Abstract: To advance the resource utilization of coal gangue as subgrade fill, this study systematically investigates the coupled effects of fractal dimension and confining pressure on the mechanical behavior of gangue through static triaxial shear tests. Using gangue from Nayong County, Guizhou, five gradations with fractal dimensions (D=2.1~2.45) and four confining pressures (50~300 kPa) were designed. Results show that the gangue is dominated by the SiO₂-Al₂O₃-Fe₂O₃ system (>85%), indicating good engineering applicability. All specimens exhibited dilatant plastic failure. Strength parameters reveal that cohesion c increases from 76.87 kPa to 101.81 kPa (32.47% growth) with rising fractal dimension, whereas the internal friction angle φ varies negligibly (<12%). Embedded within the Mohr-Coulomb framework, a semi-empirical model is developed to quantitatively predict the strength of graded gangue. The model clarifies the underlying mechanism by which fractal grading governs strength through particle filling and interlocking, providing a theoretical basis for the optimal design of coal-gangue subgrade fills.

Key words: Coal gangue, Fractal dimension, Triaxial shear test, Gradation effect, Subgrade filler, Mechanical properties