to find a single surface and a conjugated surface could also have been used.

Figure 8 - SLIDE analyses in A global no circular surface, and in B multiple no circular surfaces.

The PHASE2 analyses, as seen in figure 9, shows the differences in displacement between undercut and non undercut slopes. In 9 A, the total displacement for a shallow (2.5 m) undercut slope can be seen as well as the concentration in the toe of the slope. In figure 9 B, the level of displacement for a non undercut slope can be seen and this figures shows levels of 3 times less than in A. In both cases a displacement concentrated in the toe occurs.

Figure 9 - PHASE2 analyses showing the total displacement in A in an undercut slope and the total displacement in `B for non undercut.

0 Santiago Chile, November 2009 Slope Stability

DISCUSSION

It is typical for all types of FHO to have, at least, one strong anisotropy, and this direction can be represented by the orientation of granuloblastics hematite, tabular hematites, specularite, and the intragranular void orientation. In lower deformation areas the anisotropy is represented by bedding (Bedding FHO) and in stronger deformation areas by foliation (Foliated FHO). The breccias are associated with faults or a non tectonic collapse (Breccia FHO).

It is typical for Bedding and Breccia FHO to have bimodal composition in terms of shape, size of hematite crystals and void content. On the other hand, Foliated FHO is more homogeneous and has a stronger anisotropy.

Figure 10 shows the anisotropy in various scales.

Figure 10 - Various scale valuation of FHO anisotropy.

Table IV - Summary of more important FHO characteristics.

The FHO lab test shows a high density and high porosity (more than 25% of voids) in a very thin sand composition rock. The triaxial tests present low average in frictional angle (33 to 34) but in terms of cohesion these average is almost 30 % (0,065 to 0,092 MPa).

Geomechanical characterization, using the classical geomechanical tables (RMR or GSI) appear to be inadequate to determine the difference between the three FHO types. It is probable that the low matrix resistance and the high anisotropy importance makes the geomechanical characteristics of this material very close to soil, and as a result, it requires an adjusted rock mass classification.

If the type of FHO can be accurately determined in the field, the resistance parameters can be better understood and used to improve the understanding of the failure mechanism. The table below shows the mains characteristics of three types of FHO.

CONClUSIONS

The orientation of minerals and void imposes a strong anisotropy in the rock and this information must be taken into consideration in slope analyses. The stress resistance parallel to foliation is, at least, 30% lower than that when perpendicular and the main failure mechanisms of FHO are shearing and controlled by anisotropy.

By analyzing the shear mechanism with Limited Equilibrium analyses, it was possible to represent the failure mechanism and valuated


the importance of anisotropy in these mechanisms. This was possible across a range of slopes including very high and very low slopes.

Lower resistance parameters parallel to anisotropy (bedding and foliated) imposes planar failure geometry in a shear mechanism. On the other hand, higher resistance parameters perpendicular to anisotropy improve circular failure geometry. In these cases the probability of stress deformation is increased, especially in the higher slopes. Shear failure mechanisms are typical, especially when the anisotropy is unfavorable for stability. In special cases, the stress caused by geometry changes (under cutting or tridimensional geometry effect) can predominate.

Higher density and apparent cohesion, especially the high frictional angle of FHO, make this typology a strong fine sand material, with soil characteristics of a low resistance rock material.

In reality, for Vales iron ore mines in the iron Ore Quadrangle that do not have excessively high slopes, the use of Limited Equilibrium analyses and Mohr Coulomb resistance criteria is sufficient to understand the failure mechanism. However, in the case of slope undercutting or high slopes, it is very important to use Finite Elements analysis, especially in slopes where the anisotropy does not control the failure mechanism or when the Breccia FHO is concentrated in the toe of slopes.

In these cases, the high resistance of FHO could not fail by shear mechanism as usual, but high stress and low deformation could produce a catastrophic failure.

To reduce the risks, the FHO stress strain parameters have to be well understood for the three types and the use o Finite Elements analyses has to take part in daily valuations done by the geotechnical team.

aCKNOWleDGeMeNTS

The development of this paper was based on a master degree presented in Ouro Preto Federal University in July 2009 as a part of the NUGEO program. Acknowledgements are due to Rodrigo Figueredo and Paulo Franca and all the discussions and orientation. Special thanks to Fabio Magalhes, Milene Sabino for the suggestions, Fabiana Cruz and Hope Adams for his valuable suggestions in reviewing the text and the support of Vale.

ReFeReNCeS

1. T.V.C. Aquino; P. R. Franca; R. P. Figueiredo and S.N.O. Brito. Tombamento em Filitos, um exemplo da Mina do Pico: Metodologia de Anlise Computacional utilizada nos taludes da MBR. 5 Simpsio Brasileiro de Aplicaes de Informtica em Geotecnia. Belo Horizonte, Minas Gerais. 185-191p, 2005

2. Z. T. Bieniawski. Engineering Rock Mass Classifications. John Wiley & Sons, New York, USA, 251 p, 1989.

3. Jr F.Chemale, C. A Rosiere and I.Endo, The tectonic evolution of the Quadriltero Ferrfero, Minas Gerais, Brazil. Precambrian Research, 65, p. 25 54. 1994.

4. J.V.N, Dorr. Physiographic, stratigraphic and structural developmente of the Quadriltero Ferrfero, Minas Gerais, Brazil. USGS. 110 p. (Professional Paper 641A).1969.

5. P.R.B, Franca. Analysis of Stability Using Limit Equilibrium and Numerical Methods With Case Examples from The Aguas Claras Mine, Brazil. Master of Science. Queens University. Kingston, Ontario, Canada.203p.1997.

6. N. Hertz. Metamorphic rocks of the Quadriltero Ferrfero, Minas Gerais, Brazil: U. S. Geological Survey Professional Paper 641C,.p78. 1978.

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