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) 100 (slightly serious) 1000 (near misses)

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 Circular Toppling direction Direction of failure Toppling Circular

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

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 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

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: 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

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: 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)

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 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: lkawali@srk.co.za) Study can be extended to other mining areas; platinum and chrome (extends to surface mining and failures have been recorded)

References Boonen, P. 2003 Advantages of Using Logging-While-Drilling Data in Rock Mechanical Log Analysis and Wellbore Stability Modelling. Harries, N., Noon, D. and Rowley, K. 2006 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. 2009 Joint poststack P- and PS-wave impedance inversion and an example from northern China. Society of Exploration Geophysicists v. 28, no. 3, pages 332 -338. Simmons, J. V. and Simpson, P. J. 2007 (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. 2006 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. 1998 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. 2008 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. 2005 Foundations of Engineering Geology, 2nd Edition, Spon Press

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