1. Contribution to Assessing the Risk of Unexpected High Wall Failure in South African Opencast Coal Mines
Liisa Kawali

2. Outline Introduction Failure/consequences Failure types/origins
Traditional investigation methods
Complementary methods
Conclusions

3. Introduction
High wall failures in opencast mines continue to occur even with much improved geotechnical logging and damage prevention blasting techniques.

4. 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)

6. Failure/Consequences1
(fatality)
10 (serious)
100 (slightly serious)
1000 (near misses)

7. Big failures are spectacular and costly
BUT
the cumulative effects of small incidences can be huge!

8. 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

9. Failure Types Wedge Plane Toppling Circular Toppling direction
Direction of failure
Toppling
Circular

10. Origin of failures Rock mass Discontinuities and Structures
Competent/incompetent horizons
Intact or fractured
Discontinuities and Structures
Joints
Beddings plane
Faults
Stresses (pre-existing or induced)
Seismic activity

11. Unexpected High Wall failures
(Simmons and Simpson, 2007)

12. 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

13. Pre-split barrels

14. RSA Coal Mines
High horizontal to vertical stress ratios have been recorded
100 m
(Stacey and Wesseloo, 1998)

15. Failure Hypothesis
Changes in stress fields due to opencast mining can lead to extension strain failure due to low material strength
Occurrence of fractures provides additional release surfaces which reduces overall face competence
New fractures may form during blasting – if instability appears immediately and loose material cleaned during overburden removal
Fractures formed after excavation will cause unforeseen and unidentified instability

16. 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)

17. Effect of Excavation on Stress
σH2
σH1
For example:
At 100 m below surface before mining takes place:
σz = 2.4 MPa
σx = σy = 5 MPa (k ratio = 2)
Following extraction of a cut:
Average vertical stresses = decrease to <1 MPa (0.6 MPa for 30 m high face)
Horizontal stresses on wall parallel to x = increase about 50%
Horizontal stresses in y direction = reduce to zero

18. From Geotechnical logs:
Basic geotechnical data
TCR
SCR
RQD
Detailed geotechnical data
IRS
Fractures
Joint /conditions
Roughness
Alteration
Fill/type Q
GSI
RMR

19. Extra Information Needed
Elastic properties
Insitu stress

20. Complimentary Methods to Geotechnical Logging
Geophysical methods:
Resistivity and ultrasonic: actual borehole failures and breaks
Sonic logs: correlation between rock strength & Young’s modulus
Acoustic TeleViewer (ATV) – Joint orientation
Neutron logs: lithological interpretation
Seismic velocity: speed of shock waves through rock
- High seismic velocity = strong rock
- Low seismic velocity = more fractured (Waltham, 2005)
Other methods: Logging While Drilling (LWD)
- Includes (Gamma rays, resistivity, borehole pressure & formation tester tools (Boonen, 2003)

21. 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)

22. 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)

23. Conclusions
Current investigation methods are sufficient in identifying risks/weaker horizons
High levels of data collection needed to identify stress related risks
Geophysics can compliment logging and lab testing
Understanding magnitudes and distribution of regional stresses is important to assess risks
Regional stress map in RSA coalfield is proposed to identify small scale anomalies (SRK initiative:
Study can be extended to other mining areas; platinum and chrome (extends to surface mining and failures have been recorded)

24. References
Boonen, P Advantages of Using Logging-While-Drilling Data in Rock Mechanical Log Analysis and Wellbore Stability Modelling.
Harries, N., Noon, D. and Rowley, K Case Studies of Slope Stability Radar used in Open Cut Mines. The South African Insititute of Mining and Metallurgy, International Symposium on Stability of Rock Slopes.
Moashan, C., Shifan, Z., Zhonghong, W. and Lanfeng, L Joint poststack P- and PS-wave impedance inversion and an example from northern China. Society of Exploration Geophysicists v. 28, no. 3, pages
Simmons, J. V. and Simpson, P. J (Potvin, Y., editor) Extension, Stress and Composite Failure in Bedded Rock Masses. Slope Stability 2007: Proceedings of the 2007 International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering.
Simmons, J. V. and Simpson, P. J Composite failure mechanisms in coal measures’ rock masses – myths and reality. The Journal of the South African Institute of Mining and Metallurgy, Vol 106, Non-Refereed Paper.
Stacey, T.R. and Wesseloo, J Evaluation and Upgrading of Records of Stress Measurement Data in the Mining Industry. Safety in Mines Research Advisory Committee.
Steffen, O.K.H., Contreras, L.F., Terbrugge, P.J. and Venter, J A Risk Evaluation Approach for Pit Slope Design. 42nd US Rock Mechanics Symposium (29/06 – 02/07, 2008).
Van der Merwe, J.N. and Madden, B. J. authors 2002 (Buddery, P., editor) Rock Engineering for underground coal mining: A practical guide for supervisors at all levels.
Waltham, T Foundations of Engineering Geology, 2nd Edition, Spon Press

25. Questions?