Dunn, MJ, Harris, RO, Boitshepo, B & Chiwaye, HT 2022, 'Assessing the stress field for the Jwaneng underground project', in Y Potvin (ed.), Caving 2022: Proceedings of the Fifth International Conference on Block and Sublevel Caving
, Australian Centre for Geomechanics, Perth, pp. 123-138, https://doi.org/10.36487/ACG_repo/2205_06
The Debswana Diamond Company (DDC) Jwaneng Mine in Botswana, is a large open pit diamond mine extracting three kimberlite pipes. Open pit operations will cease in the early 2030s; studies are currently underway to transition to an underground operation by the time open pit mining ceases. It is anticipated that underground mining will be undertaken using a combination of sublevel caving (SLC) and other caving methods; mining will extend to a depth of ~1,000 m below surface.
The Jwaneng Mine is structurally complex with many large geological structures and a variable rock mass, with strengths ranging from weak kimberlites (25 MPa) to very competent dolomites (>250 MPa).
Understanding the in situ stress regime is an important input for underground mine design and has a significant impact on designs analyses including numerical modelling. The magnitudes, ratios and directions of the principal stresses influence the choice of mining method, excavation stability, stand-off distances, extraction sequence etc. Various evaluations have been undertaken and this includes stress measurements using the Sigra in situ stress test (IST) and the deformation rate analysis (DRA) methods. In addition, borehole ovality analysis using televiewer data has been undertaken on ~100 boreholes to assess the orientation of the maximum horizontal stress.
In addition to these measurements, the literature has been reviewed and independent reviews of all work have been undertaken. The outcomes from the various methods do not always agree and the interpretation of the stress field in a complex geological environment is not straightforward and has associated uncertainties. The interpretation of the data is an iterative process with different approaches applied with the aim of developing reasonable stress field inputs and sensitivities for design analysis. The process followed and the outcomes will be discussed in this paper.
Keywords: in situ stress field, stress magnitude, Sigra in situ stress test, deformation rate analysis, ovality analysis
Adam, J 1982, ‘Stress-relief buckles in the McFarland quarry, Ottawa’, Canadian Journal of Earth Science, vol. 19, pp. 1883-1188.
Andreoli, MAG, Bumby, A, Ben-Avraham, Z, Delvaux De Fenffe, D, De Wit, M, Durrheim, R, … Tabola, K 2015, ‘The stress field of southern Africa’, 10th Annual AfricaArray Workshop.
Barnett, WP 2009, Jwaneng Country Rock Model Update, report, SRK Consulting.
Barnett, WP 2020, Dynamic 3-D Litho-Structural Model Update 2019, report, SRK Consulting.
Beukes, NJ 2006, Stratigraphy of the Transvaal Supergroup at Jwaneng Mine in Botswana, Jwaneng Phase 2 Report, KLM Consulting Services (Pty) Ltd, Lanseria.
Bird, P, Ben-Avraha, Z, Schubert, G & Andreoli, M 2006, ‘Patterns of stress and strain rate in southern Africa’, Journal of Geophysical Research, vol. 111.
Creus, PK, Basson, IJ, Stoch, B, Mogorosi, O, Gabanakgosi, K, Ramsden, F & Gaegopolwe, P 2017, ‘Structural analysis and implicit 3D modelling of Jwaneng Mine: insights into deformation of the Transvaal Supergroup in SE Botswana’, Journal of African Earth Sciences, vol. 137, pp. 9–21.
Dight, P & Hsieh, A 2019, Jwaneng Mine in situ Stress Measurement Report, Australian Centre for Geomechanics, Perth.
Handley, MF 2013, Pre-mining stress model for subsurface excavations in southern Africa, The Southern African Institute of Mining and Metallurgy, vol. 113, pp. 449–471.
Harris, RO 2022, Ovality Analysis – Open Pit Influence, internal Debswana report.
Harvey Rock Physics, 2020, Stress Orientation Determination at Jwaneng Mine, consulting report.
Harvey Rock Physics, 2021, Stress Orientation Determination at Jwaneng Mine – Extension Study, consulting report.
Hooker, VE & Bickel, DL 1974, Overcoring Equipment and Techniques Used in Rock Stress Determination, report of investigation 8618, US Bureau of Mines.
Gabanakgosi, K, Mosebi, K, Mogorosi, O, Barei OM, Ntlotlang, K & Thokweng, T 2018, ‘A multi-disciplinary approach to dip slope mining in Jwaneng Mine, Botswana’, Proceedings of the 2018 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Seville.
Lee, M, 2020, In Situ Stress Setting Review, Appendix A in Jwaneng Underground Concept Study 2020 Rev1, pp. 135–152.
Maloney, S 2020, Jwaneng Mine Early Development Project - Geotechnical Stress Measurement Assurance, MIRARCO, Sudbury.
Manzunzu, B, Midzi, V, Mulabisana, TF, Zulu, B, Pule, T, Myendeki, S & Rathod, GW 2019, ‘Seismotectonics of South Africa’, Journal of African Earth Sciences, vol 149, pp. 271–279.
Meghraoui, M & IGCP-601 Working Group 2016, ‘The seismotectonic map of Africa’, Episodes, vol. 39, no. 1, pp. 9–18.
Reusch, F, Beck, D & Tyler, D, 2008, ‘Quantitative forecasting of sidewall stability and dilution in sub-level caves’, MassMin 2008: Proceedings of the Fifth International Conference and Exhibition on Mass Mining, Luleå University of Technology, Luleå, pp. 453–460.
Stacey, TR 2007, ‘Slope stability in high stress and hard rock conditions’, in Y Potvin (ed.), Slope Stability 2007: Proceedings of the 2007 International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 187–200,
Stacey, TR & Wesseloo, J, 1998, ‘In situ stresses in mining areas in South Africa’, Journal of the South African Institute of Mining and Metallurgy, vol. 98. pp. 365–368.
Stewart, I, Smith, D, Keloth, R, Xhao, X & Gray, I 2020, In situ stress testing on Boreholes GT279, GT280, GT281, GT282, & GT285, Sigra Job Reference Number: 497, Sigra, Acacia Ridge.
Stewart, B & Gray, I 2019, In situ stress testing on Boreholes GT76 & GT100, Sigra Job Reference Number: 361, Sigra, Acacia Ridge.
Venter, D, Bosman, K, Clanahan, R & Christensen, J 2019, Jwaneng Post Cut 9 Mining Studies – Concept Stage, geotechnical study report.
Wallach, J, Benn, K & Rimando, R 1995, ‘Recent, tectonically induced, surficial stress-relief structures in the Ottawa – Hull area, Canada’, Canadian Journal of Earth Science, vol. 32, pp. 325–333.
Wiles, TD 2006, ‘Reliability of numerical modelling predictions’, International Journal of Rock Mechanics and Mining Sciences, vol. 43, pp. 454–472.
Zajac, B 1997, The State of Stress as Inferred from Deviated Boreholes: Constraints on the Tectonics of Offshore Central California and Cook Inlet, Alaska, PhD thesis, Caltech, Pasadna.