STABILITY OF OPEN STOPES INTERCEPTED BY

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STABILITY OF OPEN STOPES INTERCEPTED BY
GRAPHITIC SHEARS AT ASHANTI GOLDFIELDS COMPANY
LTD. (AGC), OBUASI OPERATIONS IN GHANA PRINCIPAL
INVESTIGATOR ALBERT ADU-ACHEAMPONG MINING
ENGINEERING DEPARTMENT MACKAY SCHOOL OF
MINES RENO, NEVADA
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• OVERVIEW OF THE PRESENTATION
• LOCATION OF THE MINE, GENERAL GEOLOGY
• GEOLOGICAL DATA COLLECTION
• WEDGE STABILITY ANALYSIS
• LABORATORY TESTING OF ROCK
• STOPE SIZES AND DESIGN AT THE MINE
• ROCK MASS CLASSIFICATION
• GRAPHICAL PRESENTATION OF ENVELOPES
• STABILITY GRAPH METHOD
• NUMERICAL MODELING
• RECOMMENDATIONS

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LOCATION OF THE MINE AND GENERAL GEOLOGYOF
SOUTHERN GHANA Indicates the location
of the AGC, Obuasi operations in Ghana Indicates
AGCs interest in Africa
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VERTICAL PROJECTION OF THE MINE SHOWING SOME OF
THE MINING BLOCKS WITH BLOCKS 2 3 INDICATED BY
THE BLUE ARROWS
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• GEOLOGICAL DATA COLLECTION AND STEREOGRAPHIC
  ANALYSIS
• The dip and dip direction measurements were
  taken at
• Blocks 2 (38 1, 39 1 and 39 Levels) and 3 (35
  and 35 1 Level)
• In addition, the following were recorded for each
  measurement
• Infilling and form
• Aperture
• Roughness
• Spacing
• Continuity
• Orientation of drift axis

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• CONTOUR PLOT AND MAJOR PLANES FOR 381, 39 1 AND
  39 LEVELS (BLOCK 2)
• Dominant major planes observed
• Block 2 (48/318 and 84/215) dip/dip direction
• Block 3 (81/185 and 51/304) dip/dip direction

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WEDGE STABILITY ANALYSIS WITH UNWEDGE 1. When
water pressure is set to zero for block 2 with
drift axis of 115 degrees for block 2
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2. When water pressure is set to 0.00327 MPa with
drift axis of 115 degrees for block 2
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DIPS AND DIP DIRECTIONS WITH THE DIMENSIONS OF
THE WEDGE (SAME VALUES IN BOTH CASES)
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• ROCK QUALITY DESIGNATION (RQD)
• CALCULATION
• The RQD was calculated based on the spacing of
  the discontinuities
• measured during the field mapping using the
  relation
• where,
• t is the threshold
• ? is the discontinuity frequency
• where,
• N is the number of discontinuities that
  intersected the scanline
• L is the length of the sampling line

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• LABORATORY TESTING OF ROCK
• Direct shear test
• Typical graphitic sample

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• Half of the graphitic sample yet to be cast
• Reasons for casting the sample
• To prevent it from disintegration since the
  graphite was
• friable
• To ensure that it can be sawed for direct shear
  testing
• To ensure that the contact surfaces remained
  intact

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SUMMARY OF THE DIRECT SHEAR TEST
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SUMMARY OF THE UCS TEST Sample UCS
Average UCS Standard (MPa) (MPa)
Deviation (MPa) ADUB2 80.11 ADUB1 104.99
ADUB3 121.37 91.53 18.79
ADUC1 81.66 ADUC5 91.12 ADUC6 69.95 ADUA5
48.32 ADUA6 52.14 67.41 29.82 ADUA7 101.76
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SUMMARY OF THE TRIAXIAL TEST
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• STOPE SIZE AND DESIGN AT THE MINE
• Plan showing the hanging wall drive, crosscuts
  and the ore
• zone at the mine (38 1 Block 2)

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• SUPPORTING SYSTEM AT THE MINE
• AGC employs both artificial and natural support
  to hold up the
• excavation. Among the supports used are
• Shotcrete
• Rockbolts (grouted rebar split sets)
• Cablebolts (design angles between 55 85
  degrees)
• Backfill

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• ROCK MASS CLASSIFICATION, GRAPHICAL
• PRESENTATIONS (ENVELOPES) OF ROCK
• STRENGTHS AND STABILITY GRAPH METHOD
• TO ASSESS STOPE STABILITY
• OBJECTIVES OF THE ROCK MASS CLASSIFICATION
• Identify the most significant parameters
  influencing the behavior
• of the rock mass
• Derive quantitative data and guidelines for
  engineering design
• Provide a basis for understanding the
  characteristics of the rock
• mass class

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• TUNNELING QUALITY INDEX (Q)
• The Q calculated for the area mapped lies
  within
• - 3.29 to 13.01 for RQD threshold value of 0.1
  m
• - 1.64 to 5.13 for RQD threshold value of 1.0
  m
• These were calculated using the relation
• where,
• RQD is the Rock Quality Designation
• Jn is the joint set number
• Jr is the joint roughness number
• Ja is the joint alteration number
• Jw is the joint water reduction
• SRF is the stress reduction factor

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ASSESSMENT OF SUPPORT REQUIREMENTS FOR
RQD THRESHOLD VALUES OF 0.1 1.0 m. BLUE
ARROWS INDICATED De ESTIMATED FROM A 30 m HIGH
STOPE RQD threshold of 0.1 m RQD
threshold of 1.0 m
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SUMMARY OF MAXIMUM UNSUPPORTED SPANS FOR A 30 m
HIGH STOPE WITH RQD THRESHOLD OF 0.1 1.0 m
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ROCK MASS RATING (RMR) The following six
parameters were used to classify the rock mass
using the RMR system 1. Uniaxial compressive
strength of rock material 2. Rock Quality
Designation (RQD) 3. Spacing of
discontinuities 4. Condition of
discontinuities 5. Groundwater conditions 6.
Orientation of discontinuities. The RMR had the
ratings of 65 - 70 (good rock) for RQD
threshold value of 0.1 53 - 65 (fair rock) for
RQD threshold value of 1.0
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ESTIMATION OF GEOLOGICAL STRENGTH INDEX
(GSI) FROM THE MODIFIED TUNNELING QUALITY INDEX
(Q) THE GSI WAS ESTIMATED FROM THE
RELATION SUMMARY OF THE GSI
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ESTIMATION OF ROCK MASS PROPERTIES Reliable
estimates of strength and deformation
characteristics of rock masses are required for
almost any form of analysis used for underground
excavations mi is a material constant
for the intact rock mb is the
value of the constant m for the rock
mass s and a are constants which
depend upon the characteristics of
the rock mass
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• STABILITY GRAPH METHOD
• This accounts for the key factors that influence
  open stope design
• rock mass strength
• stresses around an opening
• the size, shape and orientation of an opening
• It was also used to determine whether an opening
  will be
• Stable with support
• Stable without support
• Unstable even if supported

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INPUT PARAMETERS FOR THE STABILITY GRAPH
METHOD The design procedure was based upon 1.
Modified stability number (N) Q is the
modified Q A is the rock stress factor B is the
joint orientation adjustment factor C is the
gravity adjustment factor 2. Hydraulic radius
(HR)
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ROCK STRESS FACTOR (A) JOINT ORIENTATION
ADJUSTMENT FACTOR (B) GRAVITY
ADJUSTMENT FACTOR (C) ? is the dip of the
stope surface
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INPUT PARAMETERS FOR THE STABILITY GRAPH METHOD
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• NUMERICAL MODELING
• The following computer programs were used in the
  analysis
• PHASES
• FAST LANGRANGIAN ANALYSIS OF CONTINUA (FLAC)
• OBJECTIVES OF THE NUMERICAL MODELING
• To assess the stability of the stopes
• To investigate the influence of uncertainties in
  rock parameters on
• the results
• Study the influence of the in situ stress field
  in particular the ratio
• of horizontal to vertical stress
• Perform numerical analysis using the
  Mohr-Coulomb and
• Hoek-Brown failure criterion

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• THE MODELING OF STOPE INSTABILITY
• The following model simplifications were made
• Three-dimensional conditions modeled in two
  dimensions
• Simplified modeling of the ground and mining
  process
• Input parameters used in the analysis for a 940 m
  depth and unit
• weight of 0.027 MN/m3

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Pillar failure determined by safety factor
contour plots for k1.3 cavity monitoring device
with a cohesion of 10 MPa instead of at the
mine looking south 22.09 MPa in the Table
below The Youngs modulus of the
graphite was assumed to be 2,000 MPa. The
analysis also considered 3,000, 5,000 and 10,000
MPa
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• INFLUENCE OF IN SITU STRESS STATE ON THE
• STABILITY OF THE STOPE
• A knowledge of the magnitude and directions of in
  situ or induced
• stresses is essential due to the following
  reasons
• Rock at depth is subjected to stresses resulting
  from the weight of
• the overlying strata and from locked in
  stresses of tectonic origin
• The stress field is locally disrupted and a new
  set is induced in the
• rock surrounding an opening
• It is an essential component in understanding
  excavation design

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Summary of stope back stresses and displacements
for the various values of k when the Youngs
modulus and the Poissons ratio of the graphite
were 2,000 MPa and 0.4 respectively
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NUMERICAL MODELING WITH PHASES USING
THE HOEK-BROWN CRITERION Input parameters for
the Hoek-Brown used to assess stability 1 The
parameters in columns C, D and E were used to
assess stability when k 0.5 and 2.0 2 UCS
value obtained from the mine (After Goel
Wezenberg, 1999)
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Safety factor contour plots with Safety factor
contour plots with the parameters in columns A, B
the parameters in columns C, D and E from the
Table below and E from the Table below
(k1.3) (k1.3)
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Safety factor contour plots using Safety factor
contour plots using the Hoek-Brown failure
criterion the Hoek-Brown failure criterion with
the parameters in columns with the parameters in
columns C, D and E from the Table C, D and E
from the Table below below (k0.5) (k2.0)

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NUMERICAL MODELING USING FLAC FLAC is a 2-D code
which uses explicit finite difference
solution procedures to translate a set of
differential equations into matrix
equations. This analysis considered a 50 m high
stope dipping at 65 degrees and 20 m
wide Material properties used for FLAC analysis
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MESH USED FOR FLAC ANALYSIS WITH 50 m HIGH AND 20
m WIDE STOPE EXCAVATED
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Plasticity indicators for k 1.3 when the
parameters in column C were used for the graphite
with all other parameters remaining constant.
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Displacement vectors at k 1.3 when the
parameters in column C were used for the
graphite with all other parameters remaining
constant.
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Plasticity indicators for k 1.3 after stepping
the model to 3100 time steps when the parameters
in column D were used for the graphite with all
other parameters remaining constant to determine
the effect of shear and bulk modulus on
stability of the stope
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• OBSERVATIONS
• The geological data collected and analyzed
  established that the
• major planes 81/185 and 51/304 dip and dip
  directions were
• dominant at Block 3 whereas 48/318 and 84/215
  dip and dip
• directions were dominant at Block 2.
• The analysis performed with UNWEDGE indicated
  that the factors
• of safety calculations were sensitive to water
  pressure.
• The direct shear test established that the
  graphite has virtually no
• cohesion with low internal friction angle
• The rock mass as well as the ore zone behaved as
  a brittle material
• Failure was explosive with sudden dropped of
  energy

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• The laboratory test performed (for instance,
  uniaxial compressive
• strength (UCS) test) indicates variability.
• The Q-rating was between 3.29 (poor) and 13.01
  (good) for RQD
• threshold value of 0.1. This gave maximum
  unsupported spans
• between 10.5 and 30 m respectively.
• The rock mass had ratings between 65 - 70 (good
  rock) and
• 53-65 (fair) for RQD threshold values of 0.1
  and 1.0 respectively.
• Two modes of failure were observed depending
  upon the
• stress ratio. Shear failures were observed at
  higher stress ratios
• and tensile failure at lower stress ratios.
• The Hoek-Brown failure criterion predicted the
  mode of failure at
• the mine better than the Mohr-Coulomb failure
  criterion

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• Varying the Poissons ratio of the graphite did
  not affect affect the
• safety factor contour plots and the stress
  distribution.
• Varying the Youngs modulus of the graphite did
  affect the
• stress distribution and the safety factor
  contour plots.
• Initiation of displacement at the hanging wall
  led to an up dip
• propagation of failure at the crown of the
  stope.

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• RECOMMENDATIONS
• Detail numerical analysis needs to be carried
  out should
• mining progress towards an aquifer.
• Stress measurements should be carried at the
  mine to help model
• the stopes accurately.
• More samples need to be taken at areas where
  there have been
• extensive ground falls to determine the
  strength of the rock mass
• and the ore zone.
• The hanging wall Ts should be adequately
  supported before
• mining proceeds to reduce the extent of tensile
  zone at the hanging
• wall.

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• The Hoek-Brown failure criterion predicted the
  mode of failure
• better than the Mohr-Coulomb failure
  criterion. This should be used
• to estimate the induce stresses around the
  stope for stability graph
• analysis.
• It will be desirable to use 3-D codes for the
  numerical modeling.

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• IN BRIEF, I HAVE DISCUSSED WITH YOU THE
• FOLLOWING
• LOCATION OF AGC MINE AND GENERAL GEOLOGY
• GEOLOGICAL DATA COLLECTION
• WEDGE STABILITY ANALYSIS
• LABORATORY TESTING OF ROCK
• STOPE SIZES AND DESIGN AT THE MINE
• ROCK MASS CLASSIFICATION
• GRAPHICAL PRESENTATION OF ENVELOPES
• STABILITY GRAPH METHOD
• NUMERICAL MODELING
• RECOMMENDATIONS

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ACKNOWLEDGEMENTS I wish to express my profound
gratitude to the Management and Staff of Ashanti
Goldfields, Obuasi Operations in Ghana and all
those who in diverse ways have helped me to reach
this far especially my committee members (Prof.
Jaak Daemen, Prof. Pierre Mousset-Jones and Prof.
Bob Watters), Rick Blitz, Ian Firth, Davood
Bahrami and Gail Scalzi. I really appreciate all
the help you have been offering. My thanks also
go to anyone that I could not mention his or her
name. May God bless you all.