| Samenvatting: | Many studies focus on brittle–ductile transition stress in intact rocks; however, in real life, we deal with rock mass which contains many discontinuities. To fill this gap, this research focuses on the brittle–ductile transition stress of rock mass by considering the influence of different Geological Strength Index (<i>GSI</i>) values on the brittle–ductile transition stress of rock mass. In other words, the Hoek–Brown failure criteria for rock mass were reformulated mathematically including the ductility parameter (d), which is defined as the ratio of differential stress to minor stress. Then, the results were analyzed and plotted between <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mstyle scriptlevel="0" displaystyle="true"><mfrac><mrow><msubsup><mrow><mi>σ</mi></mrow><mrow><mn>3</mn></mrow><mrow><mo>*</mo></mrow></msubsup></mrow><mrow><msub><mrow><mi>σ</mi></mrow><mrow><mi>c</mi></mrow></msub></mrow></mfrac></mstyle></mrow></semantics></math></inline-formula> and <i>GSI</i>, considering different (<i>d</i>) and Hoek–Brown material constant (<i>m<sub>i</sub></i>) values. The brittle–ductile transition stress, <i>σ<sub>3</sub></i><sup>*</sup>, was determined by intersecting the Hoek–Brown failure envelope with Mogi’s line, with ductility parameters d ranging from 3.4 (silicate rocks) to 5.0 (carbonate rocks). Numerical solutions were derived for <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mstyle scriptlevel="0" displaystyle="true"><mfrac><mrow><msubsup><mi>σ</mi><mn>3</mn><mo>*</mo></msubsup></mrow><mrow><msub><mi>σ</mi><mi>c</mi></msub></mrow></mfrac></mstyle></mrow></semantics></math></inline-formula> as a function of <i>GSI</i> using Matlab, and the results were fitted with an exponential model. The analysis revealed an exponential relationship between <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mstyle scriptlevel="0" displaystyle="true"><mfrac><mrow><msubsup><mrow><mi>σ</mi></mrow><mrow><mn>3</mn></mrow><mrow><mo>*</mo></mrow></msubsup></mrow><mrow><msub><mrow><mi>σ</mi></mrow><mrow><mi>c</mi></mrow></msub></mrow></mfrac></mstyle></mrow></semantics></math></inline-formula> and <i>GSI</i> for values above 32, with accuracy better than 3%. Increased ductility reduces rock mass strength, with higher d values leading to lower <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mstyle scriptlevel="0" displaystyle="true"><mfrac><mrow><msubsup><mrow><mi>σ</mi></mrow><mrow><mn>3</mn></mrow><mrow><mo>*</mo></mrow></msubsup></mrow><mrow><msub><mrow><mi>σ</mi></mrow><mrow><mi>c</mi></mrow></msub></mrow></mfrac></mstyle></mrow></semantics></math></inline-formula>. The diminishing returns in confinement strength at higher <i>GSI</i> values suggest that rock masses with higher <i>GSI</i> can sustain more confinement but with reduced effectiveness as <i>GSI</i> increases. These findings provide a framework for predicting brittle–ductile transitions in rock engineering.
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