a study of the stability mechanism within shallow mining operations that will impact on the...
TRANSCRIPT
A study of the stability mechanism within shallow mining operations that will impact on the sustainability of Platinum MinesThe CSIR Research and Innovation Conference
Natural Resources and Environment
Mr Bryan Watson
Senior researcher
27 February 2006
Slide 2 © CSIR 2006 www.csir.co.za
Agenda• Introduction
What is rock engineering and the goals behind the programme?
• SignificanceWhat are the potential benefits of this
research?
• Site descriptionStoping conditions under which the research
was conducted.
• Observations & instrumentationData collection for numerical models and
development of behavioural theory.
• Elastic modellingResults of a numerical model compared to
observations and instrumentation.
• SolutionsAnalytical and inelastic numerical
modelling.
• ImplicationsHow the findings can be used.
• ConclusionsImpact on short term profitability and
efficiency and contribution to long term sustainability.
Slide 3 © CSIR 2006 www.csir.co.za
Introduction
• What is rock engineering?
Study of rock behaviour and support requirements around man-made excavations. Pillar
30 m
Haulage tunnel
Wooden poles(elongate support)
Slide 4 © CSIR 2006 www.csir.co.za
Problem statement
• Support in shallow platinum mining operations is typically provided by in-stope pillars - a significant percentage of ore reserves are locked up in these pillars, which reduces the life of mine.
• If all pillars created in a single year across the Platinum industry were reduced in size by 1.0 m, approximately R1,0 billion profit could be realised annually.
• There is potential for increasing life-of-mine and thus a positive contribution towards sustainability.
Slide 5 © CSIR 2006 www.csir.co.za
Plan of instrumentation site
Slide 6 © CSIR 2006 www.csir.co.za
Strike slip fault to north of site
Slide 7 © CSIR 2006 www.csir.co.za
FOG adjacent to stability pillar
Slide 8 © CSIR 2006 www.csir.co.za
Support in the evaluated stope
Slide 9 © CSIR 2006 www.csir.co.za
Geotechnical and instrumentation resultsStress measurements
Mottledanorthosite(Middling 3)
Spottedanorthosite(Middling 2)
Anorthositicnorite(Middling 1)Pyroxenite
BastardReef
Geological logUCS
StrengthMPa
12,64
12,05
8,98
1,741,15
Geotechnical log
Joints/m>450
Joints/m<450
208
174
139
119
1138
2
00
0
0
0
71
1
0
0
14
8
8
0
2
4
6
8
10
12
-20 -10 0 10 20 30
Stress (MPa)D
epth
(m
)
Stress measurements
Mottledanorthosite(Middling 3)
Spottedanorthosite(Middling 2)
Anorthositicnorite(Middling 1)Pyroxenite
BastardReef
Geological logUCS
StrengthMPa
12,64
12,05
8,98
1,741,15
Geotechnical log
Joints/m>450
Joints/m<450
208
174
139
119
1138
2
00
0
0
0
71
1
0
0
14
8
8
0
2
4
6
8
10
12
-20 -10 0 10 20 30
Stress (MPa)D
epth
(m
)
Slide 10 © CSIR 2006 www.csir.co.za
Virgin stress condition
K-ratio
-1600
-1400
-1200
-1000
-800
-600
-400
-200
0
0 0.5 1 1.5 2 2.5 3
Dep
th b
elo
w s
urf
ace
(m)
X
Slide 11 © CSIR 2006 www.csir.co.za
30 mm
31 mm
Stressmeasurements
N
Elastic convergence
Slide 12 © CSIR 2006 www.csir.co.za
Elastic stress results
0
2
4
6
8
10
12
14
-20 -10 0 10 20 30
Horizontal stress (MPa)
He
igh
t a
bo
ve
sto
pe
(m
)
TXXTYYSIG1
0
2
4
6
8
10
12
14
-20 -10 0 10 20 30
Horizontal stress (MPa)
He
igh
t a
bo
ve
sto
pe
(m
)TXXTYYSIG1SIG3
K-ratio = 1,2 K-ratio = 0,5
0
2
4
6
8
10
12
-20 -10 0 10 20 30
Stress (MPa)
Dep
th (
m)
Slide 13 © CSIR 2006 www.csir.co.za
Strike section showing possible plate formation
Slide 14 © CSIR 2006 www.csir.co.za
Open vertical joint
Slide 15 © CSIR 2006 www.csir.co.za
Standard beam solutions
• Built-in ends: ht
gL 4
2
max
2
4
max 32Et
gL
• Freely-supported: ht
gL 4
3 2
max
2
4
max 32
5
Et
gL
Slide 16 © CSIR 2006 www.csir.co.za
Freely supported beam
-30
-20
-10
0
10
20
30
40
50
0 2 4 6 8 10 12 14
Intact beam thickness (m)
Str
ess
(MP
a)
0
50
100
150
200
250
Def
lect
ion
(m
m)
Base of beamTop of beamMeasuredMeasuredDeflection
Estimated beam thickness from underground observations
and geotechnical
logging7 m - 8,75 m
Slide 17 © CSIR 2006 www.csir.co.za
Modified beam analysis (analytical solution)
• Built-in ends: ht
gLk 4
2
max
2
4
max 32Et
gLk
• Freely-supported: ht
gLk 4
3 2
max
2
4
max 32
5
Et
gLk
f
x
W
SRWk 1
Slide 18 © CSIR 2006 www.csir.co.za
Freely supported beam using modification
-60
-40
-20
0
20
40
60
0 2 4 6 8 10 12 14
Intact plate thickness (m)
Str
ess
(MP
a)
0
50
100
150
200
250
Def
lect
ion
(m
m)
Min stressMax stressMeasuredMeasuredDeflection
Estimated plate
thickness from
underground observations
and geotecnical
logging7 m - 8,75 m
geotechnical
Slide 19 © CSIR 2006 www.csir.co.za
Comparison between two freely supported beam methods
-60
-40
-20
0
20
40
60
0 2 4 6 8 10 12 14
Intact plate thickness (m)
Str
ess
(MP
a)
0
50
100
150
200
250
Def
lect
ion
(m
m)
Estimated plate
thickness from
underground observations
and geotecnical
logging7 m - 8,75 m
geotechnical
-30
-20
-10
0
10
20
30
40
50
0 2 4 6 8 10 12 14
Intact beam thickness (m)
Str
ess
(MP
a)
0
50
100
150
200
250
Def
lect
ion
(m
m)Estimated beam
thickness from underground observations
and geotechnical
logging7 m - 8,75 m
Slide 20 © CSIR 2006 www.csir.co.za
Shear plane
Slide 21 © CSIR 2006 www.csir.co.za
Inelastic modeling showing compression and tensile stress zones
Slide 22 © CSIR 2006 www.csir.co.za
Inelastic stress distribution above the centre of the panel
0
2
4
6
8
10
12
-20 -10 0 10 20 30
Horizontal stress (MPa)
Hei
gh
t a
bo
ve s
top
e (
m)
Applied K = 0
Applied K = 0.5
Applied K = 0.75
Applied K = 1
Undergroundmeasurements
Zone of vertical joints
0
2
4
6
8
10
12
14
-20 -10 0 10 20 30
Horizontal stress (MPa)
He
igh
t a
bo
ve
sto
pe
(m
)
TXXTYYSIG1
K-ratio = 1,2
Slide 23 © CSIR 2006 www.csir.co.za
Implications of the findings
Slide 24 © CSIR 2006 www.csir.co.za
Significance of findings
Pillar
Slide 25 © CSIR 2006 www.csir.co.za
Conclusions
• The numerical and analytical analyses show that a complex beam or plate structure developed over the stope.
• The hangingwall behaviour is best described by a modified version of the freely supported beam theory.
• Stable stope spans may be determined by comparing the stress developed at the centre and edges of the beam to the rock strength.
• In-panel pillars left in old workings could possibly be partially or completely mined out on retreat at relative low cost.
• Extraction ratios of current workings can be improved slightly using the same procedure as for old workings.
Slide 26 © CSIR 2006 www.csir.co.za
Contributing authors:
• DP Roberts• S Coetzer• N Singh• F Flanagan
Are acknowledged for their contribution