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CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
Analysis of Large-ScalePit Slope Stability
— The Aitik Mine Revisited
Jonny Sjöberg Itasca Consultants ABDiego Lope Álvarez SKB (previously Itasca)
Patricio Gómez Itasca S.A.Per-Ivar Marklund (retired, Boliden)
Sara Suikki Itasca summer intern
The Spanish Mining Council is organising the 2018 International Symposium in Slope Stability in Open Pit
Mining and Civil Engineering at the XIV International Congress for Energy and Resource Mining in Seville.
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• Design of overall slopes is important – for open pits of all scales –
and further underlined by the push towards larger, and deeper pits
• Evolution of design tools:
❖From simple empirical and limit equilibrium methods to sophisticated
numerical computer modelling
❖Rapid increase in computer capacity, but also an increased understanding of
the behaviour of large-scale slopes
❖Still severely data- and knowledge constrained…
• Illustration of design development and current state-of-the-art
using the Aitik open pit mine as a case in point.
Things we all know…
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
Aitik Mine The Aitik Mine
• Owned and operated by
Boliden (from the start)
• Mineralization discovered
in the 1930s
• Mining commenced 1968 –
Boliden's first open pit
• Currently the largest metal
open pit mine in Europe
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
The Aitik Mine 1968 – 2 Mtpa (ore)
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
Early 70’s – 6 Mtpa (ore)
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
Footwall
Hangingwall
Mill &
workshop
2000 – 18 Mtpa (ore)
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
2010 – 36 Mtpa (ore) Workshop
Crusher
Mill
Railway terminalConveyor belts
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
Today – 40 Mtpa (ore)
3000 x 1000 m, 420 m depth, main pit + satellite pit
Main pit hangingwall
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
Main pit footwall
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
Geological setting
North
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
Geology & Geomechanics
North
Hangingwall
• Generally strong, metamorphic rock, with
some weaker units (biotite/muscovite schist)
• Medium to good rock quality; poor quality in
schist units
0
10
20
30
40
50
60
70
80
90
100
0
1000
2000
3000
4000
5000
6000
7000
8000
AM
PH
IBO
LE
-BIO
TIT
E…
AM
PH
IBO
LE
GN
EIS
S
MIC
A G
NE
ISS
BIO
TIT
E S
CH
IST
DIO
RIT
E
PE
GM
AT
ITE
SE
RIC
ITE
SC
HIS
T
AM
PH
IBO
LIT
E
TU
RB
IDIT
E
GA
BB
RO
AN
DE
SIT
E
(bla
nk)
CO
NG
LO
ME
RA
TE
FE
LD
SP
AR
-EP
IDO
TE
ZO
NE
CO
RE
LO
SS
UN
IDE
NT
IFIA
BLE
RO
CK
…
Length_RMR
Mean_RMR
Max_RMR
Min_RMR
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
Structural Characteristics
NORTH
ENDFOOTWALL
NORTH
FOOTWALL
MIDDLE
FOOTWALL
SOUTH
SOUTH
END
HANGINGWALL
NORTH
HANGINGWALL
MIDDLE
HANGINGWALL
SOUTH
• Well-developed foliation
+ cross-joints and
vertical jointing
• Hangingwall contact =
old thrust fault with clay
• No other large-scale
structures identified
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• First major design study in 1976
with follow-up in 1985
• Bench design based on structural
control; Double-benching (2 x 15
m) with offset chosen to match
foliation dip
• Catch bench width based on
original Ritchie criterion – 11 m for
90% of the cases
• Overall slopes not assessed
The early stages…
30 m
11 m
7 m3 m
Berm
Impact Zone
Double Bench
1 m69°
Foliation
Drilling Offset
5.5 m 22.5 m
Effective Bench
Face Angle = 60°
Actual Bench Face
Plane
11 m
Backbreak
Interramp slope angle = 46°
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• Problems:
❖Vertical blast damage
❖Quality control / hole deviation
❖Local crest failures &
bench widths not attained
• Solutions:
❖Blast damage model (PPV)
❖More buffer rows
❖Pre-split with decoupled charges
Blasting can make things better…
8000
4000
2000
1250
PPV [mm/s]
6.0 m
Unacceptable damage
Heave
Diggability limit
Measurable heave
7.5 m 5 m
Back-break
Bench crest
Backbreak limit
990 k
g
990 150
59300
990 k
g
855
280
280
150
290
311
311
165
165
165
311 m
m
311
165
165
165
165
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• Issues for hard rock slopes:
❖No failure observations = no calibration of properties possible
(except that failure should not develop for current mining geometries)
❖Failure may be rapid and uncontrollable (brittle rock)
• Remedies:
❖Develop methodology for modeling and parameter assessment and
apply to other slopes with failure
❖Design methodology must be conservative => use residual strength
parameters
Large-scale slope stability at Aitik…
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• Project on large-scale slope stability in hard rocks (1990s) &
revision of overall slope angles:
❖Empirical case study database
❖Updated geomechanical model
❖2D numerical modeling – continuum models, rock mass strength +
residual strength parameters + ubiquituos joints (foliation)
• Results
❖Stability assessment to 500 m depth
❖Increase of interrramp and overall angles (+5°) with drainage program
❖Additional modeling for deeper (750 m) pit => additional drainage
Large-scale and 2D – at Aitik…
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• New strategic plan – and deeper mining (2015-2016)!
• New analyses:
❖Numerical modeling, Factor-of-Safety calculations, 2D (& 3D)
❖Perfectly-plastic material model (no softening; peak strength = residual
strength); Hoek-Brown material model
❖Rock mass strength values estimated empirically using
characterization (GSI) and Hoek-Brown failure criterion
❖Experiences and practices from Itasca analysis of large pit slope used
to supplement and refine estimates
❖Acceptance criterion: FoS for the Overall Slope Angle > 1.2
2D and deeper – and the importance of "disturbance"…
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• Design values for GSI and UCS:
❖mean – 0.5 std.dev
❖Corresponds to 30-35 percentile
❖Accounts for heterogeneity in large-scale rock masses, and the ability for the rock to
fail through the weaker components (based on experience and empirical evidence)
• Variation of D (disturbance factor) with depth
❖D=1.0 everywhere proven too conservative
❖Blast damage highest close to slope face
❖Stress relief close to slope face
❖Possible stress damage at depth
2D and deeper – and the importance of "disturbance"…
H (m)
15 (m)
D = 1
D = 0
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
2D and deeper – and the importance of "disturbance"…
PROFILE A PROFILE B
PROFILE
Y4500
PROFILE
Y4000
Geographic
North
+Y
Mine North
+X77.75°
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
2D and deeper – and the importance of "disturbance"…
Current Pit
Previously Planned Final Pit
Level 750/800/850 m Pit
DESIGN PROFILE A
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• Groundwater conditions: ❖Partly depressurized in upper portion; face seepage in lower portion
❖Scenario 1: Depressurized 100 m horizontal distance (upper 2/3 of
slope height)
❖Scenario 2: Depressurized 150 m horizontal distance (upper 2/3 of
slope height).
2D and deeper – and the importance of "disturbance"…
100 m
H
H/3
Water table level
Water table level
Partially depressurized slopes (100 m)
150 m
H
H/3
Water table level
Water table level
Partially depressurized slopes (150 m)
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
Results & design recommendationsHW overall
slope angle []
Pit bottom
[level]
Depressurization
[m]
48 800 m 100 (2/3 H)
52 800 m 150 (2/3 H)
50 850 m 150 (2/3 H)
54 850 m 150 (H)
54 * 600 m 100 (2/3 H)
54 * 600 m 100 (2/3 H)
FW overall
slope angle []
Pit bottom
[level]
Depressurization
[m]
47 800 m 100 (2/3 H)
45 850 m 100 (2/3 H)
47 850 m 150 (2/3 H)
47* 600 m 100 (2/3 H)
* Middle-southern portion
(lower final slope heights)
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• How conservative are 2D-analyses?
• 3D-check for Aitik geometry:
❖Simplified slope, undrained conditions
❖FoS (3D) = 2.0 FoS (2D) = 1.2 for the same case
• Geometry matters (a lot)!
2D or 3D? Or both?
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• Increased reliability in bench slope design❖Presplit blasting on footwall (inclined, 70°) and for modified ramp design
❖Increased safety of benches; maintained catch bench design
❖Revised acceptance criteria
• Improved interramp slope design❖Increase on footwall interramp slope angle from improved blasting
❖Implemented allowed maximum interramp height = 200 m (6–7 double benches)
❖Rock mass stability => steeper interramp angles allowed
• Overall pit slope design❖Verified overall pit slope angles: 47° for footwall; 54° for hangingwall, for a
maximum pit depth of 850 m
❖Depressurized conditions required (150 m horizontal distance)
Aitik findings….
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• Analytical methods are standard for bench-scale design
• Numerical modeling remains the preferred tool for large-scale
stability assessment
❖3D models, DFN input, coupled (water-stress) modeling, etc. are all
increasingly used
❖Computational speed less of an issue…cloud computing emerging!?
• Advances in data collection not on par with advances in
modeling capabilities
❖Are we using the right tools? Volumetric coverage?
❖Is there "unconventional" data to be used? As "proxies" for rock
mechancis parameters?
The future…
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
• Validation efforts required:
❖Partial validation is also of value!
❖Acceptance criteria for validation
The future…
Reality of
Interest
Mathematical
ModelValidation
Verification
Confirmation
Computer
Model
Simulation
Outcomes
Assessment Activities
Modeling, Simulation &
Experimental Activities
CIVIL ● MANUFACTURING ● MINING ● OIL & GAS ● POWER GENERATION
The funding by Boliden is gratefully acknowledged
Special thanks to Anton Bergman, Jansiri Malmgren
& Mats Larsson for review of this paper