Transcript No Slide Title

Contribution to Assessing
the Risk of Unexpected High
Wall Failure in South African
Opencast Coal Mines
Liisa Kawali
Outline
• Introduction
• Failure/consequences
• Failure types/origins
• Traditional investigation methods
• Complementary methods
• Conclusions
Introduction
High wall failures in opencast mines
continue to occur even with much
improved geotechnical logging and
damage prevention blasting
techniques.
Why Unexpected High Wall Failure?
• Major economic impact
• Economic impacts on Production
• Damage to equipment
• Injury or loss of lives
• Industrial action
• Impact on the environment
• Stakeholder resistance
(Steffen et al, 2008; Harries et al, 2006)
Failure/Consequences
1
(fatality)
10 (serious)
Frequenc
y
High
100 (slightly serious)
1000 (near misses)
Lo
w Lo
w
Consequen
ce
High
Big failures are spectacular and
costly
BUT
the cumulative effects of small
incidences can be huge!
Large Open Cast Mine Slope Stability
4 Seam Bench
30 m
Example of rock face or slope
made by pre-split blasting to give
a stable relatively smooth finish
Failure Types
Wedge
Plane
Toppling
direction
Direction of
failure
Toppling
Circular
Origin of failures
Rock mass
– Competent/incompetent horizons
– Intact or fractured
Discontinuities and Structures
– Joints
– Beddings plane
– Faults
Stresses (pre-existing or induced)
Seismic activity
Unexpected High Wall failures
(Simmons and Simpson, 2007)
Unexpected High Wall Failures
 Also known as composite failures
 Not common in RSA, but in USA and Australia
 Very difficult to predict
 No obvious features for prediction
 Involves:
a) Joints not structurally related
b) Sliding on unrecognized defects
c) Cooler time of day/rapid temperature change
d) Within hours of heavy rain/extended dry weather
 Failure at bench scale
Little information to public domain in RSA mines
Pre-split barrels
RSA Coal Mines
High horizontal to vertical stress ratios have been recorded
100 m
(Stacey and Wesseloo, 1998)
Failure Hypothesis
1. Changes in stress fields due to opencast mining can
lead to extension strain failure due to low material
strength
2. Occurrence of fractures provides additional release
surfaces which reduces overall face competence
3. New fractures may form during blasting – if instability
appears immediately and loose material cleaned
during overburden removal
4. Fractures formed after excavation will cause
unforeseen and unidentified instability
Extension Strain
Where:
E = Young’s Modulus (~3 GPa for coal)
σx, σy, σz = stresses in three orthogonal directions (σz is vertical)
υ = Poisson’s ratio (~0.3 for coal)
Effect of Excavation on Stress
σH2
σH1
For example:
Following extraction of a cut:
At 100 m below surface before mining takes place:
Average vertical stresses = decrease to <1 MPa (0.6 MPa for 30 m high face)
σz =stresses
2.4 MPa
Horizontal
on wall parallel to x = increase about 50%
σx =stresses
σy = 5 in
MPa
(k ratio= =reduce
2) to zero
Horizontal
y direction
From Geotechnical logs:
• Basic geotechnical data
TCR
SCR
RQD
• Detailed geotechnical data
IRS
Fractures
Joint /conditions
Roughness
Alteration
Fill/type
Q
GSI
RMR
Extra Information Needed
Elastic properties
Insitu stress
Complimentary Methods to
Geotechnical Logging
Geophysical methods:
a) Resistivity and ultrasonic: actual borehole failures and breaks
b) Sonic logs: correlation between rock strength & Young’s modulus
c) Acoustic TeleViewer (ATV) – Joint orientation
d) Neutron logs: lithological interpretation
e) Seismic velocity: speed of shock waves through rock
- High seismic velocity = strong rock
- Low seismic velocity = more fractured (Waltham, 2005)
f) Other methods: Logging While Drilling (LWD)
- Includes (Gamma rays, resistivity, borehole pressure & formation
tester tools (Boonen, 2003)
Example of Logging While Drilling (LWD)
Effect of 3 µsec/ft increase in
compressional slowness in a
sandstone on the computed
poisson’s ratio
(Maoshan et al 2009
and Boonen, 2003)
Weaker layers, high
gamma readings
More work is required to define links
between the geophysical signature
with geotechnical properties from the
lab testing (calibration)
Van der Merwe and
Madden, 2002)
Conclusions
1. Current investigation methods are sufficient in identifying
risks/weaker horizons
2. High levels of data collection needed to identify stress related
risks
3. Geophysics can compliment logging and lab testing
4. Understanding magnitudes and distribution of regional
stresses is important to assess risks
5. Regional stress map in RSA coalfield is proposed to identify
small scale anomalies (SRK initiative: [email protected])
6. Study can be extended to other mining areas; platinum and
chrome (extends to surface mining and failures have been
recorded)
Questions?