Slope stability assessment is an essential aspect of mining and civil engineering. In this study, Songwe open-pit mine in Malawi was investigated to establish possible pit slope instability. In performing the analysis, an integrated approach entailing rock mass characterisation, kinematic and numerical methods were applied. Based on rock mass classification system, Songwe Hill carbonatite rock mass is characterised as a good rock but still it possesses numerous random discontinuities that present a complex challenge in geotechnical engineering. Dip 6.0 software was used in carrying out kinematic analysis based on the attributes of discontinuities. The results show that there is a 16% likelihood of planar failure in the divided slope sections of the planned pit. Thus, slope angle optimisation to 41° has been proposed as a counter-measure to minimise the potential risk of planar failure. At the optimised angle, the risk of planar failure could be reduced by 44%. On the other hand, wedge failure was found to be improbable since no joint intersections were found in the critical zone of potential failure. For numerical analysis, finite element code was applied using FLAC3D 5.0 application. The results demonstrate that overall slope angle of 41° would offer a favourable balance between safety and mining economics as mining operations progress to deeper horizons thereby avoiding a costly push back solution due to instability.
References
[1]
Eberhardt, E. (2003) Rock Slope Stability Analysis—Utilization of Advanced Numerical Techniques. Earth and Ocean Sciences at UBC, Vancouver.
[2]
Llano-Serna, M.A., Williams, D.J. and Ruest, M.R. (2016) Analysis of Kennecott Utah Copper’s Biggham Canyon Mine Pit Wall Slides. Tailings and Mine Waste 2016, Keystone, 2-5 October 2016, 787-794.
[3]
Smith, J.V. and Arnhardt, C. (2016) A New Assessment Method for Structural-Control Failure Mechanisms in Rock Slopes—Case Examples. AIMS Geosciences, 2, 214-230. https://doi.org/10.3934/geosci.2016.3.214
[4]
Goodman, R.E. (1989) Introduction to Rock Mechanics. 2nd Edition, John Wiley and Sons, Hoboken.
[5]
Hoek, E. (2005) Rock Slope Engineering. 4th Edition, The Institute of Mining and Metallurgy, New York.
[6]
Wyllie, D.C. and Mah, W.C. (2004) Rock Slope Engineering. 4th Edition, Taylor & Francis Group, New York.
[7]
Sjoberg, J. (1996) Large Scale Slope Stability in Open Pit Mining. Lulea University of Technology: Division of Rock Mechanics, Lulea.
[8]
Read, J. and Stacey, P. (2010) Guidelines for Open Pit Slope Design. SCIRO Publishing, Clayton. https://doi.org/10.1071/9780643101104
[9]
Mogorosi, O., Gabanakgosi, K., Barei, O.M. and Thokweng, T. (2017) Slope Stability Challenges and Solutions for Mining Kimberlite Resources Hosted in Structurally Complex Country Rock: Dip Slope Mining at Jwaneng Mine, Botswana. International Kimberlite Conference, Gaborone, Vol. 11, 1-3.
[10]
Tunono, A.B. and Dimbungu, L. (2010) Jwaneng Open Pit Mine Cut 8 South East Wall Slope Design. The Southern African Institute of Mining and Metallurgy, Johannesburg, South Africa.
[11]
Stead, D. (2015) The Influence of Shales on Slope Instability. Rock Mechanics and Rock Engineering, 49, 635-651. https://doi.org/10.1007/s00603-015-0865-0
[12]
Tuckey, Z., Stead, D. and Eberhardt, E. (2013) Combining Field Methods and Numerical Modelling to Address Challenges in Characterising Discontinuity Persistence and Intact Rock Bridges in Large Open Pit Slopes. Australian Centre for Geomechanics, Perth. https://doi.org/10.36487/ACG_rep/1308_07_Tuckey
[13]
Stacey, T.R. (2007) Slope Stability in High Stress and Hard Rock Conditions. Australian Centre for Geomechanics, Perth.
https://doi.org/10.36487/ACG_repo/708_Stacey
Zhou, X., Chen, J., Chen, Y., Song, S., Shi, M. and Zhan, J. (2016) Bayesian-Based Probabilistic Kinematic Analysis of Discontinuity-Controlled Rock Slope Instabilities. Bulletin of Engineering Geology and the Environment, 76, 1249-1262.
https://doi.org/10.1007/s10064-016-0972-5
[16]
Singh, T. and Rao, K.S. (2016) Kinematic Stability Analysis of Multi-Faced Rock Slopes in the Himalayas. Proceedings of the Conference on Recent Advances in Rock Engineering, Amsterdam, The Netherlands, 281-284.
[17]
Obregon, C. and Mitri, H. (2019) Probabilistic Approach for Open Pit Bench Slope Stability Analysis—A Mine Case Study. International Journal of Mining Science and Technology, 29, 629-640. https://doi.org/10.1016/j.ijmst.2019.06.017
[18]
Kincal, C. (2014) Application of Two New Stereographic Projection Techniques to Slope Stability Problems. International Journal of Rock Mechanics & Mining Sciences, 66, 136-150. https://doi.org/10.1016/j.ijrmms.2014.01.006
[19]
Fleurisson, J.A. (2012) Slope Design and Implementation in Open Pit Mines. Geological and Geomechanical Approach. Procedia Engineering, 46, 27-38.
https://doi.org/10.1016/j.proeng.2012.09.442
[20]
Al Mandalawi, M., You, G., Dowling, K. and Dahlhaus, P. (2016) Kinematic Assessment of Slopes at Handlebar Hill Open Cut Mine. International Journal of GEOMATE, 10, 1575-1583. https://doi.org/10.21660/2016.19.09509
[21]
Croll, R., Swinden, S., Hall, M., Brown, C., Beer, G., Scheerpers, J. and Trusler, G.E. (2014) NI 43-101 Pre-Feasibility Report. MSA Group (Pty) Ltd., Johannesburg.
[22]
Garson, M.S. and Smith, C.W. (1965) Carbonatite and Agglomeratic Vents in the Western Shire Valley. Geological Survey Department, Malawi.
[23]
British Geological Survey (2009) Malawi Potential of Malawi. Malawi Government, Malawi.
[24]
Broom-Fendley, S., Brady, A.E., Horstwood, M.S., Woolley, A.R., Mtegha, J., Wall, F. and Gunn, G. (2017) Geology, Geochemistry and Geochronology of the Songwe Hill Carbonatite, Malawi. Journal of African Earth Sciences, 134, 10-23.
https://doi.org/10.1016/j.jafrearsci.2017.05.020
[25]
Witley, J.C., Swinden, S., Trusler, G. and Dempers, N. (2019) Songwe Hill Rare Earth Elements (REE) Project, Phalombe District, Malawi. MSA Group, Randburg.
[26]
Ulusay, R. (2015) The ISRM Suggested Methods for Rock Characterization, Testing and Monitoring: 2007-2014. Springer International Publishing, Berlin.
https://doi.org/10.1007/978-3-319-07713-0
[27]
Bieniawski, Z.T. (1989) Engineering Rock Mass Classification: A Complete Manual for Engineers and Geologists in Mining, Civil and Petroleum Engineering. John Wiley and Sons, Toronto.
[28]
Itasca (2012) Fast Lagrangian Analysis of Continua in 3Dimensions User’s Guide. Fifth Edition, Itasca Consulting Group Inc., Minneapolis.
Singh, B. and Goel, K.R. (2011) Engineering Rock Mass Classification: Tunneling, Foundations, and Landslides. Butterworth-Heinemann, Oxford.
https://doi.org/10.1016/B978-0-12-385878-8.00011-2
[31]
Hoek, E., Carter, T.G. and Diederichs, M.S. (2013) Quantification of the Geological Strength Index chart. American Rock Mechanics Association, San Francisco.
[32]
Kliche, C.A. (1999) Rock Slope Stability. Society for Mining, Metallurgy, and Exploration, Inc., Littleton.
Bye, A.R. and Bell, F.G. (2001) Stability Assessment and Slope Design at Sandsloot Open Pit, South Africa. International Journal of Rock Mechanics & Mining Sciences, 38, 449-466. https://doi.org/10.1016/S1365-1609(01)00014-4
[35]
Singh, T.N., Puri, S. and Pandey, A. (2003) Stability of Rock Slopes in Complex Rock Formation. In: Deformation Characteristics of Geomaterials, A.A. Balkema, Lisse, 947-954.
[36]
Melnikov, M.N., Kozyrev, A.A., Reshetnyak, S.P., Kasparian, E.V. and Rybin, V.V. (2003) Geomechanical and Technical Substantiation of an Optimal Slope Angle in the Kovdor Open Pit. In: Udd, J.E. and Bekkers, G., Eds., Mining in the Arctic, CIM, Kola, Russia, 321-327.
[37]
Sonmez, H. and Ulusay, R. (1999) Modifications to the Geological Strength Index (GSI) and Their Applicability to Stability of Slopes. International Journal of Rock Mechanics and Mining Sciences, 36, 743-760.
https://doi.org/10.1016/S0148-9062(99)00043-1
