Authors: Baczynski, NRP

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Baczynski, NRP 2020, 'Hoek–Brown rock mass: adjusting Geological Strength Index for directional strength', in PM Dight (ed.), Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 901-912,

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Rock mass shear strength is often directional due to sets of geological defects that co-align with failure paths. In the limit, the entire path may be defined by co-aligned defects and rock mass strength in that direction is equal to defect strength plus the strength of any intact rock bridges that may exist between such defects. Conceptual considerations (Baczynski 2018) and a large number of case studies (Baczynski 2019a, b) indicate strong linear relationships between relative occurrences (%) of co-aligned defects and intact rock bridges and the adjustments required in the Geological Strength Index (GSI) input to Hoek–Brown equations to quantify the directional shear strength. GSI adjustment is a two-step process. The general rock mass GSI is first negatively adjusted for relative portion (%) of failure path length that is defined by geological defects coaligned with the path. GSI is then positively adjusted for relative portion (%) of failure path length defined by intact rock bridges between the co-aligned defects. GSI (directional) is computed as GSI (general rock mass) minus GSI (defect adjustment) plus GSI (bridge adjustment). For general design, GSI (defect adjustment) is 0.4 × (occurrence (%) of co-aligned defects) and GSI (bridge adjustment) is 1.2 × (occurrence (%) of intact rock bridges along the failure path). Correlation coefficients for the proposed adjustments are typically 0.75 to 0.95. GSI adjustment factors may be further refined by mi and rock type. The geotechnical data that needs to be collected to enable GSI adjustment is discussed. Indicative step-path case study results used to develop the recommended GSI adjustments are shown. Example data for probabilities (%) of occurrence for defects and intact rock bridges and their respective lengths are tabulated. Use of the Rosenblueth method to develop statistical models is explained.

Keywords: Hoek–Brown rock mass, directional strength, data collection, case studies, GSI adjustment

Baczynski, NRP 2000, ‘STEPSIM4 Step-Path method for slope risks’, Proceedings of GeoEng 2000: An International Conference on Geotechnical & Geological Engineering, vol. 2, International Society for Rock Mechanics, Lisbon, p. 86.
Baczynski, NRP, deBruyn, IA, Mylvaganam, J & Walker, DJH 2011, ‘High rock slope cutback geotechnics: a case study at Ok Tedi Mine’, in E Eberhardt & D Stead (eds), Proceedings of Slope Stability 2011: International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering, Canadian Rock Mechanics Association, 13 p.
Baczynski, NRP 2018, ‘Step path adjusted Hoek Brown GSI chart’, Proceedings of the 10th Asian Rock Mechanics Symposium, Society for Rock Mechanics and Engineering Geology, Singapore, paper 0590, 12 p.
Baczynski, NRP 2019a, ‘GSI adjustment for directional Hoek–Brown strength quantified by case studies’, Proceedings of the 14th ISRM Congress, International Society for Rock Mechanics, Lisbon, 8 p.
Baczynski, NRP 2019b, ‘GSI adjustment for directional Hoek-Brown strength calibrated by step-path case studies’, Australian Geomechanics, vol. 54, no. 3, pp. 51–78.
Bar, N & Baczynski, NRP 2019, ‘Slope failure risk by engineering geological logic and the simplified step-path method’, Proceedings of the 13th Australia New Zealand Conference on Geomechanics, Australian Geomechanics Society, Perth, 5 p.
Brady, BHG & Brown, ET 2004, Rock Mechanics for Underground Mining, 3rd edn, Kluwer Academic Publishers, Berlin.
Rosenblueth, E 1975, ‘Point estimates of probability moments’, Proceedings of the National Academy of Sciences, vol. 72, no. 10, pp. 3812–3814.
Truzman, M 2007, ‘Statistical summary of rock mass characterization for tunnels of the Caracas-Tuy Medio railroad project’, Proceedings of the XXIII Pan-American Conference on Soil Mechanics and Geotechnical Engineering, vol. 2, Sociedad Venezolana de Geotecnia, Chacao, pp. 877–882.

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