Modification of Rock Stress Factor for the Mathews Stability Graph Method Based on Hoek–Brown Criterion and Its Application

During underground mining, stope stability is affected by excavation-induced stress redistribution and nonlinear degradation of rock mass strength. The conventional Mathews stability graph method employs an empirical stress factor A, which does not explicitly consider the nonlinear relationship between stress conditions and rock mass strength. In this study, the generalized Hoek–Brown criterion was introduced to define the maximum stress factor (MSF), and a modified stress factor A′ was developed by considering tensile and shear failure mechanisms. The proposed method incorporates the nonlinear stress–stability relationship of underground stopes and improves the reliability of stability assessment. A copper mine in southwest China was selected as a case study. The rock mass quality indices and stress parameters of ten representative stopes were obtained through field investigations, stope roof stress measurements, discontinuity surveys, and laboratory tests. The modified stress factor A′ was incorporated into the Mathews stability graph to account for excavation-induced stress redistribution and rock mass strength degradation. Compared with the conventional method, the modified approach generally reduced the stability numbers of the investigated stopes, with an average reduction of approximately 25.6% (excluding D1780-1, where the confinement strengthening effect resulted in a slight increase in stability number). The revised stability classifications show good consistency with the FLAC3D simulation results and field observations, providing supporting evidence for the application of the proposed method to underground stope stability assessment in the studied mine.