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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, [email protected]
Rodrigo Peluci de Figueiredo
Federal University of Ouro Preto, Ouro Preto, Brazil, [email protected]
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.