Confinement-dependent transient mi for the Hoek-Brown (H-B) strength criterion

The growing global demand for mineral resources necessitates deeper excavation, where increasing in situ stresses pose significant geomechanical challenges. Consequently, an accurate strength assessment of deep-seated rock masses is crucial for ensuring the safety and reliability of underground excavations. The Hoek-Brown (H-B) strength criterion remains a widely applied rock strength criterion. However, its conventional form struggles to characterize the full nonlinear evolution of rock strength from pre-critical to critical conditions across varying confining stresses. To address this limitation, this study proposes a Confinement-dependent Transient m_i Model (CTM) as an enhancement to the H-B criterion. By incorporating confinement-sensitive variations of m_i, calibrated using low-confinement triaxial test data, the enhanced H-B criterion effectively describes the complete nonlinear strength response. Its predictive accuracy is validated against the classical H-B model using 160 triaxial compression tests spanning 14 rock types. The results demonstrate that the enhanced criterion significantly improves the agreement between experimental and theoretical strength predictions. Additionally, the study provides a mechanistic interpretation of the transient m_i parameter under varying confining pressures, establishing its role as a brittleness index—where higher m_i values indicate greater brittleness, and lower values reflect enhanced ductility under differential stress conditions.