Rock Mechanics for Natural Resources and Infrastructure

SBMR 2014 – ISRM Specialized Conference 09-13 September, Goiania, Brazil

© CBMR/ABMS and ISRM, 2014

SBMR 2014

Borehole Camera And Extensometers To Study Hanging Wall

Stability – Case Study Using Voussoir beam - Cuiabá Mine

Reuber Ferreira Cota

Anglogold Ashanti, Sabará, Brazil, rfcota@angolgoldashanti.com.br

Rodrigo Peluci de Figueiredo

Federal University of Ouro Preto, Ouro Preto, Brazil, rpfigueiredo@demin.ufop.br

SUMMARY: Cuiabá mine, owned by Anglogold Ashanti Córrego do Sítio Mineração, is located in

Sabara-MG, with excavations more than 1100m below surface, is one of the most important

underground gold mines in Brazil. In recent years, there have been significant problems of

instability of the hanging wall (HW) in some stopes (production excavation). In order to understand

and anticipate the problems of instability of the hanging wall, a monitoring system was

implemented consisting of televised boreholes, in the walls of the excavation. This was in addition

to the large number of Multiple Point Borehole Extensometers (MPBX) and SMART cables

(Stretch Measurement to Assess Reinforcement Tension) installed in the mine. This paper presents

an example of the identification from monitoring, in the Fonte Grande Sul orebody – level 10.2

stope (about 680m below surface), with evidence of instability in the hanging wall. The observation

of borehole cracks, shears, failures, and displacements, indicated the beginning of instability in the

hanging wall, which allowed measures to be taken to stabilize this area. A detailed follow-up

confirmed the stabilization after actions have been implemented. In order to exploit the data

collected during the process of study and to attempt to validate a simple method for evaluating the

stability of the hanging wall in schist, a stability study was performed using the voussoir arch

theory. Despite the identification of the thickness of the beams formed within the hanging wall, the

geological complexity, evidenced by interbedded rocks with different elastic characteristics and

strength, folds and boudinage, which was beyond the simplification of the calculations, did not

allow a proper assessment of the stability of the studied area using the voussoir arch theory.

KEYWORDS: Underground mine, rock mechanics, borehole camera, extensometers, voussoir

beam.

1 INTRODUCTION

Cuiabá mine, owned by Anglogold Ashanti

Córrego do Sítio Mineração, located in Sabara-

MG, with excavations more than 1100m below

the surface, is one of the most important

underground gold mines in Brazil, with annual

production of approximately 9 tons of gold.

Following an increase in production in

beginning of 2007, problems of instability,

mainly in hanging wall, were identified in

mining excavations (stopes). A study to

understand and predict these problems was

implemented comprising of constant monitoring

with borehole cameras of boreholes with

borehole cameras and extensometers MPBX

(Multi Point Borehole Extensometer) and

SMART (Stretch Measurement to Assess

Reinforcement Tension) cables.

A case study was conducted in Fonte Grande

Sul (FGS), Level 10.2 stope, located between

700m and 665m below surface, which showed

that working with the data collected by

extensometers and the filming of boreholes, can

be a powerful tool to recognize timely evidence

of instability of the hanging wall, allowing that

mitigation activities can be implemented to

stabilize the area.

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After processing the data related specifically

to filmed boreholes, it was possible to recognize

the thicknesses of the beams formed by the

shear and cracks identified inside the holes in

schistose rock in the hanging wall, thus,

together with the identification of other

parameters, an assessment was done on the

applicability of the analogy of voussoir arch

theory for the stability assessment of the

hanging wall for the study area.

1.1 Location

Cuiabá mine is located close to Sabara city,

Minas Gerais state (Figure 1), 35 km from Belo

Horizonte.

Figure 1. Cuiabá mine location.

1.2 Objectives

The main objectives of this work are listed as

follows:

• Study the evolution of deterioration in the

stability of the hanging wall during the process

of mining activities;

• Assess the effectiveness of the monitoring

systems used, extensometers and filming of

boreholes, as tools to identify timeously, the

indications of instability in the hanging wall;

• Evaluate the effectiveness of mitigation

activities for the stabilization for area with

evidence of instability;

• To study the applicability of the analogy of

voussoir arch theory as a mechanism for the

stability of the area with indications of

instability.

2 CASE STUDY – 10.2 FONTE GRANDE

SUL STOPE (FGS)

For a better understanding of all the

mechanisms involved in this study, site

characterization is presented, covering location,

geometric characterization of the mining panel

as well as the geological and geotechnical

characteristics.

After describing the study case, the

interpretation of data obtained mainly by the

filmed holes, it was possible to define the

thickness of the beam, caused by the separation

of cracks and schistose shears located in the

hanging wall.

With the geomechanical characterization of

the hanging wall, the geometry of the

excavation and knowledge of the thicknesses of

the beams, it was possible to evaluate the

applicability of the method voussoir modified

by Diederichs and Kaiser (1999a), for the study

of stability of schistose hanging wall.

2.1 Case Study Location

Case study is located in Fonte Grande Sul

(FGS) orebody, level 10.2 between 700 and

665m below surface (Figure 2).

Figure 2. Case Study location.

2.2 Characterization

2.2.1 Stope Geometry

Stope 10.2 FGS has a vertical height of 35m,

strike of 460m, with the predominant dip of the

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Lithology

Amount

of

samples

Uniaxial Compressive

Strength (MPa) Poisson

(ν)

Elastic

Module E

(GPa) Minimun Average Maximum

Grafite

phylite 9 12 63 149 0,13-0,25 12-51

Sericite

schist 52 36 66 141 0,15-0,26 39-78

orebody at 30 °. The mining method applied is

cut and fill with backfilling of waste rock and

hydraulic fill.

2.2.2 Geological and Geotechnical

Characterization

Fonte Grande Sul orebody is located in the

normal limb of the large tubular fold in Cuiabá

mine. Orebodies located in the normal limb, in

general have the following lithological

sequence, from bottom to top, schistose meta-

basalts and schistose meta-andesite, forming

foot wall; banded iron formation with sulfides,

defining the ore (in the study area the thickness

range between 7 and 8m); a layer of graphite

phylite and sericite schist, both forming the

hanging wall.

Two major discontinuity families have been

identified in the area. One family is defined by

schistosity that is the main structure of the

mine; the other family is defined by

discontinuities with the same direction of

schistosity, but with a 180o difference of plunge

direction (Figure 3).

Figure 3. Family of discontinuities (Software DIPS 5.0).

Geomechanical classification of wall hanging

was carried out for 10.2 FGS stope using the

Rock Mass Rating (RMR) classification

(Bieniawski, 1989) and Q - Rock Tunnelling

Quality Index (Barton et al, 1974.). It can be

noted that there is little variation in the quality

of the rock mass along the wall hanging (See

Figure 4). The average hanging wall RMR was

rated between 60 and 41. Q values for almost

all of the hanging wall exposures were

classified in bad rock mass with scores between

1 and 4.

Figure 4. Rock mass map for 10.2 FGS orebody.

Laboratory tests were done to characterize

hanging wall rocks. These results can be

visualized in the table 1.

Table 1- Laboratory test results for hanging wall rock.

2.2.3 Support Characterization

Plain strand Cablebolting, 9.5m in length and

maximum axial strength of 25t to 27t were used

in the hanging wall on a 1.5x1.5m pattern

(Figure 5). Plates and barrels were installed on

the cables. Approximately 1500 cables are

installed on a monthly basis at Cuiabá mine.

Figure 5. Stope 10.2 FGS with visualization of support.

Ore Hanging Wall

Foot Wall

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2.3 Visual Scale of Cracks and Shears

Borehole camera monitoring was started in

Cuiabá mine for the identificiation and

classification of cracks and shears. Following

the collection of a large dataset, it was possible

to make a visual scale of cracks and shears

(Figure 6). This scale allow different levels of

cracks and shears.

Borehole camera monitoring of holes in the

hanging wall (HW), with mining evolution,

assisted to identify intense shearing in the

schistose mainly when the holes were more

further from stope face.

.

Figure 6. Visual scale of cracks and shears for holes with

a diameter of 5 cm.

2.4 Monitoring and Actions Evolution

For more adequate understanding of the case

study, it is necessary to know the chronology of

events that occurred, from the identification of

signs of instability to the rehabilitation of the

area with mitigation actions. The development

of research and interventions are listed as

follow:

• 15/11/2007 - A SMART cable (9.5m long) and

an extensometer MPBX (15m long) were

installed, about 2m far from each other (along

the direction of the layer) on the hanging wall.

In this moment the vertical height of the mining

panel was about 15.5 m. The geometry of the

stope can be seen in Figure 7.

• 05/12/2007 - Significant displacement was

recorded (2.8 to 2.5 cm near the surface of the

hanging wall) after the production blast. The

behavior of the displacements recorded by the

SMART cable and MPBX was similar (Figure

8). For stable areas, in general, a typical

displacement is of ± 1.5cm near the HW face

after blasting. Therefore this high displacement

was a first indication of instability in this area.

Figure 7. Vertical cross section in 10.2 FGS.

Figure 8. Displacement graph (A) SMART cable and (B)

MPBX.

• 29/01/2008 - Monitoring with borehole

camera allowed the captuirng of images inside

the borehole in the hanging wall. The inspected

hole had a length of 15.5m and it was almost

perpendicular to the schistosity. Shears of level

1 were identified approximately 7.0, 1.1, 0.5

and 0.05 m, measured from the face of the HW

(Figure 9).

• 07/02/2008 - Another important displacement

was recorded by the SMART cable and MPBX

(about 3cm near the face of the HW - Figure

10). There was no activity at this location

during this period. It indicated, again, that this

area was with serious stability problems in the

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hanging wall (HW).

Figure 9. Vertical section with cracks and shears inside

hole 1 on 29/01/08.

Figure 10. Displacement graph A) SMART cable and B)

MPBX.

• 14/02/2008 – The borehole camera was used

in borehole 1 and level 1 shears were identified

approximately 8.0, 7.5, 7.0, 6.1, 4.5, 4.0, 3.5,

3.0, 2.5, 1.1, 0.8. , 0.75, 0.6 and 0.5 m measured

from the surface of the HW (Figure 11).

Figure 11. Vertical section with cracks and shears inside

hole 1 on14/02/08.

After the identification of the deterioration of

rock mass conditions, cablebolting were

installed plus the installation of plates and

barrels as reinforcement. Hydraulic filling was

also done in this stope.

• 14/03/2008 – Borehole 1 was again filmed.

Progresses in shears were identified when

compared to the previous recording (Figure 12).

Figure 12. Vertical section with cracks and shears inside

hole 1 on14/03/08.

• 29/03/2008 – Production blasting was done at

the study area.

• 09/04/2008 - Borehole 1 was again filmed to

evaluate the condition of the HW after blasting.

High level of shearing was identified 7m

(measured from the surface) blocking the hole

(Figure 13). The extensometer MPBX

apparently reached the limit of measurement,

not detecting further displacement (Figure 14).

Figure 13. Vertical section with cracks and shears inside

hole 1 on 09/04/08.

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Figure 14. MPBX displacement graph with televised hole

information.

Recordings with the borehole camera allowed

the study to gradually evaluate the evolution of

shearing over time. Some images of the

development of shears can be viewed as a

function of time for certain positions within the

rock mass in Figure 15.

Figure 15. Shear evolution over time for 1.1 and 7 m into

the HW.

In order to continue monitoring this site more

holes were drilled for monitoring of with the

borehole camera. Cablebolting were installed on

denser pattern of 1.0 x 1.0 m and 9.6m length.

Extensive mechanical rock scaling was

required to remove broken rock material . A

brow of ± 2.5m was formed in the HW with a

length of ± 10m , along the direction of

schistosity, and 13m along the dip (Figure 16).

Figure 16. Vertical section with cracks and shears and

HW picture with brow after scaler machine working.

A new MPBX was installed and other holes

were drilled for borehole camera monitoring.

The evolution of shear and crack and the

identificiation by means of borehole camera

filming in holes 1, 2, 3, 4, 5 and 6 can be seen

in Figure 17. The quantity and magnitude of

cracks and shears decreased after reinforcement

with cableblolting was done. This is the most

important indicator with respect to the borehole

camera information, indicating the

improvement of the HW geomechanical rock

mass condition.

Figure 17. Cracks and shear evolution. In the hole 6 there

was no cracks or shears.

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A comparison between the displacements

collected by the 1st MPBX, the phase of

instability of HW, and the 2nd

MPBX installed

subsequently after installation of cablebolting

reinforcement can be seen in Figure 18.

Figure 18. Displacement graphs (A) MPBX with

instability indication and (B) MPBX 2 after reinforcement

installation.

Through the analysis of Figure 18, we can

identify significant difference in displacement

between the instruments. It is noteworthy that

the measuring time for MPBX 2 is much larger

than for MPBX 1, furthermore the larger

amount of blasting that occurred in the study

area. The MPBX 2 installed after the

application of reinforcement show typical

displacement for areas without signs of

instability.

Mining activities has been successfully

completed in the area. This was due to the

identification of signs of instability, the

installation of reinforcement and constant

monitoring using extensometers and borehole

camera. A current photo of the study area can be

seen in Figure 19.

Figure 19. Study area after mining activities.

2.5 Voussoir Arch Theory

In order to exploit the data collected during the

process of study and to attempt to validate a

simple method for evaluating the stability of the

hanging wall in schist, a stability study was

performed using the voussoir arch theory.

First of all it was necessary to find the rigid

limit below hydraulic/rockfill floor to be

considered. For trying to solve this problem, 6

displacement graphs from MPBX and SMART

cables were analyzed after they were covered

with hydraulic/rockfill in the same stope; five

extensometers did not identify displacement

after blast number 3 (Figure 20) after being

covered, or 6m along ore dip below the floor.

Then span considered was normal span plus 6m.

Figure 20. Displacement graph with blasting and moment

of covered extensometer.

In order to minimize errors related to

lithologic recognition in all televising of holes

in the study area, the interbedded graphite

phylite was recognized immersed in sericite

/chlorite schist (Figure 21).

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Parameter Simulation 1 Simulation 2

Span (m) 25 25

Thickness (m) 2.5 4.7

Rock Mass Elastic Modulus (GPa) 13.3 13.3

Specific Weight (KN/m3) 28 28

Bedding angle 30 30

UCS (MPa) 63 63

Support pressure (KPa) 87.1 109

Buckling Limit (<35%) 2% 2%

Crush Safety Factor 369.7 80.8

Figure 21. Graphite phylite immersed in sericite/chlorite

schist.

The thickness of the beam to be used is 2.5 m

on the graphite phylite which was removed with

the scaler equipment. Another simulation used a

beam thickness of 4.7 m because of the

crack/shear identified in the first and second

hole at the same depth within the hanging wall

(Figure 22).

Figure 21. Beam thickness used.

All parameters used in the voussoir analogy e

results for both simulation can be visualized in

the table 2.

The results do not indicate instability for both

simulation.

Table 2- Parameters used in the voussoir analogy to the

simulation 1 and 2 and the results obtained.

3 RESULTS

The timely detection by the extensometers and

the images obtained by borehole camera

monitoring in the area with signs of instability,

allowed measurements could be done to

stabilize the site. The success achieved after the

implementation of stabilization measures,

attested by monitoring, demonstrates the

efficiency of extensometers and borehole

camera as important tools to minimize the risk

of fall of ground in the hanging wall.

The analysis performed by calculation using

the analogy of voussoir indicated a very stable

situation. This result disagrees with the data

obtained by monitoring. The difference between

the stability analyzes, voussoir and monitoring,

may be associated with frequent intercalations

of rocks with different elastic properties and

strengths that were identified within the hanging

wall (Figure 23), beyond the possibility of

improper choice of the beam thicknesses

studied.

Figure 22. Example of graphite phylite intercalation.

REFERENCES

Diederichs, M.S., Kaiser, P.K. (1999a). Stability of Large

Excavations in Laminated Hard Rock Masses: The

Voussoir Analogue Revisited, International Journal of

Rock Mechanics and Mining Sciences, Canada, v. 36, 97-

117p.

Diederichs, M.S., Kaiser, P.K. (1999b). Tensile Strength

and Abutment Relaxation as Failure Control Mechanisms

in Underground Excavations, International Journal of

Rock Mechanics and Mining Sciences, Canada, v. 36, 69-

96p.

Hutchinson, D.J, Diederichs, M.S. (1996). Cablebolting

in Underground Mines, Bitech Publishers Ltd, British

Columbia, Canadá, 406p.